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Compression numbers, what do they mean? UPDATED: PICS. Massive carbon in lifter vall?

Discussion in 'The Garage' started by 73k5blazer, Jun 19, 2005.

  1. 73k5blazer

    73k5blazer Unplug the matrix cable from the back of your head Premium Member

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    Ok, this may be a little off topic being it on my '86 C10 4.3L v6 engine, but I thought I might draw upon the more experianced in this area:
    I've been having what feels like the engine is misfiring, so I replaced the cap, rotor, coil, pickup coil, module, plugs, wires and it got better, but still not right.
    The thing pops while idles and is spitting fire, like it's running rich, very rich. I don't think it carb (I could be wrong) it was new 5000mi ago, and ran perfect for 4300mi, this problem cropped up and has gotton steadily worse in the last few hundred miles.
    So at last resort (should have done it first) I ran a compression check, and I had to check three times to verify the numbers I was getting.
    Heres what I got:
    Cyl No. 1: 148
    2: 145
    3: 126
    4: 189
    5 : 149
    6 : 148

    So I got one really high, and one really low (the two middle cylinders). 1) can anyone with a service manual for 1986 tell me what the compression is supposed to be, I don't have GM manual for that year (or anything beyon '84 before this engine was offered), looks like generally it's supposed to be around 150 (looking at the numbers for older 6's and 8's) and 2) what's going on with one really high and one really low. I've seen low before, bad news I know, but what's up with the high?
     
    Last edited: Jul 5, 2005
  2. Mad-Dog

    Mad-Dog 1/2 ton status

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    189 psi static is not that high....actually it's about right and the others are low.
    static cylinder pressure should be uniform across the board or within 10%,
    the cylinder with the highest reading probably has some excessive carbon buildup helping to seal it up a little better than the others.
     
  3. Iron_Weasel

    Iron_Weasel Registered Member

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    Minimum pressure is 698 kPa (100 psi) @ 200 rpm.

    When doing a compression test, you have to make sure that you turn the engine over the same number of revolutions per cylinder, engine at normal operating temperature, spark plugs removed, and throttle wide open.
    If you fail to do any of those things, then you run the risk of getting skewed numbers.

    I'll assume you did a dry test. So, you might want to run a wet compression test and pay careful attention to what the reading is on cylinder #3 & #4. If the pressure goes up on #3, then you have rings that aren't sealing properly. If you do a wet test and the number doesn't increase, then you've most likely got a problem with the valves/valve seals.

    As for cylinder #4, as Mad-Dog said, it's most likely that you have an abnormally high carbon buildup on the piston or the combustion chamber itself.

    Finally, the actual numbers aren't as much as an issue as the uniformity of the numbers. I could take the engine in my K10, run a compression test on it here (approx. 5000 feet above sea level), then take it down to sea level and run it again and get a totally different reading from the same engine. The difference between the high and low numbers is about 32% while the difference on #3 is 15 and #4 is 26%.

    Essentially, #3 isn't sealing as well as it should and #4 most likely has carbon buildup. Without pulling the heads and checking it out that way, it's really hard to tell. But, having excessive carbon buildup will cause misfiring.
     
  4. 73k5blazer

    73k5blazer Unplug the matrix cable from the back of your head Premium Member

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    Thanks for that. I'll try the wet test and see what I get.
    It has started blowing blue smoke, bad, at startup in the last few hundred miles too. I did crank same number of revs each cylinder, and like I said, I went around a total of three times, and the numbers I posted are average numbers for each cylinder, all three tests produced pretty much the same numbers for each cylinder though. I had the pedal to the floor during all tests. I did not have the other spark plugs removed, only the one I was testing with with compression gauge hooked to it.
    I kinda think I need to pull the heads and go through them. But mabey a few more tests are in order first to see whats going one.
     
  5. Iron_Weasel

    Iron_Weasel Registered Member

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    I'd highly recommend doing a dry compression test with all of the spark plugs removed. After that, see what your numbers are. Then do a wet test on #3 at the bare minimum.

    The reason you want all the plugs out of the engine and the throttle blocked open is so that the engine spins freely as possible. When you have the spark plugs in place, you're forcing the engine to overcome the compression being generated in the other 5 cylinders and it will give you an inaccurate reading. So just pull all 6 plugs and then do the test the same way you did it this time.
    Your average reading (with the numbers provided) is 148 - so to stay within the 10% "acceptable" varience of the cylinders, your lowest reading should be no less than 133 and your highest no higher than 163.

    Blue smoke is usually a sign of burning oil, so if you do indeed have bad rings ,that could explain the low reading on the #3 cylinder. In which case a wet test will give you a better idea of the condition of #3 without pulling the heads.
     
  6. beater_k20

    beater_k20 Banned

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    i would also recomend a leakdown test, to see where your problem really lies before tearing anything apart.

    it was mentioned before that "a problem with the valves/valve seals". valve seals have absolutely nothing to do with what kind of compression an engine makes. they are only there to prevent excessive amounts of oil from running down the valve stems and comtaminating the intake charge.
     
  7. 1979jimmy350

    1979jimmy350 1/2 ton status

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    if you are only blowing smoke on start up i would think it would most likley be valve seals. I have a 4.3 that blows smoke on start up when i had my exaust manifolds off i could see oil running down that exaust valves. This is a commen thing with the older 4.3's
     
  8. Iron_Weasel

    Iron_Weasel Registered Member

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    I was always told that if the valve seals were worn, they could affect the compression in addition to oil control. :dunno:
    Guess thats what I get for thinking at 10pm on a Sunday. :laugh:
     
  9. dyeager535

    dyeager535 1 ton status Premium Member

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    Where are the valve seals in relation to the valve seats? :)
     
  10. Iron_Weasel

    Iron_Weasel Registered Member

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    Yeah...good point.
    I claim temporary retardation. :grin:
     
  11. 73k5blazer

    73k5blazer Unplug the matrix cable from the back of your head Premium Member

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    OK, new test run. All plugs removed, engine warm. Carb tied to WOT.

    No. dry wet
    1 167 175
    2 165 178
    3 120 145
    4 168 180
    5 168 175
    6 157 168

    So with those numbers, my rings look ok, but I need a head job?
     
  12. Iron_Weasel

    Iron_Weasel Registered Member

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    Those numbers are more uniform than the first set you posted, and the increase in PSI is to be expected when you add oil to the cylinder for a wet test. #3 still looks to be a problem though - 19% difference there.

    Take Beaterk20's advice - do a leakdown test on that cylinder and you'll get a better idea of why you have lower compression in that cyinder.

    Generally, when you do a leakdown test, you can hear in one of three areas:

    1) Crankcase: Remove the oil filler cap. If you can clearly hear
    a whooshing/howling by listening at the oil filler cap, you're losing
    compression through the rings.

    2) Tailpipe: It is your exhaust valves if you can hear it (or even
    feelit) at the tail pipe.

    3) Intake manifold: If it's your intake valves if you can clearly
    hear a whooshing/howling by listening at the throttle body/intake manifold.
     
  13. 73k5blazer

    73k5blazer Unplug the matrix cable from the back of your head Premium Member

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    I don't have access to a leakdown tester, but just hooking air to it I hear it coming from the crankcase pretty easily, it's blowing out my PCV opening. Not gushing, but a pleasent breeze. I'll see if I can build or get my hands on a tester.
    I did quickly assemble a small gauge on the end before my quick connecter I had screwed into the cylinder, there is a quick drop to around 75 or 80 (regulator set to 100) then it's back up to almost 100, which is expected, thats what the regulator is supposed to do, keep it at what you set it to, which is why a leakdown tester has that restricter valve. But looking at that, I guess one could say roughly 20-25% drop. There doesn't appear to be any coming from exhaust or carb, just the crankcase.
     
  14. 73k5blazer

    73k5blazer Unplug the matrix cable from the back of your head Premium Member

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    OK, I got lucky my neighbor whom I hardly ever talk to was out and came strolling over, I asked him if he happen to have a leakdown tester, and he said yes!
    It shows no more than 8%. I was wrong before though, I had the throttle closed, once I opened that again, I got the carb whoosing noise, a still a little from the PCV opening.
    8% isn't that bad. So whats up, why's this thing running like dog doo-doo. I was expecting to see something like 20%, through the exhaust or something. 8% seems like it's in range. Just for fun, I ran compression again on that side (1,3,5) and got pretty much the same numbers. Here's another thing, I have air coming out of cylinder 1 hole when applying air to cyl. 3. Not much, but definatly there, could that be running back through the intake?
     
  15. 73k5blazer

    73k5blazer Unplug the matrix cable from the back of your head Premium Member

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    I ran a leakdown on cyl. 1, and it show a 3-5%. I think this guys leakdown tester gauge is a little rusty, it's literally rusty, it looks old. He said he can't remember when he bought it, he thinks was in the early '80s. But anyway, there is only a slight crankcase venting, no carb, no exhaust, no other spark holes. Back to cyl 3, and carb whoosing and slight crankcase breeze, and some through cylinder 1 spark plug hole.
    So, with that, it looks as though I have a bad intake valve on three, either chipped or not seating right? Does that coicide with what I see when the thing runs which is exhaust poping and sparks shooting out the tailpipe?
    Here's what I think at this point:
    Gas leaks through bad intake valve on power stroke after plug fires, more gets leak/sucked through bad intake valve on exhaust pushout stroke and gets dumped into exhaust and causes the poping and flaming? No good? Any other theories these numbers lead to?
     
  16. Iron_Weasel

    Iron_Weasel Registered Member

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    One thing I should point out just in case: When doing a leakdown test, the cylinder you're testing has to be at TDC on the compression stroke. If at all possible, test the engine when warm because a cold engine won't hold air as well as a warm engine (plus you drive your K5 with a hot engine 99% of the time anyway).
    10% and lower is the "acceptable" range for leakage for a good cylinder.

    Just to make sure I understand what you're saying.....you have air leaking through the carburetor AND the crankcase?
    Do you have the throttle open or closed when you get the carb whoosing noise?

    If it's coming back through the crankcase with the throttle closed, typically that's a sign that your rings aren't sealing as well as they should. This could explain the low compression numbers. However, in a lot of cases where the rings are bad, when you do a wet test the PSI will jump considerably...I've seen as much as 35 - 45PSI. 28PSI difference between dry & wet is still a pretty good amount though.
    Also, air coming out through the carburetor would typically indicate an intake valve not seating correctly.

    Air coming out of #1 is odd though..it could very well be leaking through a bad intake valve, through the intake runner, push on a weak intake spring/valve on #1, and then come out the #1 spark plug hole.
    Or..worst case scenario...the head gasket is blown between those two cylinders - though it's unlikey with the 167 dry PSI on #1 you already mentioned.

    It's a good theory and certainly a possibility. However, without pulling the heads and checking how well the valve is seating and the spring rate on that intake valve, it's just educated guessing.
     
  17. 73k5blazer

    73k5blazer Unplug the matrix cable from the back of your head Premium Member

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    Yep you understood correctly, through carb and through crankcase. I thought about that for a while. The PVC vent tube is on that side, but if I put my palm over it, it builds enough pressure and pops off my loose press fit rubber oil cover/cap on the other valve cover, so it is crankcase and not valve seals.
    Duirng cylinder 1 test, I still felt air through crankcase, but not as much as on cylinder 3, mabey cylider 3 has bad valve and bad rings, who know. I think I need to pull the heads and see whats going one. In that truck though, it's almost a waste to spend the extra time doing heads on the vehicle, it's almost just as easy to pull the entire engine and tranny (remmeber this is my C10, with tiny little bench press weight 3spd manual trans).
     
    Last edited: Jun 20, 2005
  18. 73k5blazer

    73k5blazer Unplug the matrix cable from the back of your head Premium Member

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    Well I did a leakdown test with a real tester on Number 3 and it was showing about 35% loss, the others were holding steady at around 15% loss. So I took the motor out, and low and behold, I don't really see any major damage. What I do see is something I've never seen before, a whole lot of carbon buildup in the lifter valley. What the heck causes that? Whatever it is, it doesn't look good that stuff is falling into the cam area, is completly covering one lifter and a couple of the pushrods have shiny mark where they have been rubbing on the stuff. Anyone see this before? What's the cause? The cylinder walls are immaculate as far as I can tell. One of the center cam lobes looks a little scratched (probably from that crud falling in there). The heads look ok, the valve stems have a little oil on them, nothing major, but some of that carbon scale buildup looked like it came through the pushrod hole and was built up a little there.
    It almost looks as though I could clean this thing and put some new rings and gaskets, mabey some valve seals and it would be fine. Question is, what caused this? My best guess is intake gasket? THis crud was then causing the intake valve on no. 3 to stay open.

    [​IMG]
    [​IMG]
     
    Last edited: Jul 5, 2005
  19. big83chevy4x4

    big83chevy4x4 3/4 ton status

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    i have a 4.3 vortec that looks twice as bad as yours, it was so bad that i couldn't get the distributor out of the motor because it was built up so bad. this motor was run on quaker state since it has 30,000 miles on it (had 325,000 when we pulled it). ive heard that quaker state and penzoil oil does this to just about every motor. i think its parafin wax buildup.
     
  20. 4X4HIGH

    4X4HIGH 1 ton status Premium Member GMOTM Winner

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    That is quite typical of penzoil or very poor oil change intervals.
     

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