Chronic Overheating

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Written by B S Fine

It is a fairly common assumption these days that old cars will have overheating problems. But is that assumption justified? Arguably it is, because old cars like the MG Z Magnette were not designed to cope with modern traffic conditions, especially sustained high speeds on motorways. Nor were they designed to run on modern fuels, with a composition very different from what was delivered at the pumps in the fifties. On the other hand, there is ample evidence that a well sorted car takes these conditions in its stride. So when we say “old”, do we not really mean “worn out” or “poorly maintained”? Perhaps that overstates the case too. What is clear is that no owner of one of our cars should see his temperature gauge needle persistently climbing towards “Hot” and rationalise it by saying “it’s an old car”. 

Overheating is unfortunately a problem that can arise for a range of different reasons and the best approach to solving it is to be systematic and observant. The causes can be simple or complex, so by carefully analysing symptoms and eliminating possible causes, the problem can successfully be overcome. 

It may seem obvious but just because your temperature gauge tells you your engine is too hot, don’t assume you have a problem with the engine. The gauge and the sensor head are 50 years old too! Check the true situation with an independent thermometer. You can get digital meters now that will verify if a problem really exists. 

If your car regularly runs a bit over normal, but is able to cope with extreme conditions, again, I would say don’t get too hung up about it. It may be the gauge and even if it is really running a bit hot, it is unlikely to be a problem. An engine that is seriously hot will start running roughly and pinking badly 

If a car suddenly starts to overheat on the road, it is generally easily traceable to a component failure like a broken fan belt (the ignition warning light comes on too), a physical obstruction of the radiator air flow (e.g. a newspaper blown up off the road) or a sudden loss of coolant (loose joint, broken hose, failed gasket all normally accompanied by much steam). This article is more aimed at the problem of chronic overheating, when the car simply fails to cope with any adverse conditions such as sitting in traffic, climbing a steep hill or sustained fast travel. 

The first steps in solving the problem are about carrying out all the normal service and maintenance operations involved in ensuring that the system is in top condition: cleaning out and solvent flushing, replacement of suspect hoses, checking joints and gaskets. Some older engines may be chronically blocked with corrosion or sludge that simply will not shift and the only solution to this is an engine-out rebuild, but that is a last resort. 

Sometimes the problem may be accompanied by other symptoms, such as loss of coolant. Some coolant loss is inevitable when an engine regularly overheats, because steam escapes via the radiator overflow, but if the loss seems excessive, then this may suggest a leak somewhere that is causing loss of system pressure. If the pressure is lost, the water boils at a lower temperature and gives the impression of overheating. Look for the physical signs of leakage: drips, steam, water-staining in the car or underneath it. 

If there is no coolant loss, then the problem can only arise from a system that is not working efficiently or an engine that is generating too much heat. If the latter is the cause, then it is not really a cooling system problem at all. 

If you are satisfied that the system is clean and unobstructed, then the coolant may not be circulating quickly enough.  Take the thermostat out and do a road test. If the overheating disappears, then the thermostat is your problem. Carry out a proper check of the thermostat by immersing it in a saucepan of water and raising the temperature. Make sure it opens. If you can obtain a heavy-duty thermometer, check that it is opening at the right temperature. The normal is 190? F or 88?C but a few degrees hotter is no problem. 

Check the water pump. Is it correct for the engine? There has been a lot of merging of part numbers since the fifties and water pumps, in particular, have tended to be redesigned so that they fit a number of applications. This keeps costs down.  Different  materials have been introduced and pump rotors in particular are often now plastic. So you may have a pump that looks right but has too small a rotor to fit the engine. At worst, the rotor may have detached itself from the shaft, though this would normally be accompanied by noise. Is the fan pulley the right size? It is unlikely to be wrong, but too large a pulley will turn the pump too slowly. 

If there is no obvious problem with the cooling system, move on to checking for excessive heat generation in the engine. 

A common cause is ignition timing that is too far advanced. Generally this would be accompanied by other symptoms such as pinking (a.k.a. “pinging”). Your engine may be spot-on when statically timed, but what happens if you put a good timing light on it and time it dynamically? Your distributor may look right but a previous owner may have swapped it for any old DM2 that may be set up for a completely different engine. You can only tell by checking the small serial number stamped on the side. Even if the serial number is OK, would you know if a previous owner had rebuilt it with any old springs or bob-weights? Dynamic timing will allow you to plot the advance curve. Unfortunately, this is a case where referring to old data or recommendations will not necessarily help you because of the changes to modern fuel, which is generally more tolerant of timing advance. However, by plotting the degree of advance at, say, intervals of 500rpm you will at least be able to see when the advance starts, how much it moves with engine speed and when it stops. This data will show whether the advance is likely to be suitable for your engine and its degree of tune. It will also reveal uneven or jumpy advance caused by wear in the distributor. Refer to the relevant article in the Ignition section to tune your distributor. 

Running too weak a fuel mixture will also cause overheating. Again, we often check mixture at tick-over, but what is happening at 3,500rpm? Using a Colourtune will help you identify problems with mixture across the whole rev range. Are the carburettors worn or maladjusted? Are the needles right? The problem may not be related to carburettor adjustment but to fuel starvation at high revs. A tired fuel pump or a partially blocked fuel line may be causing partial starvation and weak mixture at high revs. This would normally be indicated by loss of power at the top end. 

Modern fuel may accept greater advance, but it is also more volatile, so vaporisation can be a problem. Again, it may not be obvious at tick-over but it is a self-perpetuating problem at high revs, because partial vaporisation in the fuel line or the float chamber will cause a weak mixture, which causes heat, which causes vaporisation, and so on. Check the heat screen between the carburettors and the exhaust manifold. Is it well insulated? If not, add more heat resistant mat. You can also wrap your manifold with purpose-made heat-resistant cord (obtainable from performance parts suppliers). Wrapping the float chambers and sleeving the fuel pipes protects them from engine-bay heat too. If you are tuning your car for high performance or racing, bonnet louvres may be needed to disperse the heat. 

Another mechanical cause of overheating is binding brakes. This puts undue strain on the engine and is also normally accompanied by performance problems. If your car has been standing for a while, check that all the wheel cylinders are releasing properly, especially the rears, which provide the hand-brake function. After a run, the affected brake drum will be searingly hot so check carefully.