Low compression?

[Brunoandrade98]

Hi all,

After trying to get to the bottom of a lumpy idle, I decided to buy a tester and do a compression test on all cylinders.

The car was warm when I tested it and the results were consistently low, with cylinders 4, 2 and 1 reading between 110-120 PSI. Cylinder 3 reading was about 130 PSI.

The spark plugs from cylinders 4 and 2 had oil on the threads. No noticeable signs of oil contamination in the tips of the plugs (any of them).

This might not be the cause of my lumpy idle, but it’s certainly less than ideal…

There’s probably a few things that could cause that, but the only thing I’d be able to check and fix myself is the timing belt, if the low compression is caused by bad timing. This is unlikely as the belt is only 2 years old and the csr ran fine after changing it.

What’s the going rate for a decent 197 engine?
Alternatively, what’s the rough cost of a megane engine swap? Assuming I remove and install the engine myself?

 

[Sean197]

Put a few drops of oil down and redo a 'wet' compression test. If the figures rise then piston rings are the likely issue, but your figures are certainly low.

 

[Foxspeed]

did you spin it over the same time for all cylinders and make sure the throttle was fully open?

 

[Brunoandrade98]

did you spin it over the same time for all cylinders and make sure the throttle was fully open?

I actually haven't, no! Didn't even think about that, to be honest. Since it's an electronic throttle body, does it really open while the car is cranking (with the foot down on the accelerator), or do I need to hold it open with my hand?

I imagine it will make a difference in the readings, but I can't imagine it making too much of a difference.
I can't really imagine it making it enough of a difference to push the results to a more healthy reading like 160 minimum...But I'd loooove to be proven wrong.

Will try and see. If it's bad, which I think it will be, I'll do a wet compression test.

If the compression does not change wet vs dry, does this point to timing?

 

[RSRowe]

It’s been ages since I tested one, but I’m pretty certain of the 2 I’ve done, both were ‘low’ like yours. Both made good power on the dyno and ran fine and used next to no oil with hard use on track.

Throttle open (using the pedal) is definitely required, and you will see a bit of a change doing a wet test no matter the health of the engine (granted if it’s 30psi difference, then you have a problem).

The number of engine revolutions needs to be consistent too remember.

Them all being low doesn’t really point at the timing being the issue. Removing the cam caps at the end of the head would be the definitive answer to that though (you would need to buy new caps though) or a leak down test if you have the equipment.

 

[Brunoandrade98]

So, just re-ran the test and the results are in (not good).

With the throttle wide open, I ran a dry test first and all 4 cylinders measured a very consistent 125-130 PSI - 1 point for consistency...

I then poured oil into cylinders 2 and 4, as the others had debris on the spark plug seats, so I didn't feel comfortable throwing dirt into the cylinders - wouldn't want to potentially ruin the cylinder walls and get bad compression, would I? - and re-ran the test on these cylinders.

Wet, cylinder 2 measured at 180 PSI, and cylinder 4 measured 165 PSI.

At this point, I believe I have my answer...

 

[Brunoandrade98]

It’s been ages since I tested one, but I’m pretty certain of the 2 I’ve done, both were ‘low’ like yours. Both made good power on the dyno and ran fine and used next to no oil with hard use on track.

Throttle open (using the pedal) is definitely required, and you will see a bit of a change doing a wet test no matter the health of the engine (granted if it’s 30psi difference, then you have a problem).

The number of engine revolutions needs to be consistent too remember.

Them all being low doesn’t really point at the timing being the issue. Removing the cam caps at the end of the head would be the definitive answer to that though (you would need to buy new caps though) or a leak down test if you have the equipment.

Thanks for the input! Read my reply above this one for an update.

Honestly, the car never felt that punchy, so I suspect it's definitely lost a few horses and would have already been like this when I bought it.

I serviced my car twice a year in my ownership, so I can say I probably didn't cause the wear. The engine is also only on 76K miles.

I unplugged the fuel pump for the test, not the injectors, so do you reckon there's a chance that petrol was sprayed into the chamber, cleaning any leftover oil and affecting the compression? Long shot, I think I'm just in denial