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QI overhauled the engine of my 55,000-mile Spitfire last year. The crankshaft and bores were fine, so I just fitted standard-size bearing shells and piston rings, plus a new radiator and water pump. I retained the old Kenlowe fan. It now overheats above 50mph and the fan runs constantly at this speed. Below 50mph the gauge sits at the halfway point. I’ve tried another water pump and refitting the old radiator. What do you suggest next?
David White, via email
AFirst of all, let’s consider what constitutes normal behaviour. Classic temperature gauges usually have the ‘cold’ marker at 40°C. The ‘normal’ zone covers 60-100°C. The red zone starts around 100-110°C, at which point the coolant will start to boil. On some cars, you can add 10°C to all those figures.
Classic thermostats often have a stated temperature of 80-85°C, which dictates the normal running temperature. When working hard or sitting in summer traffic, the car may run up to 95-100°C, which remains within the design range of the engine and cooling system.
You’d reasonably expect an electric fan to come on at 90-95°C – ie, the upper third of the ‘normal’ zone – and turn off having reduced the temperature by 5°C or so. An electric fan should not normally run continuously for long periods, especially with the car in motion.
Concerning your Spitfire, we suspect your fans may be set at too low a temperature and also that the car is not, technically, overheating at all. However, we note that it’s running much hotter post-rebuild than before. This can be a feature of runningin a rebuilt engine, as frictional losses are higher. As you haven’t done that much to yours, though, we’d rule this out.
Look around the engine bay for coolant hoses that may have kinked, folded or collapsed during removal and refitting. Make sure the fan is sucking air through the grille and blowing it towards the engine. If it’s blowing the wrong way, this will defeat the flow of air through the grille as the car drives along. Check the temperature stamped onto the thermostat and make sure it opens by about 8mm when heated in a pan of water. As a rule of thumb, it will be fully open only when it’s 10-15°C above its stated temperature.
Is the car using coolant at all? If so, look for external leaks. Sometimes these evaporate away without leaving a telltale stain or puddle. If you’re sure there are none, a head gasket leak may be responsible – and it
may also be directly responsible for the hot running. Defective gaskets are rare, but do exist. It’s possible that the wrong gasket has been fitted and it doesn’t exactly line up with the coolant passages in the block and head. It’s also possible that a warped or pitted block or head has compromised its sealing.
If there’s little or no coolant loss, then perhaps the engine is simply not running efficiently? Check the ignition timing and tune the carburettors properly. Check the camshaft timing (see manual).
If any aftermarket ‘improvements’ have been fitted such as an alternative camshaft or nonstandard carburettors, then the car will need setting up on a rolling road before it’ll run correctly.