Question about 1989 GMC S-15 Jimmy

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What are the main causes of compression loss in the #4 cylinder?

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  • ecavella Sep 05, 2010

    Well we tore the motor down to the block an had the heads reworked an got new spark plugs an an found the problem we had before.. We actually had a broken valve.... It had a chip missing out of it.. we replaced that valve and got everything put back together.. Well we couldn't get the timing just right.. or the valve lash right.. we finally got the timing right an it fires right up with no unexplained noise.. however when you take it down the road an come to a complete stop, when you try to hit the gas to go it wants to cut out.. so then we took the number 4 cylinder spark plug out it has nothing on it.. still looks brand new.. barely any gas smell on it...

  • ecavella Sep 05, 2010

    1989 GMC s15 Jimmy 4.3L V6 (vin Z).. ok.. so if I compress the spring I should be able to tell if the valve is bent or not??

  • ecavella Sep 05, 2010

    Ok, so we jus took it down the road an it acts fine now... seems like nothin else is wrong with it.. Could it have been the valve stuck open cause the rocker was too tight an it fixed itself after we loosened the rocker arm a little??

  • ecavella Sep 05, 2010

    ok.. thank you so much.. I will get back to you if it happens again today in a few minutes... Thank you again.. you were a very big help...

  • ecavella Sep 05, 2010

    one more thing.. could timing have alot to do with it?? Cause when the timing is too advanced it doesn't seem to lose compression, but when the timin seems right it starts to do it again...

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  • GMC Master
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Burned exhaust valve or blown head gasket

Posted on Sep 05, 2010

  • 11 more comments 
  • ecavella Sep 05, 2010

    Well we tore the motor down to the block an had the heads reworked an got new spark plugs an an found the problem we had before.. We actually had a broken valve.... It had a chip missing out of it.. we replaced that valve and got everything put back together.. Well we couldn't get the timing just right.. or the valve lash right.. we finally got the timing right an it fires right up with no unexplained noise.. however when you take it down the road an come to a complete stop, when you try to hit the gas to go it wants to cut out.. so then we took the number 4 cylinder spark plug out it has nothing on it.. still looks brand new.. barely any gas smell on it...

  • Jeff Turcotte
    Jeff Turcotte Sep 05, 2010

    so, are you saying the valve timing might have been off after reassembly? Might have bent the intake valve on #4. Seems you have an "interference" engine, which explains the original broken valve. Could have also put a hole in the piston--hope not. Either way, unless you see something obvious when you take off the valve cover, I think the intake and head have to come off again :-(

  • ecavella Sep 05, 2010

    Well wouldn't there be evidence of a hole in the piston?? Such as oil on the spark plug?? Cause there is no evidence on it..

  • Jeff Turcotte
    Jeff Turcotte Sep 05, 2010

    maybe--not sure. As the piston is moving up, gas would be going down thru the hole. I'm not saying it's the piston, but you have no compression, right? Has to be opening in there--either gasket, valve, or piston. Valve is most likely, as gasket leak would probably get oil or water on the plug, as you noted. If the valve is bent, you should be able to tell from taking off the valve cover, but I still think the head has to come off to fix, no? ;-(

  • ecavella Sep 05, 2010

    the problem we had before was in the #3 cylinder... the problem now is in #4 on the opposite side.. An yes I think the timing was probably already off before we put the rocker arms on an stuff.. By the way.. do you know the valve lash for it??

  • ecavella Sep 05, 2010

    What would it look like if the valve is bent jus by taking off the valve cover?? an it is gettin a little compression now, just not too much.. could it have anythin to do with a push rod or anything else??

  • Jeff Turcotte
    Jeff Turcotte Sep 05, 2010

    OK, a little compression--maybe the valve is bent sligthtly. If you can take off the spring or at least compress it some you could see if the valve closes decisively or maybe starts to rub the guide before closing. I can look up lash if you give me year, model, and engine

  • Jeff Turcotte
    Jeff Turcotte Sep 05, 2010

    OK, I looked at your valves. If you have the the cam off the lobes, see if you can push the valves in and let them out again. I've never done this before, so not sure how well it will work. I just think if the valve was bent it would not close completely and maybe you could feel it as you let up on it.

    You have hydraulic lifters, and I copied the rocker tightening proceduires below from autozone.com

    4.3L (VIN Z) ENGINE

    This engine utilizes hydraulic valve lifters which means that a valve adjustment is NOT a regular maintenance item. The valves must only be adjusted if the rockers arms have been disturbed for any reason such as cylinder head, camshaft, pushrod or lifter removal.

    On 1993 models, the 4.3L (VIN Z) engine may be equipped with either of two rocker arm retaining systems. If your engine utilizes screw-in type rocker arm studs with positive stop shoulders, no valve lash adjustment is necessary or possible. If so equipped, please refer to the 4.3L (VIN W) valve lash information, found later in this section. If however, your engine utilizes the pressed-in rocker arm studs, use the following procedure to tighten the rocker arm nuts and properly center the pushrod on the hydraulic lifter:

    To prepare the engine for valve adjustment, rotate the crankshaft until the mark on the damper pulley aligns with the 0° mark on the timing plate and the No. 1 cylinder is on the compression stroke. You will know when the No. 1 piston is on it's compression stroke because both the intake and exhaust valves will remain closed as the crankshaft damper mark approaches the timing scale.


    Another method to tell when the piston is coming up on the compression stroke is by removing the spark plug and placing your thumb over the hole, you will feel the air being forced out of the spark plug hole. Stop turning the crankshaft when the TDC timing mark on the crankshaft pulley is directly aligned with the timing mark pointer or the zero mark on the scale.

    With the engine on the compression stroke, adjust the exhaust valves of cylinders No. 1, 5

  • Jeff Turcotte
    Jeff Turcotte Sep 05, 2010

    could be there was something caught in the valve. So is compression good now? Hard to say what's going on, sorry.

  • Jeff Turcotte
    Jeff Turcotte Sep 05, 2010

    yes, it is possible that the rocker was down too tight, but I have never seen this with hydraulic lifters.
    Hope that was it. :-)

  • ecavella Sep 05, 2010

    can you copy all that information on that page for me I can't seem to get it off the auto zone website an I need a little further information.. Mine is also an 89 Vin Z so it's not showin that part.. If you could just copy all of the material on that subject an paste on here so I can print it out.. please an thank you...

  • Jeff Turcotte
    Jeff Turcotte Sep 05, 2010

    even though it mentions 93, the page applies to 89. This interface will not do a good job with copy and paste. Can you send me your email address? Else, you should be able to access on autozone by registration--it's free.

  • Jeff Turcotte
    Jeff Turcotte Sep 05, 2010

    When checking compression, you are cold cranking the engine and the spark timing is inconsequential. But I think maybe you're saying it starts to miss a little when you ****** the timing? The engine likes advanced timing, so maybe it just runs better all around and that's what you're noticing. The reason they don't advance timing more is it will cause detonation and overheating. On the power stroke where you might advance the timing, both valves should be well closed, but you're starting to push me into theoretical areas that I haven't studied for a long time.
    Only remaining concern I have is if that exhaust valve is leaking, it will eventually burn. But if it was really just the rocker arm, you may be fine. Just redo the rocker arms/lash and see how it goes...??

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compression_gauge.jpg
Engine Cylinder Compression Gauge
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