zeeboost

Intermittent loss of compression

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    So this one is an odd one to me as I've never experienced symptoms like this, and I'm curious what some of the more experienced L28 gurus think. 

     

    I recently rebuilt an L28et for a friend of mine, which started from an external head gasket leak. Took the head (p90a) to a reputable machine shop, partly because they were the only ones around here that would actually straighten the head before resurfacing. He had the head for a few months (I believe he worked on it in his free time), but once I received it back, took roughly 3 months before it was running in the car.

    First startup everything was great, drove it easily for a couple miles, brought it back and parked it. Next day while cranking, I could hear one cylinder skipping (no compression), and once the engine fired up it had a dead misfire for roughly 10 seconds, but then it cleared up and drove great. This happened a couple more times, only after the engine sat. During the misfire, I did hear a slapping sound every once in a while that I've heard in the past from sticking valves when trying to resurrect an old engine. I never confirmed that's what it was, but the sound is very distinct. Finally the condition got to the point where I had no compression on startup (sounded like a blender), took at least 30 seconds of cranking on it before it would start to pop, then finally catch and fire up...which again after a few seconds of running rough, would smooth out and run great. That weekend we ended up taking his car on a 200 mile trip, mostly highway cruising, but also some mid rpm pulls under boost as I was tuning it while he drove it. The next morning I noticed it fired right up, didn't skip a beat, and it hasn't acted up ever since (this was a month ago and there have been a few hundred more miles added to the car). But a new symptom appeared, lots of smoke, especially on startup and high vacuum conditions.

    It's burning oil, and I'm suspicious it's the valve guides as it has the classic symptoms of it. The engine never smoked before I tore it apart, so I don't see it being the rings. Those were the only parts on the bottom end I didn't touch during the rebuild because compression numbers were good and the engine had no oil consumption. I did replace the turbo with a low mileage used unit, but they usually have different smoking conditions than high vacuum. So I'm wondering if the valve guides were initially too tight, causing the valves to intermittently stick open, and now that they're worn, they're allowing oil to enter the cylinder. I've never really seen that happen before though. I plan on popping the spark plugs out and going in with a borescope to see if I can find evidence of oil running down from the cylinder head.

     Has anyone here had experience with similar symptoms? I'm assuming the p90a valves are identical to the p90? Curious if maybe he installed the wrong guides by catalog error or something. Anything else aside from the guides that could be responsible for the smoke? If you made it this far, thanks for reading.

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    Did the guides get replaced? or did they get reamed? If the machinist reamed them, they could have been too tight, but reamed guides wear faster and can allow blowby

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    don't know much about the subject but when i read the headline i thought: sticking valves… Maybe the problem go's away after a good warm-up.. the valves are a bit thight in the guides.. 

    Also.. isn't a P90a head equipped with hydrolic valves?  P90 is not.. the P90a is!  (i think… 🤔)

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    Correct, the P90A has hydraulic lifters if still original and does not have the timesert solid lifter mod.

    Edited by S30Driver
    additional info

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    If it's a P90a head and does not have the hydrolic valves and has been modified.. think you now know the problem… :confused:

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    2 hours ago, Patcon said:

    Did the guides get replaced? or did they get reamed? If the machinist reamed them, they could have been too tight, but reamed guides wear faster and can allow blowby

    That's a good question, I had assumed since the head is over 35 years old that they would've been replaced, but I'll see if I can get my friend to dig up the receipt and see what's on there.

     

     

    1 hour ago, dutchzcarguy said:

    If it's a P90a head and does not have the hydrolic valves and has been modified.. think you now know the problem… :confused:

    This head still has the hydraulic lifters, which the machinist thought I was just misinformed when I told him that, until he saw them for himself. Considering all the other Z heads were solid lifters, it's understandable. But if they were a problem then I would still have compression when cranking it over, it'd just be running rough. 

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    It's my understanding that if the hydraulic lifters are failing, they will fail "short", not too long.

    By that, I mean... The failure mode is they leak down and then fail to pump up to the proper length. Valve clearances will be too small. And if the valves never open (or only open a tiny bit) because the lifters are too short, If that's severe enough, I'm assuming that could cause low compression.

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    4 minutes ago, Captain Obvious said:

    It's my understanding that if the hydraulic lifters are failing, they will fail "short", not too long.

    By that, I mean... The failure mode is they leak down and then fail to pump up to the proper length. Valve clearances will be too small. And if the valves never open (or only open a tiny bit) because the lifters are too short, If that's severe enough, I'm assuming that could cause low compression.

     

    Guess I should've clarified, the intermittent compression I'm referring to would be from excessive cylinder pressure leakage, thus causing the "skip" or lack of resistance inside the engine while cranking. If the lifters were not pumping up, that would result in low compression numbers if I had a gauge connected, but I should still hear resistance while cranking. Hope that helps.

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    11 hours ago, zeeboost said:

    It's burning oil, and I'm suspicious it's the valve guides as it has the classic symptoms of it.

    I plan on popping the spark plugs out and going in with a borescope to see if I can find evidence of oil running down from the cylinder head.

    How about a valve seal?  Maybe one bound up then tore.  Might be able to see signs from under the valve cover.

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    I don't have anything to offer that hasn't already been stated, but wouldn't a leak down test pinpoint the issue? Perhaps a hiss out of the intake or exhaust.

     

     

     

     

     

     

     

     

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    6 hours ago, Zed Head said:

    How about a valve seal?  Maybe one bound up then tore.  Might be able to see signs from under the valve cover.

    Yes you are correct, that's what I was referring to. I was curious if the valve guide seal / valve stem seal could be so tight that it can actually cause a valve to hang open. 

    5 hours ago, Reptoid Overlords said:

    I don't have anything to offer that hasn't already been stated, but wouldn't a leak down test pinpoint the issue? Perhaps a hiss out of the intake or exhaust.

     

    I have not experienced any misfires or running rough within the last few hundred miles, so a leakdown test should pass through all 6 cylinders at this point. It can still pass with bad valve guide seals though. 

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    I doubt a seal would stick a valve under running conditions . Does sound like the oil burn is a valve seal though. 

    This is a problem I’m having now is finding the source of oil. Like you said , a leak down test won’t help find it. 

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    So one or more of the valves were sticking before the engine warmed up completely, if they were sticking closed that could indicate a failed or lazy lifter, too tight valve guide or misaligned guide, or a burr. If sticking open that points to too tight vale guide or misaligned guide or a burr. That slap you heard could have been the piston hitting the valve and closing it.

    If the mechanic over reamed the guides the valves could be binding because of too much slop or misalignment, if he knurled the guides they could just be too tight because he didn't ream enough and as the engine warms up they expand and allow easier movement, it's called morning sickness.

    Now it's drinking oil, anyway I look at it you still have to pull the head, examine and measure those guides.

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    So you're thinking that the sticking is more from the guide, but not the seal itself? At this point I figured at a minimum I would be removing the changing out the stem seals, since I'm suspicious they're the culprit for the oil burning. With the seals off I can do a rough visual on the movement of the valve inside the guide to see if there's any obvious excess slop. If I have to pull the head off then it's going back to the machine shop, and I don't know how long that will take again...frankly I'm really just hoping the stem seals will do the trick. Though it was kind of odd that literally all cylinders had no compression on startup towards the end of this issue. 

    Thank you all for the replies, sounds like it's pretty narrowed to either valve stem seal or guide. 

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    There’s also the option that the machinist used bronze inserts vs knurling the guides and they were/are too tight. Maybe material from wearing on the inserts ruined the seals ? Several possibilities 

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    Thank you all for the replies, I'll follow up with this thread once I get time to tear into the engine. 

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