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Need help with a 4.3 compression issue

Discussion in 'The Garage' started by newyorkin, Jun 26, 2006.

  1. newyorkin

    newyorkin 1 ton status

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    I haven't posted in 2 months... And as I'm scouring the web for answers, I just realized I can still post in the CK5 garage... :woot:

    Here's my problem, I hope one of the brethren can help...

    I just bought a 1998 Blazer with a 4.3l. It needed the intake gasket, so I did that, and now can't get rid of this other problem, therefore I can't get it inspected.
    PCM keeps throwing a cylinder #4 misfire code, and it idles, well, like it's misfiring.
    I've swapped out the injector spider, spark plug, plug wire, fuel filter, and none have made a difference. No (locatable) vaccuum leaks. I thoroughly scrubbed the entire intake when I had it off for the gasket job.

    It has a rough idle and lots of misfiring at idle and low speed, but smooth at high speed.
    In a 5 minute compression test, cylinder #4 is at 130psi, cylinder #2 (it's neighbor) is at 180psi. I figure 130 is still pretty high, and within the 70% tolerance if 180 is the highest, although this was a rushed man's comp test.
    The plug for #4 was also very oily. I cleaned it off before reinserting it, and when I restarted the truck later, it ran great for about 3 minutes.
    So I'm wondering, is the problem really poor compression, or very fast plug fouling from bad stem seals?

    It's been about 10 years and I've forgotten much, so I think I probably didn't even do the comp test right (it's been awhile, I had to go back to my mom's house and dig through her garage to find the tester).
    Can I get a roughly accurate reading by only taking the plug out of the cylinder I'm testing, or do all plugs need to be removed for proper cranking?
    How do I read the test properly? 1st comp cycle it went to maybe 90psi, 2nd comp cycle to 110psi, 3rd to 130-140psi and stayed around there. I was thinking that meant bad rings, but then #2 acted the same way and landed at 180. Is that normal for a comp test, or is it supposed to spike up to full compression on the first cycle? I'm sure it's possible they both have worn rings, but 180 is pretty good compression for worn rings. That cyl's plug looked perfect, too.
    Would leaky stem seals cause poor compression? I've run a half dozen bottles of injector/system cleaner through, so I'd hope that would get any crud off the intake valve.


    Basically my questions are:
    -Can I cheat on the compression test?
    -How do I actually properly execute a compression test?
    -How do I determine if it's rings, head gasket, or valves?


    A very worried part of me thinks the PO overheated the truck with the manifold gasket leak, and cooked some valves or warped the head in a wierd way.

    Thanks for any info, brothas. It's been lame away from the brotherhood...
     
  2. surpip

    surpip 1 ton status

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    to get it to pass, you might be able to throw some hotter temp range plugs in there, to help burn off the oil
    might beworth a shot
     
  3. surpip

    surpip 1 ton status

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    oh and to tell id it is rings, the trick i was taught was to pour a little oil down the spark plug hole, and test it again, if it dosent leak down and the comp. is higher, its the rings, cause the oil helps to seal it
     
  4. newyorkin

    newyorkin 1 ton status

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    Right, I heard that, too. I knew I was forgetting a lot...

    Thanks for the hotter plug tip, I'll see what I can find. I can deal with it running like crap, but I can't deal with a crash while it's un-inspected. I'm hoping for sympathy from the five-o with a "Failed inspection, waiting for parts" note from my buddy who does the inspections...
     
  5. surpip

    surpip 1 ton status

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    130PSI should be more than high enough, sounds like most of the problem is from the plug fouling, and if i was a betting man i would bet on the valve seal
    you can buy just one hott plug, napa will sell you just one plug
     
  6. newyorkin

    newyorkin 1 ton status

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    I dumped a bottle of Engine Restorer in this morning, then drove 30 miles to work, and have a lot more driving to do today... I'm not betting much on that sealing out the oil, but I'll stop by autozone today and look for a hotter plug...
    At the risk of sounding stoopid, how do I find a hotter plug? I typically find the replacement number and go with that, I dunno how to find something compatible but hotter... I've been going through that for my bike for 6 months...
     
  7. Can Can

    Can Can Pusher Man Staff Member Super Moderator

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    I'm not gonna help until you apologize for abandoning us..........:deal: :mad:





















    :rotfl: :rotfl:

    Nice to have you back, brother. I've heard things have been busy, so I'm not gonna hold your absence against you. As long as you promise to come back one of these days.......
     
  8. surpip

    surpip 1 ton status

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    well if you can find a smart guy at the AZ you will be in luck
    i am not sure how to find one by the numbers, but IIRC the center electrode will be longer
     
  9. diesel4me

    diesel4me 1 ton status Premium Member

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    oil returns???

    Did the heads look all gunked up around the valve springs??,if so,the oil return holes might be clogged too,and make oil puddle up around a valve stem,causing it to be sucked in and foul the plug...or a bad guide,or valve stem seal...

    Are the plug wires new??..one bad wire can cause the plug to fire...and so can a pick up coil (if the 4.3 you have has one)...

    I'd say your compression is ok if all are over 110 psi or more..I've seen motors with 60 lbs in one cylinder still run good...

    I've seen lots of 4.3's have carbon build up in the EGR valve passages where it bolts on,causing plugs to carbon up solid,and hold the EGR valve open,making it stall at idle--might have nothing to do with your symptoms,but I'd take a peek anyway..my friend changes a lot of those "spiders" at his shop too,they seem to croak often...400 bucks for a new one from GM!!:eek1:
     
  10. newyorkin

    newyorkin 1 ton status

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    :ignore: :ignore: :shame: :shame: :whistle:
    Ok ok, I'm sawwy... I've missed the brotherhood, extremely tempted to re-up in the last couple days, especially after Brandon's IM. But I know as soon as I do, I'll be hanging in the lounge all day and screwing up at work... :doah: I figure another month or two, when the guy training me retires, I'll be back...


    Surpip, I gots the plug, it's a longer electrode indeed. I'm going to swap it out and see how it goes tonight. Thanks for the tip, hombre!

    D4M, I haven't taken the valve covers off yet. It's a vortec, and there's a bunch of junk over them that I don't want to pull off yet.

    I gotta admit, the intake gasket was super-easy, as was the spider. On my older vehicles, this stuff usually took all day...
     
  11. surpip

    surpip 1 ton status

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    no worries man, let me know how it works
     
  12. beater_k20

    beater_k20 Banned

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    Ratch, i'm not sure who told you the 70% tolerance, but that's WAY off. if you've got one at 180 the absolute lowest you should have in any other hole is about 150. need to figure out why it doesnt have the compression, and where its going.
     
  13. newyorkin

    newyorkin 1 ton status

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    Beater, it was in a manual, and I saw it again somewhere else, but it was supposedly manufacturer's specs. They didn't list actual acceptable compression ranges, just that the lowest should be within 70% of the highest.
    I don't even know if 180 is the highest, it was just the adjacent cylinder, I was checking to make sure there wasn't a head gasket loss between them.
     
  14. 3 on the tree

    3 on the tree 1/2 ton status

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    I just put a fresh head on my 70 250. The old head showed 130psi on all cylinders but 6. #6 showed 110, with a steady miss and popping out the tailpipe. When I pulled the old head, #6 exhaust valve was sunk over 1/8" into the seat!!!! If I see more than a 20% difference tween high and low readings, I start looking for a problem.
     

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