Asking on behalf of the owner of our Chump e30, since I can't figure it out.
It's an ETA 325e, if that matters (doubtful). After VIR 12-hour car ran great (and felt faster than usual, actually). They drove it home from my house when we got back. Day after, Jim decided to do a compression test just out of curiosity. 5 cylinders read decent for a 200k engine (IIRC he said 125 or so, cold), but one he said read 60. He repeated the test several times and got the same result.
He then did a leakdown test on all cylinders and it came out fine across the board.
So the question is: what could cause a leakdown test to come out good, but a compression test to show one cylinder substantially lower than the others?
Bent rod is about the only thing I can think of.
A crapload of carbon build up on 5 cylinders?
I would redo the comp test and make sure the engine is warm and throttle open. Fuel relay removed too.
Bent rod or a wiped cam lobe.
Kenny_McCormic wrote:
Bent rod or a wiped cam lobe.
Or a gummed up intake valve, but that's unlikely on a race motor.
How exactly was the leak down test done? I've never done one, but I've heard that if you do it with the piston at the bottom of the bore, the rings will seal better than they would in the middle, because there is less cyl wall wear at the bottom.
Try doing the test in the opposite order? As in start with the questionable cylinder first. The battery will get weaker as you go through the test, so do that cylinder first and see if the results change.
Did he do another compression test after the leakdown test? I suppose its possible that a little piece of crap got stuck on a valve and blown out when he did the leakdown.
'Round our shop it's not unusual to have an engine with valve train problems have a good cranking speed compression test and good leakdown test, yet run like ass.
A 'running' compression test helps a lot, thus: say cyl 2 has a misfire but its neighbor 4 is OK. Pull cyl 4 plug ONLY, put the compression tester in, start the engine for a few seconds, record the reading. Now reenable #4, pull the plug on #2 and do this all over again. If cyl 2 has lower compression it very likely has a broken valve spring or sticking valve, but it will show good compression on a cranking test! Seen that more than once.
Leakdown tests have their place but only work well when a valve or ring has an appreciable problem (burnt valve, broken ring etc) and serves mainly to identify whether it's upstairs or down in the basement.
I have also stood there and watched a cylinder with a wiped cam lobe show HIGHER compression than all the others (Hemi in a cop car).
Kenny_McCormic wrote:
Did he do another compression test after the leakdown test? I suppose its possible that a little piece of crap got stuck on a valve and blown out when he did the leakdown.
Not sure, he bought a new compression tester today (the other one was really old) thinking it might be faulty or something. Haven't heard further. I linked him to this thread though.
I hate to point out the obvious first, but check the valve adjustments before doing anything major. If the intake valve has too much lash it can effect the compression numbers, but the valve will still seat fine and pass a leak-down test with flying colors. Always start with the basics first.
Curmudgeon wrote:
I have also stood there and watched a cylinder with a wiped cam lobe show HIGHER compression than all the others (Hemi in a cop car).
How does that work? Valve never actually opens so lots of pressure can build and only escape every other stoke?
In reply to Giant Purple Snorklewacker:
Wipe the intake lobe, air can't be drawn in, low compression.
Wipe the exhaust lobe, air can't be let out, compression rises.
Giant Purple Snorklewacker wrote:
Curmudgeon wrote:
I have also stood there and watched a cylinder with a wiped cam lobe show HIGHER compression than all the others (Hemi in a cop car).
How does that work? Valve never actually opens so lots of pressure can build and only escape every other stoke?
Kenny McCormic is right. I would never have believed it if I had not seen it with my own eyes.
