low compression in 1 cyl but not leaking down

sl2ner · 07-10-2011, 04:29 PM

ok guys so ive been using the forum for awhile now but this is my first post. ive got a 84 944 N/A. just replaced the head gasket, rebuilt the head, and the car is running pretty bad up until about 3 or 4k rpm.

compression check shows about 190 psi on all the way across accept cyl 4. its 45 psi.
i used a air compressor to pump the cylinder up to 125 psi and it held it.

so why is cylinder 4 holding 125 psi of pressure but only building 45 psi on its own. any ideas? could it be a collapsed lifter not letting the valve open and let enough air in??

Guest765 · 07-10-2011, 06:26 PM

bad valve seals which seat during the more gradual compression from the air compressor, but get blow-by during the relatively faster compression of turning the engine over?
could also be a bad spot in the cylinder wall which is letting air escape during a compression test but doesn't let the air out when the piston is stationary during the leakdown?

HondaDustR · 07-10-2011, 07:45 PM

Quote:

Originally Posted by sl2ner

could it be a collapsed lifter not letting the valve open and let enough air in??

That's very possible. Do you hear any clacking during startup or anytime while running? There's also a slight chance the cam lobe is bad on that cylinder not opening the valve far enough. It could also be a bad cylinder, but I would imagine that would trash the rings as well, making it impossible to seal well no matter what the position. Seems like valve seats also would just leak no matter what if they do leak.

Guest765 · 07-10-2011, 08:22 PM

Quote:

Originally Posted by HondaDustR

That's very possible. Do you hear any clacking during startup or anytime while running? There's also a slight chance the cam lobe is bad on that cylinder not opening the valve far enough. It could also be a bad cylinder, but I would imagine that would trash the rings as well, making it impossible to seal well no matter what the position. Seems like valve seats also would just leak no matter what if they do leak.

not 100% on the cylinder, think of it like this;

if there is a spot which has been worn down for whatever reason, it would not necessarily damage the rings if it was just a low spot and didn't have a ridge or burr on the edge. it would just be a 'dead spot' in the compression cycle.

op, did you do the oil teaspoon/tablespoon test during your compression test?

krystar · 07-11-2011, 08:17 AM

but did u test the leakdown along the entire cylinder wall? it might hold perfect up at TDC, but midway have a gap.

sl2ner · 07-11-2011, 10:38 AM

Quote:

Originally Posted by krystar

but did u test the leakdown along the entire cylinder wall? it might hold perfect up at TDC, but midway have a gap.

that test would be a bit of an issue seeing the valves would be open at other points letting air out. i would have to take the timing belt off and close that cylinder. i could see having other issues as well with this idea.

Quote:

Originally Posted by fwayfarer

not 100% on the cylinder, think of it like this;

if there is a spot which has been worn down for whatever reason, it would not necessarily damage the rings if it was just a low spot and didn't have a ridge or burr on the edge. it would just be a 'dead spot' in the compression cycle.

op, did you do the oil teaspoon/tablespoon test during your compression test?

very possible, but kinda hard to check i would think. no i havent tried the oil method yet because i didnt think about a lower spot on the cylinder possibly being damaged so i didnt try, just ruled that out since it was holding air pressure when i pumped it up. ill try it though.

Quote:

Originally Posted by HondaDustR

That's very possible. Do you hear any clacking during startup or anytime while running? There's also a slight chance the cam lobe is bad on that cylinder not opening the valve far enough. It could also be a bad cylinder, but I would imagine that would trash the rings as well, making it impossible to seal well no matter what the position. Seems like valve seats also would just leak no matter what if they do leak.

ill have to listen for the clatter next time i look at it but i dont think if any that it has much clatter, i would of noticed and found that problem.

Quote:

Originally Posted by fwayfarer

bad valve seals which seat during the more gradual compression from the air compressor, but get blow-by during the relatively faster compression of turning the engine over?
could also be a bad spot in the cylinder wall which is letting air escape during a compression test but doesn't let the air out when the piston is stationary during the leakdown?

well im not saying its impossible that the valve seals are bad for some reason but they are brand new, i just had the head rebuilt and i gave them new everything pretty much other than springs, valves, and lifters.

krystar · 07-11-2011, 12:42 PM

Quote:

Originally Posted by sl2ner

that test would be a bit of an issue seeing the valves would be open at other points letting air out. i would have to take the timing belt off and close that cylinder. i could see having other issues as well with this idea.

during entire compression stroke, both valves are closed

sl2ner · 07-11-2011, 10:44 PM

your right, duh. my bad

thanks man

sl2ner · 07-12-2011, 01:20 PM

did a compression test with some oil and got the pressure from 45 psi to 75 psi. bottom end problems? weird that it happened during a head gasket installation. i made sure the cylinders were all clean before putting the head on. hmm....

sl2ner · 07-14-2011, 10:25 AM

anyone else have a idea?

bump

Yubanfueled · 07-14-2011, 10:30 AM

Someone with a VW Diesel low compression on 1 cylinder renewed his engine by lapping the valves, soaked rings in favorite solvent, and checked for a solid - no leakdown occurring - hydraulic lifter.

This seemed to be the problem: after sitting one lifter had no give at all.
After lapping and one new lifter: runs fine.

He had a 40% drop in #2 cylinder. I was trying to diagnose a 20% drop in #4 cylinder in my VW. Hope it helps.

Guest765 · 07-14-2011, 01:30 PM

Higher compression numbers with oil points to psiton rings or cylinder wall. Doesn't sound good sorry

krystar · 07-14-2011, 07:04 PM

so before the head gasket, it had GOOD numbers? or it just ran fine?

sl2ner · 07-16-2011, 07:08 PM

ya all the info you guys can give me helps and it just ran fine before. i had never done a compression test before on this car so i dont know before and after numbers.

all i know is the head gasket was bad but it still ran OK but now with the new gasket it runs horrible. before the head gasket took a crap it ran really good

Guest765 · 07-17-2011, 09:29 AM

Quote:

Originally Posted by sl2ner

ya all the info you guys can give me helps and it just ran fine before. i had never done a compression test before on this car so i dont know before and after numbers.

all i know is the head gasket was bad but it still ran OK but now with the new gasket it runs horrible. before the head gasket took a crap it ran really good

do the head gasket again, say a small prayer, you might have just got the ****tiest seat in the world and it's leaking around that cylinder. hope for the best.

sl2ner · 07-21-2011, 02:06 PM

lol thats kind of a expensive for a "lets just try it and see if it works" idea

Guest765 · 07-21-2011, 02:20 PM

Quote:

Originally Posted by sl2ner

lol thats kind of a expensive for a "lets just try it and see if it works" idea

okay. got a better one? it only gets worse from there.

good luck.

Guest765 · 07-21-2011, 06:59 PM

you sure theres not a piece of something (gasket?) in the cylinder or something?

whats the torque on the head bolts look like