Ask The Mechanic – Aircooled engine cooling

The summer is here and that hopefully means that
we are experiencing warmer air temperatures.
With warmer air temperatures, comes warmer
engines. Those using aircooled engines will find it
even harder to keep the engine cool during the
summer months and we have all seen the odd VW
at the side of the motorway! Don’t let that be you
(not through overheating anyway!)

Although it may seem like a small detail, to ensure
cooler engine temperatures, it is absolutely vital
that the tinware and engine compartment rubber
seals are all present and intact. This ensures that
there is cool air above the engine and hot air
below it. These are known as the cool and warm
zones. If tinware parts are missing, or the seals
around the front and back of the engine are torn
or broken, hot air will be drawn from the cylinder
heads and exhaust back into the cool zone around
the top of the engine and then sucked in by the
cooling fan and re-circulated over the cylinders
and heads, causing the engine temperature to
rise, potentially to a critical level. This can cause all
kinds of problems over time, some of which may
not be immediately obvious, from hot starting
troubles, to cracked cylinder heads, up to and
including a seized engine.
If you’ve just bought a car/bus, it is well worth
checking the condition of the tinware and seals
and also making sure that there are no foreign
bodies stuck in the cooling fan (remember to do
this with the engine turned off!)
If you are fitting a reconditioned or new engine,
don’t just rely on refitting the parts that were on
the old engine, as they may not be correct either.
The thermostat is another vital piece in the cooling
system. There is a set of flaps inside the fan shroud,
that actually block cooling air when the engine
is cold, in order to warm up the engine more
quickly. These are opened by the thermostat,
located between the cylinder barrels and if this
part is defective your engine will very quickly
overheat. Check the function of the thermostat
and flaps and if required, replace. The alternative
is to completely remove the thermostat and
flaps, which while it certainly simplifies matters,
is not ideal. It means that your engine may never
reach the correct operating temperature in cold
weather conditions.
The last few points to consider are your ignition
timing, air leaks and fuelling. Poor ignition timing
can cause your engine to run too hot, it’s unlikely
to be visible if it’s wrong but you should hear
it. Fuel mixture is equally important, so ensure
the carburettor jetting is correct for the size of
the engine, fuel starvation will raise the engine
temperature internally. Your fuel system could be
setup perfectly, but if your engine is sucking air in
elsewhere through a split hose or a broken gasket,
then the whole fuel/air mixture is compromised
and the chances of running lean and therefore
hot, are increased too. Spraying the intake system
with Wd40 whilst running will help to detect this,
an air leak will suck the spray in, using it as fuel and
changing the engine note at the same time.
I hope there are some helpful tips for members to
help stay cool this summer.