compression test
 

Hi,

noob question, does timing effect compression testing ?

if timing belt or chain is out by 1 or 2 teeth would it impact the result ?


many thanks as always

It might. Imagine that the compression is highest when the piston has fully drawn in air and then pushes the piston fully up before the spark happens. If the exhaust valve opens prematurely, the compression would be off. Not sure how much 1 or 2 teeth would show. Had an old V8 once that the camshaft gear had worn down and the chain would slip. The car would actually run with the chain off by several teeth. Now, if you really think that's the case, *don't start the car* as the BMW engine may be an interference engine. That means the valve and piston can occupy the same space in the combustion chamber -- only the timing belt/chain keeps them from be in the same place at the same time. Your engine gets damaged. Not all engines are interference engines.

yes.

engine must be properly timed in order to get accurate readings.

do you suspect your engine timing is off? if so, don't start the car as twh mentioned.

no it is all ok , the car timing was off by 2 teeth and that is the mechanic told me after he changed the belt( previous owner had it off by two teeth)

now he has sorted it and car is way quicker but then I thought my old compression testing may be incorrect now as it was done with the wrong timing

Interesting. Why do you care now about the compression? Was it low before? If it was low before and the new mechanic said it was installed improperly and the car drives better now, it may have been an incorrect reading. But, if all is well now, I wouldn't bother unless the curiosity is too much.

just curiosity really.
it was 190 psi which is very high and brilliant but wanted to check to ensure

190 psi is fine. I don't think it is really high.

Have a look here if you want to get into some details:
https://www.bimmerfest.com/forums/at...2&d=1399346098

Seems like plain old crank the engine and measure the compression may be too simple for the BMW based on reading that doc.