REPAIRS PAGE 113
ENGINE MECHANICAL PROBLEMS-SYMPTOMS
The 4-AGE and 4-AGZE engines as fitted to our cars are pretty well bomb-proof.
However, wear will eventually take its toll of even the best of designs,
so a few words on self-diagnosis.
Problems can typically be: Abnormal engine noises –
knocking, rumbling, tapping etc. Smoking engine exhaust – blue, black, white.
Poor performance – acceleration, idle etc. Oil in coolant - and vice versa.
Reduced oil pressure.
Abnormal noises - A good way to listen for these is with a
stethoscope, or failing that a long screwdriver pressed to the ear.
A top-end tapping noise is probably excessive valve
clearances, usually worse on first start-up till cold oil pressure builds. Can
get worse as oil gets hotter and thinner.
A loud metallic knocking noise – piston slap. Loudest when cold.
‘Double’ knock that doesn’t change much with load (at a given rpm) - gudgeon pin wear.
A light regular bottom end knock with the engine revs steady
– conrod bearing wear. Louder when oil warmed up.
A deeper knock – main bearing wear. More pronounced under load.
Deep knock when changing gear – worn crank thrust bearings
Smoky exhaust - Blue - excessive oil in combustion chambers.
Can be worn cylinders/rings or valve seals/guides. Black - over-rich mixture.
White - head gasket leak.
Poor performance - This can be caused by so many things that
it is impossible to categorise them precisely by a given symptom. A rough rule
of thumb though is - Lumpy idle - induction air leak after the AFM, or leaking
vacuum hose.
Poor acceleration - ignition problem, low fuel pressure,
blocked injector(s).
Oil in coolant - oil cooler leak. Head gasket - maybe.
Coolant in oil - head gasket failure.
Low oil pressure:
Worn crank main bearings. Will be accompanied by noises as
described above.
Low oil level - not as daft and uncommon as it seems! Unless
the dipstick is wiped prior to taking a level it can give a falsely optimistic
reading from trapped oil.
As might be appreciated these notes are meant as a guide
rather than a firm diagnosis. Symptoms are often blurred by other factors being
in play, and so subjective even with experience.
After all, you wouldn’t treat your own illness by just
reading guide notes and then not visiting your doctor – would you? Well I
wouldn’t!
ALAN JONES