The general warming of the climate with associated increased temperatures has also seen increases in cooling system problems, a trend that is going to continue. Increased issues in temperate climates should not be seen as any failing of the original MG designed cooling systems as it needs to be remembered that these conditions are often normal temperatures for hotter parts of the world where these MGs were routinely exported to, and MG didn’t carry out any substantial changes to the cars specification, usually just a 74 degree C stat specific in those markets that can be usefully applied to our cars in place of the 82 degree C item.
It is to be expected that some cars will be more affected than others due to variation between cars. However, it is interesting to note differences between chrome bumper cars being generally less affected than rubber bumper cars. Over many years I have noticed during testing of many chrome and rubber bumper cars on rolling roads that rubber bumper cars were more difficult to keep cool than chrome bumper cars. Some of the reason is the obvious restriction that the rubber bumpers position creates, but what was surprising is that the metal grille stone guard behind the rubber bumper has a surprisingly big impact and removing it whilst on the rolling road saw a marked improvement in the speed of cooling and reduction in temperature.
The bottom line though is that all these MGs cooling systems in good working original condition should cope with everything the standard engine and operating conditions can throw at them during normal road driving, so why do some cars temperature gauges head for the hills at the first hint of a traffic queue, wider throttle or indeed a hill? At this point it is important to say that any cooling system problem needs to be diagnosed promptly and also correctly in order for the gravity of the problem can be assessed and the appropriate solution actioned.
The easy answer is that there is a basic fault, or several faults giving a cumulatively bigger impact within the cooling system. A very quick check of the fan belt to make sure it is tight enough to drive both the water pump and if applicable the mechanical fan at the correct speed takes just seconds. Whilst around the fan it’s worth making sure any metal fan doesn’t have any loose blades as if they separate the bonnet will pose no restriction to it wanting to try and reach orbit!
A long term and creeping issue simply associated with age is the general build-up of corrosion within the cooling system, which is why the use of coolant with the recommended corrosion inhibitors and changing it within the recommended service periods is best to reduce this corrosion as far as possible. Much will be generated from within the cylinder block and heads internal cast iron material, with addition corrosion coming from other ferrous componentry within the cooling system.
This MG RV8 lower coolant pipe connects between a split bottom hose and here shows how in the lower section covered by the hose has seriously corroded due to lack of routine cooling system maintenance.
Alloy engines such as the V8 and components such as A and B series thermostat housings are less susceptible, but not immune to corrosion problems. One less common area that can affect alloy components is when the cars electrical systems earthing path is poor and this sees electrical flow seek out alternative paths and some really quite significant damage can be seen in the image of a rather sad Revotec fan controller.
Alloy components can corrode rapidly if there is any issue with the vehicle electrical system grounding as can be seen in this poor condition Revotec fan controller.
The next most obvious cooling system suspect is any obvious coolant leak and in most cases the source of a leak is simply a matter or following the wet trail! One regular failure point is to find a drip or dried coolant trail from under the small hole in the underside of the water pump main body. Such a leak shows that the pump is in need of being renewed and the leak is the failing seals around the pumps shaft, which may be aggravated by wear in the shaft bearing/bushing that allows too much lateral movement, stretching the seals ability to seal.
Another possible hidden issue on pumps that have been in service for a long time that isn’t obvious initially, but can create overheating could be corrosion of the pump impeller. This can see the pumps flow rate being noticeably reduced, so that for example during a drive when the engine is up to temp and you come to a halt and the engine idles and after a couple of minutes the temp may rise and possibly lead to overheating.
However, a quick check that can confirm is to raise engine rpms to between 1500 and 2000 and held there. If within a minute or two the temp drops then this would be pointing to the pumps flow not being good enough as higher rpms will increase the flow rate. If the pumps impeller has rusted then it won’t flow as much coolant and of course a core reason for this is often that the coolant hasn’t been changed frequently enough and so the antifreeze contents anti corrosion aspects have degraded. It is worth noting that some replacement pumps can come with a plastic impeller, which will deal with potential corrosion affecting impeller performance, but can create their own individual issues.
One that is possible is the impeller becoming loose on the shaft, so the pumps shaft still turns but the impeller doesn’t, thus leading to no coolant flow and rapid temperature rise to overheating. In the short period that the impeller starts to loosen it may only slip on the shaft and so you get a slowly increasing flow reduction with a slow increase in the overheating problem. There is much to commend the original MG design approach of a cast impeller and following coolant change recommendations and MGOC Spares follows that route.
Radiators degrade faster than most other parts in the cooling system, mainly from internal degradation or restriction that is not easy to confirm, but conversely external blockages or simple poor condition is much easier to assess. Internal problems can be build-up of corrosion debris from the inside of the engine blocking the radiator core, or long term use of hard water allowing a build-up of limescale that can not only restrict flow, but act as an insulator and so reduce heat transfer.
A quick check here is to feel the back of the radiator, (engine off), once the engine is up to temperature that should be evenly hot across the top, (or one side for crossflow radiators), then progressively feeling slightly cooler as you feel down the radiator, (or across for crossflow). A more modern and also more accurate method of checking the even temperature across the radiator can be done using an infra-red thermometer that can be bought quite economically. These also give a significant advantage of being able to check temps whilst the engine is running both sides of the radiator with no need to get close to operating cooling fans. If you see areas of noticeably cooler spots then this is a clear indication of internal restriction and reduced radiator efficiency.
At this point it is also worth noting whether you can see evidence of previous use of any proprietary cooling system ‘leak stop’ products such as K-Seal, Radweld, Bars Leaks or similar. They have always been quite effective in sealing leaks that allows the car to be driven home for the leak problem to be correctly dealt with, but and it is a quite big but these products will also quite effectively seal up any already partially restricted passageways in the radiator and reduce further the radiators efficiency.
Unfortunately you also can’t simply flush these products from the cooling system and they leave a smeared layer over the inside of the cooling system that can usually be easily seen when looking into the expansion tank or radiator. Removal once you have dealt with the leak will usually demand a more aggressive cleaning product than is available on the motoring shelves in supermarkets, and often that means a specific motor trade product.
Indeed use of these stronger flushing products is quite often needed when dealing with just years of build up within the cooling system, and here many owners will often have been deterred from following this route because of the stories they may have heard that other leaks are promoted by the flushing agent. I would answer this by simply saying that if the flushing agent does find weaknesses and create other leaks then it is just confirmation that whatever is leaking was already at the end of its service life and was due replacement anyway!
Not that these flushing agents will be the cure for all internal cooling system restrictions as the ‘sand’ like deposits from the cast iron engines tends to settle in the bottom of the coolant galleries in the engine block and over time this tends to harden off in a similar way to how sandstone develops. Removing this will often need the bare engine block to spend a significant time in an engine rebuilder hot caustic bath and also need some direct agitation to clear most of it.
Typically classic cast iron engines often suffer from this problem and the B series engines block drain is a good example with its block drain low in the block to allow for effective block draining, BUT, of course over time it tends to be blocked quite frequently. Even when the block drain tap, or later hexagon plug (bolt), is removed a steel probe is usually needed to dig a path up inside the blocks drain path to restore the ability to drain.
The MGB engine has had the block drain valve removed and a steel wire used to probe and clear the solid sediment build up so the drain point actually works.
Another situation that can be seen is that owners with problems with their cars may tend to apply an overkill approach to dealing with what may be a simple fault and spend large sums of money replacing many other items that still has a reasonable service life left. Consequently what may be a surprise but MGOC Spares prefer to sell owners only the spares they need to deal with the problem, unless the owner is sure they want to for the more expensive route.
Uprated or alloy radiators on some MGs is an expensive route that isn’t always needed, yet on others like the MGB GTV8 where the radiator was always running close to its maximum capacity an alloy radiator can provide a big improvement in cooling capacity.
On the outside of the radiator the common issues are visible debris that restricts airflow and simple erosion of the cores fins that corrode away and reduce the available surface area for exchanging heat to the passing air. Whilst the former issue can usually be clearly seen and easily resolved by cleaning, the latter needs a new or rebuilt radiator, helped by the temp test mentioned previously.
Radiator and oil cooler blinds
Owners of MGs from the pre 1980 period will recall that radiator blinds were a common accessory that many cars had fitted. These were usually flexible canvas or leather cloth covers that were mounted in front of the main radiator and could be adjusted to allow a varying degree of the radiator to be exposed to passing air. This of course reflects the average winters were then noticeably colder than we have seen in several decades and not just the infamous 1963 and 1983 extra-long and extra cold periods. It also hinted at engine being overcooled, a problem that could be as damaging to an engine as overheating, mainly due to the less effective lubricating properties of oils from that period, but the main selling point was that it improved heater performance.
As far as the cooling system is concerned a fully working thermostat achieves exactly the same today as it did in previous decades and so it will keep internal engine coolant temps up around the thermostats opening temperature. However, another related issue during some of the very cold periods is that unless the antifreeze content was high and fresh the fact that the thermostat would often stay shut for longer periods meant that there was little heated coolant being passed through the radiator, and that a real risk was for coolant to freeze as the very cold air super cooled the radiator core during driving. Obviously this could mean sudden unexpected overheating when the stat opened and coolant couldn’t flow through the radiator!
Cars like the MGB where most models came with a standard oil cooler saw the same potential overcooling of the oil raised at the same time. Indeed there is actually more issue with the oil temperature as there is no thermostat to regulate oil temp and overcooled oil was a more damaging issue in the 1960’s and ‘70’s. This was recognised within MG as there was a simple clip on oil cooler cover marketed by British Leyland Special Tuning. As mentioned this issue was compounded by the then available engine oils not being as good and in more recent decades and why today there is not the issue of worrying about overcooled oil.
Refilling with Coolant
Radiator flushing or replacement is the most common reason for having to drain and subsequently refill the radiator and the rest of the cooling system. Fortunately most classic MGs can be refilled without airlocks forming that would create reduced coolant flow, hot spots and possible overheating. However, the simple principle of raising the front of the car often helps because it is raising the cooling system fill point so it tends to help air within the coolant galleries move towards the fill point and out.
Several models also have their heaters return hose connection points as the highest point of the cooling system and in some instances reverse filling from this high point can help reduce any air locks, but usually the water pumps flow efficiency tends to push air out of the heater circuit.
Later MGs such as the 1500 Midget and 1977 model year on MGB saw the radiators sealed with no fill point that moved onto the top of the thermostat housing where there is a removable plug. A very simple trick to raise the effective level of these fill points is to use a plastic funnel that has a tapered spout and which has a diameter that is slightly less than the diameter of the fill point thread. This then allows the funnel spout to be pushed inside the fill point and the thread to cut a gentle thread in the plastic spout to provide a good seal and firmly hold it (see image). The funnel can then be filled and the level of coolant in the funnel will then be the highest point in the cooling system, so now you can let gravity do all the work to fill the system, aided by removing the heater return hose at the heater, which when you see fresh coolant stream out can then be refitted and there should be no air locks.
Midget 1500 cooling system fill point the same as late MGB, so the funnel fill method makes filling simpler.
Refilling a later MGB or Midget cooling system can be easier by using a funnel that is a tight fit in the fill point so that the coolant added can now be higher than the highest point in the cooling system so making filling easier.
This approach can be applied to V8s although as the fill point is on an angle a funnel with a flexible spout is better. This is especially beneficial on V8s that have a reputation for being more difficult to fill without air locks, although specifically on the factory cars it is important to check the small bore hose that connects between the thermostat housing and inlet manifold is not blocked (nor are the ends in the thermostat housing and inlet), as this is there to help remove airlocks.
The V8 radiator can be easier filled using a funnel so the fresh coolant is now the highest point in the cooling system, although the radiators angled fill point would make the use of a funnel with flexible nozzle more effective.
Hose degradation can often not be obvious, but a clear indicator of an age issue is when you take hold of the top or bottom hoses when the engine isn’t running and is cool, and squeeze the hose. It should be quite supple, but if you get a crunching feeling then it’s time to renew the hoses and it’s always best to replace ALL hoses as even if one feels OK, it may not be long before it becomes crunchy. The other benefit is that you have just one drain down allowing you to do a complete cooling system flush, change the hoses and add new coolant.
The thermostat is usually a boringly reliable item, although failures can occur over time. The most common is the stat jams slightly open and even when this is only a very small degree its surprising how much coolant is able to flow. This has the effect of not allowing the engine to reach normal levels when moving above 30mph, only creeping up once you are in stop/start traffic.
One small beneficial addition not found on many replacements today is a small jiggle valve in the thermostat flange which is present to allow easy passage of any air trapped or generated steam, but in the presence of liquid coolant the bulb end of the jiggle pin closes the small hole to prevent coolant flow. On engines that have the stat mounted in a horizontal plane the fitted position of the jiggle valve is not important, but in instances where it is mounted vertically such as on V8s, then the jiggle valve should be mounted at the 12 o’clock position as that is where any air or steam is most likely to collect. For any mounting position having this jiggle valve significantly aid the ability to bleed air and speed up filling.
Having a jiggle valve (arrowed) in the thermostat flange helps air and steam to pass to help remove airlocks, but on these vertically mounted positions in this V8 it is important to fit the stat with the jiggle at 12 o’clock position.
A common misnomer that owners sometimes follow is that when their engine suffers overheating that removing the thermostat will cure this. This can only be true if the thermostat has failed in the closed position and the cure is a simple replacement. Whilst it may seem that removing the thermostat does assist cooling it is mainly from the fact that there is now no temperature control to hold the coolant at the thermostats opening temp, so now it runs at a much lower and varying temperature and now takes longer to overheat than it did when the thermostat was fitted. The actual reality is the maximum coolant flow is reduced compared to when a thermostat is fitted, and whilst many competition engines of the cars period ran without thermostats they were crucially now fitted with a ‘Blanking Sleeve’ (still available) that returns lost coolant flow.
A less common thermostat failure is when it fails in a fully shut position and here the temps rise very rapidly into the ‘Hot’ zone of the gauge and if not noticed the engine will start to loudly grumble as the coolant starts to boil, just as mentioned with a failed water pump impeller. It’s worth pointing out here that early Sprites and Midgets had their temperature gauge transmitter fitted into the top of the radiator, so downstream of the thermostat, so isolated from seeing temp rise until the stat has opened.
Whilst in the area of the thermostat I do also have to mention the A series engines silly little inaccessible bypass hose that runs from the underside of the thermostat housing of the cylinder head to the top of the water pump. Those of us whose regular use of A series cars from the pre 1980 period will probably have some scars from the considerable fiddle of having to replace a split hose, especially if the replacement was a straight hose that was very difficult to bend an coax over the two pipe stubs. Much easier, but still a pain was the hose with fluted sections as it bend easier. However, as with all things there are positives and negatives and whilst the fluted is easier to replace in situ, the straight hose is stronger and tends to last longer and is quite easy to fit when doing a head gasket change. Of course this hose was deleted when the 1980 A+ engines arrived with the Metro, so the engine can be made to live without it.
As previously described the temperature gauges come in two formats, capillary types for earlier cars where there is a transmitter bulb that screws into the cylinder head, (except for the early Sprites and Midgets as previously mentioned), and which is connected to the gauge in the car via a capillary tube and in service these are both accurate and reliable.
As these contain an inert gas to operate the gauge any crack or break in that capillary line means the gauge has failed with the needle stuck on its lower stop or higher with no movement as temps change. Repairing these is very much a specialist job and a complete gauge replacement on exchange is usually the way forward. So if you suddenly smell an odd whiff when routing one of these gauges lines then you may have just created a leak! This means that when handling and fitting these you have to take care not to bend or excessively move the tubing about. Additionally bends need to be gentle and the excess length needs to be carefully coiled at the bulkhead end and be free to move so that when the engine is moving on its mounting this movement can be absorbed within the coil so not to create any spot tension point. See the image of a coiled capillary tube in an MGB.
An MGB with a capillary type of temperature gauge shows here how the intended excess length of capillary tube is coiled in wider diameter loops in order that movement between the engine and the cars body can be absorbed within the whole length of the coils to avoid localised stress and failure.
Late Midgets (Aug 1977 on) and MGBs (Sept 1976 on) saw changes to instruments where the capillary type gauge was replaced by electrical versions. The accuracy and reliability of these was not as good as the previous capillary types, but should still be seen as more than adequate, which may have had some connection to the period of its development and the seismic issues affecting British Leyland at that time. The company had been on a cut any cost for several years before going bust in 1975 and the Government taking over. Cost cutting continued and it is likely that development of the electrical gauge was not as thorough as may have been and adopting off the shelf parts that would be reasonably close but not spot on was adopted.
Another aspect of the electrical gauge that has a direct impact on the gauge reliability is the addition of a voltage stabiliser that for the MGB was adopted in 1964 and Midget from late 1967, and common to all classic MGs in following years. This was introduced to provide a nominal constant 10v supply to the fuel gauge to eliminate the fluctuations due to the normal fluctuations of voltage between 12 and 14 volts. When the electric temperature gauge was adopted this was also fed by the same stabilised supply for the same reasons.
Whilst voltage stabilisers have been generally reliable if they fail by cutting the power supply then both the temp gauge and the fuel gauge readings will drop to zero. On the other hand if the stabiliser no longer provides the 10v regulated supply then small changes in the reading seen simultaneously in both gauges with voltage changes would point to the stabiliser as the source of the problem.
Cooling fans were until the arrival of the MGB GTV8 always mechanically driven and mounted on the nose of the water pump. Mechanically driven fans normally need no attention over very long term periods, although a rattle from the front of an MG with a metal cooling fan is likely to be the rubber grommets around the fan securing bolts having worn away, a simple repair. I mentioned earlier checking the security of the metal fan blades that are riveted onto the fan hub and the danger of losing a loose blade. Any movement or looseness in a blade to hub rivet should be seen as an urgent need to replace the fan.
Mechanical fans rarely give problems aside from checking for loose blades as previously mentioned. However a surprisingly common issue is that some fans can be fitted the wrong way around and if so then the carefully curved shape of the blade can’t work properly and so airflow is very poor. Put simply is that the gentle curved side of the blades needs to be facing the engine, so it pulls air through the radiator and pushes it back at the engine.
This MGA cooling fan has been fitted back to front. Note the concave side of the blade arrowed is pointing to the radiator; it should be towards the engine.
When the MGB GTV8 introduced electric cooling fans these could be fitted back to front as well. This image shows the correct fit and direction the blades turn.
Which fan was fitted was dependant on the market that the car was to be sold in so looking at an MGB would see the UK market cars fitted with a three blade metal fan whilst cars destined for a hotter climate would see a six blade metal fan. Then in early 1974 all MGB markets were upgraded to the new seven blade plastic moulded fan that can be fitted to earlier cars with the appropriate water pump pulley, spacer and bolts. As a seven blade fan moves more air than the three blade one at the same engine rpms it reflects an acknowledgement that there was a growing need for better cooling, largely due to the every troubling weakening of the mixture to meet ever tightening emission restrictions that generated more engine heat. Moving more air also makes more fan noise as witnessed in the Swiss market where the raised noise level generated their Government to object and so this market went back to the three blade fan!
With the mechanical fans the biggest downside of having the fan attached to the water pump is that it moves with the engine as it moves on its mountings. For this reason the fan has to be mounted quite a distance from the radiator core to prevent accidental fan to radiator contact during conditions of heavy braking! Whilst the mechanical fans do move a lot of air the problem is that the further away from the radiator core they sit then more air is likely to be drawn from areas away from the radiator. This because pulling or pushing air through a radiator has to overcome a resistance and airflow follows the path of least resistance.
Manufacturers were well aware of this and why it is common to find fan shrouds between the radiator and fan on many cars with engine driven fans, so directing airflow to come through the radiator. That is except when you look at many MGs including the MGA and B and why the introduction of a fan shroud by MGOC Spares some years ago was a very positive development, now also available for the MGA, that significantly increases the volume of air passing through the radiator. Oddly MG obviously knew of the issue as some early Midgets and MGC’s came with a standard shroud??
Whilst the MGB with engine driven fans never had a cowl to direct as much are the fan moved to be pulled through the radiator MG did apply this on some other cars such as this MGC, which also benefitted from an expansion tank.
MGOC Spares' very effective radiator cowl that significantly increases the proportion of air moved by the engine driven fan actually passes through the radiator.
Another small aspect that helps block an easy path for air to flow rather than through the radiator are the two seals found above the pre 1977 model year MGBs radiator. The common one is that which fits in the groove on top of the supporting diaphragm and seals against a closed bonnet, but when you look underneath that and to the top of the radiator there is a very big gap that can let quite a large volume of air the mechanical fan is moving to bypass the radiator. Here there is a very simple foam seal that is an interference fit to fill this space. (See image)
MGB radiators up to 1977 Model Year have two specific seals to stop air bypassing the radiator, the upper one in the support panel groove against the bonnet and the one between the top of the radiator and the support panel.
Electric fans offered a couple of immediate advantages over mechanical engine driven fans in that when operating they would turn at their maximum speed, rather than being tied to engine speed, so providing better airflow. They could also be mounted very close to the radiator so more of the air being moved would actually be forced through the radiator.
However, it should be noted that the 1970’s standard MG electric fans are of their time and whilst effective when in good working order they can suffer motor problems through water ingress, and other issues can affect them. (It also follows that modern electric cooling fans are both more efficient and quieter and so can offer additional cooling performance over an original MG electric fan.)
This type of electric fan mounting was popular for some years using fixings that pass through the radiator core to hold it tight against the core, however over time and is the radiator is aging then this is not a recommended way to fit a cooling can due to its weight and the torque generated by the fan operation.
Looking at the original MG electric fans one difficulty for the late MGB with just a single fan is that when the fan motor starts to age and reduce its operating speed it is difficult to notice until the drop off is significant. Unfortunately only a small drop in speed will noticeably extend the cooling down time the fan delivers, which is probably the most obvious thing an owner may pick up on. On cars with twin fans it is much easier to detect a failing fan motor as the motors degrade at different rates and it’s easy to notice when one motor is slower by simple visual comparison.
Another issue for the late 4 cylinder MGB electric fan(s) was that the switch to operate the fan(s) was a sliding fit through a seal that is in the top of the radiator and especially when a new seal or switch was fitted the switch could pop out during driving from internal cooling system pressure, thus dumping all the coolant in a single spectacular event. A metal clip is available to help hold the switch in place (image) although many DIY solutions using a couple of hose jubilee clips can be found (image). Incidentally the V8 switch was different and known as the Otter switch found on the front top of the inlet manifold, however like other items of its era its reliability wasn’t always good and so today the electronic replacement from MGOC Spares gives better long term switching consistency and reliability.
1977 on model year MGBs with the forward mounted radiator had the fan switch slid into the top tank of the radiator and could blow out in use. Circled here Is a simply DIY additional switch clamp to prevent this.
Switch on and off temps for an electric cooling fan needs to be balanced with the thermostat that is fitted. For reference the original 1977 model year on MGB thermostatic switch has a switch on temperature of 92 degrees and a switch off temp of 87 degrees, which will work well with the 82 degree or indeed the 74 degree stats. However, fit an 88 degree stat and once the fan operates then it will stay switched on until the engine is switched off as the thermostat will be keeping the coolant temperature above the fans lower switch off temperature. Note non-original alternative thermostatic switches are available, but their switch on and off temps are not always the same.
Original MG electric fans can also be fitted on their motors’ shafts backwards and the same huge loss of airflow efficiency occurs. This was compounded by a late factory workshop manual actually having a line drawing where the fan blade was shown fitted the wrong way around! Also the same effect can be created when a motor is renewed as replacement motors usually do not come with the motors wiring fitted into a moulded plug for simple connection into the cars wiring. This demands the fitter has to fit the new motor wiring into the existing plug and if he gets the two wires the wrong way around then the motor will now turn the opposite way that has the same degrading effect on airflow.
It is also important to fit these fans so that the blades rotate as close as possible to the radiator without touching it. This means it being within 5mm of the nearest point on the radiator. This is simply to provide as little space for air being moved to escape from passing through the radiator, so more air moved does positive cooling work.
An electric fan this far away from the radiator core will not work very well as too much are will be able to bypass the radiator.
I have mentioned that modern cooling fans are generally more efficient both in the volume of air moved against the power consumed, they are quieter and the blade ends all being connected by the fans moulding makes them more rigid and also forces more air through the radiator. So this can often mean that a single modern cooling fan conversion can do the work previously done by two 1970’s MG fans, so when an original fan motor fails or other issues arise then it is very reasonable to consider upgrading to a modern fan conversion such as the very well designed and made Revotec range that consists of a wide range of options, including a superb adjustable fan controller.
MG heaters are renowned for their heating inefficiency, although occasionally there will be a standard car that breaks that mould. This has driven a range of uprated heater fans to increase airflow along with uprated heater motors and also heater radiators that are able to exchange more heat to noticeably improve the average MG heater performance.
In terms of problems with the standard set up there are few with probably the internal degradation and turning to dust of the foam seals surrounding the heater matrix simply through old age. These very simple seals are present to ensure that all the air that comes through the heater box passes through the matrix, so that when the driver has set the heater control to hot the moving air actually picks up that heat and transfers it into the car.
With heaters being used less and less with classic MGs often only coming out on warmer summer days means that the common type of heater control valve as seen on MGBs, C’s and V8’s will often stay closed for very long periods. A consequence of this is that their internal seal can dry out, often helped by being insulated by slow sediment build up, and when next turned on can see leaks develop. To help deter this, its useful to always have a small flow of coolant flowing, but ideally the heater valve should have periods when it is open fully. Note the Image of a heater valve that had become fully blocked and the alloy was corroded and the hose connection stub broke off with very little pressure!
Alloy corrosion has completely blocked this heater valve that also broke away from the valve, lack of use and poor cooling system maintenance were prime drivers.
The same car with the corroded and broken heater valve also suffered significant corrosion to the water pump return pipe.
It is also possible for the a leaking heater matrix to spring a leak and this can lead to wetness within the footwells, especially noticed when cornering, and if there is a sweet smell inside the car then that is usually from the antifreeze. The heater designs are quite simple, but on the MGB, C and V8 removing the heater is not a job that owner’s relish taking on, but all spares are available and often owners take this opportunity to upgrade.
Heater fans are usually single speed, but late MGBs introduced a two speed fan, or more accurately a second slower speed. This was achieved by adding an extra wire from the switch to a resistive coil of wire mounted in the heater box, (to use the passing air to dissipate generated heat in the coil), to feed the fan motor reduced voltage so the motor turned slower. When it comes to upgrading to a two speed fan today then conversions on offer will provide a second speed that is actually faster than the fastest standard fan speed for genuinely increased airflow..
Late MGBs introduced a second heater fan speed, but not a faster speed but a half speed through a second feed passing through a coil to reduce voltage. Here the three wires into the heater box indicate this is a late two speed fan.
Please Note: The information provided in this blog is intended as a general guide only. While we aim to ensure accuracy, classic vehicles can vary, and procedures may differ depending on model and condition. Always consult your vehicle’s official workshop manual before carrying out any work. If in doubt, seek the services of a qualified professional.
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