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cylinder 1 low compression after rebuild

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JTURBOGSX

15+ Year Contributor
178
0
Jun 22, 2004
niagara falls, New_York
So long story short I took a layer off my cylinder 1 ring so I decided to do a rebuild with 2g pistons 1g rods etc

So I tore her down finshed her the other day. She fired right up no smoke no probs any who I took her first break in about 15miles putting a load on it so the piston rings sit nice I come back for shits and giggles i do a compression test 145 throughout the cylinders accept cylinder 1 130 so I said whatever Went and drove another 70 miles came back did another compression test this time every cylinder 170 and cylinder 1 is still 130 WTF.... Im not understanding why is it possible That cylinder 1 I last to seat I doubt it. I rebuild the short block not the head so Im wondering if a leaky valve seal would cause a loss like that.....What worries me is I added a cap full of oil and it jumped 25 :(
 
Just to clarify, there was damage to the piston ring in cylinder 1 before the rebuild?

If so, was there any visible damage to that cylinder? Did you hone the cylinders before install? The next step would be to do a leak down test of that cylinder. But if cylinder 1 was damaged before and now the rings won't seat, then the cylinder itself is likely an issue. Check out this link for more info on leak down testing:

http://www.dsmtuners.com/forums/articles-engine-fuel/338152-compression-leak-down-testing.html
 
yes I honed it There was no scratches on the cylinder So I felt no reason for machine work. Cross hatching was still there before the rebuild It only took a small piece off the second ring like a layer... so I rebuilt it. Is it possible its a valve seal Im hoping cylinder 1 isnt out of round :( I put a cap full of oil in it shot up
 
Did you measure the cylinders with a bore gauge? Do a leakdown test or take a compression tester hose and remove the valve, then hook it up to your air compressor hose and listen for where the air is leaking to.
 
The cylinder rignt behind the water pump wear more than the rest, a cool bore wears faster than a warm one.

Did you take any measurements of the block?

Yes, you may have a light bend in a valve if you left the cams in it and set the head on the gasket surface.

Place a vac gauge on the IM and at idle the needle should be steady, if it jumps, then you have a good chance at a valve issue
 
!! Leak down test reported a a leaky exhaust valve! I did a compression test and was only 100. I do have one question I heard A decent amount of air coming from oil cap on cylinder 1.... Would that be because The valve is leaking and its escaping from there also ?
 
If you can hear a good amount of air from the oil fill during a leak down test, then it's most likely leaking past the rings. Some is normal, especially if the engine was cold when testing. But one cylinder leaking much more air than the others is a sign that those rings aren't sealing. Since the head has to come off anyways to fix the exhaust valve, it would be a good time to give cylinder 1 a careful inspection. Get some good measurements of the bore, check for any damage to the walls or pistons, and even post up some pictures if you can.
 
If you can hear a good amount of air from the oil fill during a leak down test, then it's most likely leaking past the rings. Some is normal, especially if the engine was cold when testing. But one cylinder leaking much more air than the others is a sign that those rings aren't sealing. Since the head has to come off anyways to fix the exhaust valve, it would be a good time to give cylinder 1 a careful inspection. Get some good measurements of the bore, check for any damage to the walls or pistons, and even post up some pictures if you can.



I Under stand that my question was if the exhaust valve is open is that gonna cause it to come back through the oil cap as well Highly doubt rings are shot after 140 miles its a big time leak through the exhaust
 
I Under stand that my question was if the exhaust valve is open is that gonna cause it to come back through the oil cap as well Highly doubt rings are shot after 140 miles its a big time leak through the exhaust

NO, it won't, it will make you hear it in the exhaust..you would basically have to have a bad valve seal (rubber piece under spring) and a blocked exhaust to force it through the valve seal into the valve cover to hear it. as stated earlier any noise coming from the oil cap will be from a leak past the rings, neither valve can leak into the crank-case unless it's so severely worn or bent that it shuold have been apparent that it needed replaced when you did the new rung/piston as well as having enough restriction in the exhaust path that it's forced past the seal instead of out the pipe/manifold

if you want to know if it's the valve 100% yes or no, so this. pull the valve cover and do the leak down again, now spray something like a light oil or something on that valves seal (located under the spring) and see if it hisses, bubbles or changes the noise, if it does then you've got a damaged valve seal and valve seat, meaning possibly a bent valve and bad valve guide as well but that SHOULD have been noticable in the rebuild you've already done

Now, i'm not the engine guru that BogusSVO is but it's my opinion that you either didn't properly hone the cylinder, fit the rung gaps correctly or maybe have an out of round cylinder that was beyon repair via honing and is something that you're gonna be stuck with until you do more work.

youn can rephrase the wquestion or re-ask it until you get any answer you want but it stil won't change what's really wrong with it., if you want it to be a valve then repair the valve guides, seals and seats, and if it still leaks you know now that you were wrong but have a rebuilt head too and need to go back to people opriginal advice of it being in the cylinder or ing you replaced
 
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NO, it won't, it will make you hear it in the exhaust..you would basically have to have a bad valve seal (rubber piece under spring) and a blocked exhaust to force it through the valve seal into the valve cover to hear it. as stated earlier any noise coming from the oil cap will be from a leak past the rings, neither valve can leak into the crank-case unless it's so severely worn or bent that it shuold have been apparent that it needed replaced when you did the new rung/piston as well as having enough restriction in the exhaust path that it's forced past the seal instead of out the pipe/manifold

if you want to know if it's the valve 100% yes or no, so this. pull the valve cover and do the leak down again, now spray something like a light oil or something on that valves seal (located under the spring) and see if it hisses, bubbles or changes the noise, if it does then you've got a damaged valve seal and valve seat, meaning possibly a bent valve and bad valve guide as well but that SHOULD have been noticable in the rebuild you've already done

Now, i'm not the engine guru that BogusSVO is but it's my opinion that you either didn't properly hone the cylinder, fit the rung gaps correctly or maybe have an out of round cylinder that was beyon repair via honing and is something that you're gonna be stuck with until you do more work.

youn can rephrase the wquestion or re-ask it until you get any answer you want but it stil won't change what's really wrong with it., if you want it to be a valve then repair the valve guides, seals and seats, and if it still leaks you know now that you were wrong but have a rebuilt head too and need to go back to people opriginal advice of it being in the cylinder or ing you replaced

I'm not asking to get the anwser I want... Obviously I'm taking correct measures to diagnois the prob. One being a I have a bad exhaust valve which either got damaged on install of the head of something is keeping it open. Anyways I will take the advise of checking valve seal .....the valve is leaking bad so. Without a gauge hooked up Idk the exact percentage I'm not convinced rings are bad since its been under 500 miles and rings haven't fully seated. Nothing being reasked Idk if that's ## attempt to be a dick ? But you did anwser my question so thanks
 
I'm not asking to get the anwser I want... Obviously I'm taking correct measures to diagnois the prob. One being a I have a bad exhaust valve which either got damaged on install of the head of something is keeping it open. Anyways I will take the advise of checking valve seal .....the valve is leaking bad so. Without a gauge hooked up Idk the exact percentage I'm not convinced rings are bad since its been under 500 miles and rings haven't fully seated. Nothing being reasked Idk if that's ## attempt to be a dick ? But you did anwser my question so thanks

Sorry man, it came out wrong, no offense was intended, I just meant that if you want a reason to do the valves just do them but don't disregaurd what people advise because you think the parts are too new or for some other reason can't be the issue, I've been guilty of this in my early years of working on cars and bikes when i was a teen and usually the advice of the people around me who were informed enough to give the advice were right

I'm jus saying for all logical purposes the valve would be so hard to make the sound come out the oil cap that there'd have to be a lot more things wrong than just the valve, valve seat and valve seal for it to be the source of air and noise in the valve cover

see, any leaking air past teh valve seat would go out the manifold/turbo/exhaust, there for to get enough pressure backed up to make it come out the valve seal into the valve cover/oil cap the exhaust port or that manifold runner would have to be plugged off or the amount of leak down would have to be so high that a partially clogged bent runner would be the only way to back up enough that it would take the more resistant path of the valve guide-to-seal to inside the valve cover/oil cap

there's such a low probability of it being the valve that the advice of the ring is way more likely, and at under 500 miles you shuld be giving it more seat in time anyway although it shold be well seated rings before then anyway..

again i lean towards improperly gapped rings, improper honing and or out of round cylinder in which case more wear time may raise compression if there was no cylinder damage to the eye (out of round of course not being "eye ball test worthy)


sorry again that my last part of first post sounded dickish, I just wanted to imply that you can't rule something out just because you think it should not be the problem when the problem you're using as the "option" is way less likely to even be possible
 
Sorry man, it came out wrong, no offense was intended, I just meant that if you want a reason to do the valves just do them but don't disregaurd what people advise because you think the parts are too new or for some other reason can't be the issue, I've been guilty of this in my early years of working on cars and bikes when i was a teen and usually the advice of the people around me who were informed enough to give the advice were right

I'm jus saying for all logical purposes the valve would be so hard to make the sound come out the oil cap that there'd have to be a lot more things wrong than just the valve, valve seat and valve seal for it to be the source of air and noise in the valve cover

see, any leaking air past teh valve seat would go out the manifold/turbo/exhaust, there for to get enough pressure backed up to make it come out the valve seal into the valve cover/oil cap the exhaust port or that manifold runner would have to be plugged off or the amount of leak down would have to be so high that a partially clogged bent runner would be the only way to back up enough that it would take the more resistant path of the valve guide-to-seal to inside the valve cover/oil cap

there's such a low probability of it being the valve that the advice of the ring is way more likely, and at under 500 miles you shuld be giving it more seat in time anyway although it shold be well seated rings before then anyway..

again i lean towards improperly gapped rings, improper honing and or out of round cylinder in which case more wear time may raise compression if there was no cylinder damage to the eye (out of round of course not being "eye ball test worthy)


sorry again that my last part of first post sounded dickish, I just wanted to imply that you can't rule something out just because you think it should not be the problem when the problem you're using as the "option" is way less likely to even be possible

No worries Reason I didnt get It Bored .20 over just for the sake of the rebuild Is was I had all standard size parts and I did It with block in the car...Ive been in the game A while I should have checked the bore in cylinder 1 but I didnt have a the correct bore gauge to do so. There was no visible damage all the cross hatching was still there so I just did a ball hone. I cant be 100% if cylinder 1 is taperd but I can say this when I was doing my ring gap on all cylinders I measured the gap at top of bore middle and bottom of the cylinder on each they all had normal ware obviously the top of the cylinder a .002 wider than the the bottom nothing to worry about so in a way I Did measure the bore I guess.... Rings are set at a Nice wide gap to .20 .25 Idk if Im trusting this compression tester also Results vary every time sometimes cylinder 1 will be 100 125 and 150 ..... Always when the engine is at operating temp wot etc blah.
As far as the leakdown I dont have percentages as Im using my compression tester hose with the valve taken out. Its clearly A big leak out the exhaust valve. Engine at top dead center of course. Cylinder 4 Is alot quieter and almost no noise from oil cap id say 5% if that and Cylinder 1 Is a little more noiser prob 10-15%. The car is running very stout So im not sure If im gonna pull the head to do the valve anyways......
 
well update Valve is fixed 170 compression across the nation :D
 
Hi guys, im having the same issue but with #2 cylinder. I also performed a leak down test and found exhaust valve leaking. I saw that you replaced the valve and it fixed the issue, my question to you is did you just replace the valve, or replace everything for that valve? thank you for your help.
 
Hi guys, im having the same issue but with #2 cylinder. I also performed a leak down test and found exhaust valve leaking. I saw that you replaced the valve and it fixed the issue, my question to you is did you just replace the valve, or replace everything for that valve? thank you for your help.

It will depend on the extent of the damage.

Watch all of the "cylinder head tech" videos by jafro. They will give you a good idea of what you are looking at as far as repairs and what you will need.
 
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