Cylinder pressures

[Johnny]

My 1108 cc engine develops a mid range misfire. It seems to idle smoothly enough.

When I checked the plugs and measured the cylinder pressures I got the following results (with negligible leakdown in any cylinder):

Cylinder 1: a tan coloured plug and 190 psi
Cylinder 2: a tan coloured plug and 190 psi
Cylinder 3: a much darker plug, wet too and 235 psi
Cylinder 4: a tan plug and 240 psi.

Does anyone have any suggestions as to what may be going on please?

Johnny

 

[AdamWilkes]

The pressure numbers depend on engine compression ratio - my 1108cc F6 van is low compression (8.3:1), so my pressures would be lower than with a saloon engine (9:1 or higher).
I've read that 20 times the compression ratio in psi would be roughly new cylinder pressures.

Higher than expected pressures could indicate carbon build-up on a piston or in the combustion chamber (carbon build-up reduces volume, hence increases pressure when the piston rises in the cylinder). To me, the cylinders 3 & 4 numbers seem too high. Possibly the engine needs a de-coke (cylinder head off, clean everything inside).

Possibly there is oil leakage into cylinder 3; given the high pressure I wouldn't expect broken piston rings or head gasket leaks in there so would look to valve-shaft seal leakage. If you are going to do a de-coke, replace the valve-shaft oil seals as well & job done!

Otherwise, try 'Oil Stop Leak' or similar poured in the engine oil - it worked for me once with a 26-year-old SAAB probably on original (age-hardened) valve-shaft seals. This stuff swells & softens old seals where they may have hardened and then worn a gap.

Misfire from burning oil ? Or maybe pre-ignition due to carbon build-up in cylinders 3 and/or 4 - compression ratio now too high, or carbon staying red hot ?