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Compression Test Results Analysis

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by Panther, May 29, 2003.

  1. Panther

    Panther 1/2 ton status

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    Hey guys. I'm hoping to get some help with the results from my compression test last nite cause it only made me more confused. /forums/images/graemlins/confused.gif

    First some background. The truck has almost 250K miles on it. I believe it's the original motor so I'm not expecting much more from it. Also, it did sit for quite a few years before I bought it. But the motor runs strong in my opinion and fires right up. However it failed emissions horribly (about 3x the hyrodrcarbon limit ). The only significant engine mod I've done is the HEI swap and MSD 8.5 wires.

    I've only done the test on the odd numbered cylinders so far. I wanted to start there since the emissions test was on the driver's side pipe and it's true dual exhaust.

    I expected to find one oil drenched plug and really low compression in that cylinder, but I didn't. The (fairly new) plugs were all in the same condition: dark, but dry at the end. The threads however were quite wet looking and the side of the block is pretty grimy meaning it was wet recently.

    Here are the numbers from the tests. I did three dry tests and then two after squirting some oil in the cylinder.

    1 125 125 125 135 (w/ oil) 135 (w/ oil)
    2
    3 140? 125 125 140 (w/ oil) 140 (w/ oil)
    4
    5 130 130 130 160? (w/ oil) 135 (w/ oil)
    6
    7 125 125 125 140 (w/ oil) 135 (w/ oil)
    8

    I know the numbers are not great, but good I think for a 30 year old, 250K engine. More importantly, I don't see anything that indicates a mis-firing clyinder that would cause such high HC. My only idea is that maybe a plug is not firing at idle because it's not grounding through the oil on the threads. The inspection guy and I even played with the timing and the carb idle at the lane and couldn't tune it. But that's not surprising since it was so far off.

    I really hate to start tearing the engine apart and throwing money at it, while having no idea what's wrong. I will do the test on the other side tonite and post those results tomorrow.

    Thanks!! /forums/images/graemlins/smile.gif
     
  2. doctor4x4

    doctor4x4 1/2 ton status

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    i agree looks ok 2 me mabe if your gonna start somewere i recomend carb
    and checking vacum wich will tell ya about your heads
    or a leak down test wich also tells ya a lot about head condition ya might be getting someoil n from bad valve seals
    well good luck and hope this helps a little
    Randy
     
  3. Grim-Reaper

    Grim-Reaper 3/4 ton status Author

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    Does it have a EGR? My 75 doesn't and they ran a different test on it because it never had one. It had higher aloweables then my 79 with full emmisions gear. Always passed.
    How much oil does it use? It could be it has bad valve guide seals and not be a ring problem at all. Valve guide seals are cheap but a pain to install.

    Most tests are a idle test and a test at about 2500RPM. What one did it fail or did it fail both? If it just failed the idle test it may just need the idle mixture adjusted.

    Also fid a different air cleaner if you have that one with the small restrictive inlet. It doesn't allow enough air flow. I swapped to a 85 air cleaner and made a big difference in performance.

    What do you have the timing set at? HEI in the 70's should be 8 degrees.
     
  4. ZooMad75

    ZooMad75 1/2 ton status Premium Member GMOTM Winner

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    Grim and doctor both are right on the money. I'd also look at the condition of the carb given the discription of the plugs. I didn't see it in your profile but I'm assuming you have a Qjet on there if its stock. Qjets are notorious for leaking fuel from a couple of pressed in casting plugs on the underside of the main body of the carb. This leak can lead to a rich condition that you cant ajust out of the carb. I'd pull the carb and rebuild it. While you are there, use some JB weld and seal the plugs up. Also check the condition of the primary throttle shaft, Qjets tend to wobble out the primary throttle shaft bores and or crack, causing an air leak that can give you other problems.

    The leakdown test will be an excellent indicator of ring or valve guide wear. Its worthwile considering the compression figures are close. You didn't mention any oil consumption, but on a engine with 250k I got to belive its using some oil. I had a 307 with bad guides and it used 1qt every other time I filled the tank with gas. The oil got burnt and made passing emissions a pain. Oil or gasoline, to the emissions sniffer its all Hydrocarbons.

    As for the spark not being complete because there was oil on the threads of the plug and therefore not grounding, its not an issue. Even if the oil was not conductive, the clearance between the treads of the plug and the treads in the head is so small that would still go to ground. Actually the plug has more surface area contact to the head at the plug seat (the tapered part) where it seats to the head.

    Bottom line make sure the carb is fresh and in tune and that you do not have a oil control problem at the valve stem seals/guides.
     
  5. Panther

    Panther 1/2 ton status

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    Thanks guys. I am looking at getting a leak down test done, but will have to call around to find a place to do it.

    My Jimmy has no smog control that I know of. That includes no EGR and no cats. Honestly I should call the guy and make sure the test is set up for that. I kind of assumed it would be set up right b/c they have the truck info (I know, ass-u-me).

    As a 73 in NJ, it only gets the idle test (which is what it failed). Could an idle-air adjustment change the hydrocarbon output that much? To me (and the guy at the inspection lane who even suggested a burnt valve), being off that much is indicative of a non-firing cylinder which is dumping a cylinder's worth of gas into the exhaust.

    It has a very crappy ~10" by 3" open element filter on top. I have an air box off a newer Chevy truck that I was going to cut the intake tube about half way up on for a snorkel. Would I be better off with that? With the current open element, I do not have the passenger side valve cover PCV tube hooked up to anything (just the stock tube with the cotton filter at the end). Any chance, high oil pressure could cause a problem like this?

    I need to check the timing. I did have it too advanced (giving me a hard start), but the inspection lane guy and I did retard it to no avail.

    According to the PO, the Quadrajet was just rebuilt before I bought it by a decent shop. Whether they sured up the casting plugs, I don't know. Can I get at the casting plugs if I just pull off the carb (i.e., not pull it apart)?

    Honestly I'm not sure about the oil consumption. It's not remotely a daily driver so I've only put about 300 miles on the truck since November. It's hasn't drank enough oil for me to notice yet.

    And thanks for the info on the threads Rob; it was just an idea that popped into my head while looking at the wet plugs.
     
  6. ZooMad75

    ZooMad75 1/2 ton status Premium Member GMOTM Winner

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    If you have done one side of the engine for the compression test, you can do a leak down test. You do need the tool (that you might be able to rent) but will also need access to an air compressor. If you can get access to both, do it yourself. I have a problem trusting shops, too many people ripping people off, that is unless you know a good one you can trust.

    [ QUOTE ]
    With the current open element, I do not have the passenger side valve cover PCV tube hooked up to anything (just the stock tube with the cotton filter at the end).

    [/ QUOTE ]
    The PCV hose needs to go to a vaccum source on the carb, not filtered. The PCV needs vaccum to work right. The other hose that originally came off of the pass side valve cover was for the breather that was inside the factory air cleaner. The PCV would have been in the Driver side cover.

    That being said I dont think your problem is due to high oil pressure. If the guides are leaking, all the high oil pressure is going to do is get more oil up to the heads. The valves/guides are not pressure lubricated only splash from the rockers and pushrods. Again if the valve seals are rock hard and you do have a loose guide it will leak the oil into the cylinders.

    Again, me distrusting shops, I'd still check the carb. You can check the plugs without dis-assembling the carb too much. You have to take the throttle base plate off of the main body of the carb. There are screws that attach the baseplate to the main body that you can see once you pull the carb off the engine. You still have to take apart some of the linkage, but take it slow and you'll get through it. Once the base plate is off you'll see the plugs plain as day. You will see if the shop did a good job and sealed the plugs off or if they were still leaking you would see stains from the fuel dripping out of it.
     
  7. Panther

    Panther 1/2 ton status

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    [ QUOTE ]
    If you have done one side of the engine for the compression test, you can do a leak down test. You do need the tool (that you might be able to rent) but will also need access to an air compressor.

    [/ QUOTE ]

    Getting the tool is the real problem. I looked it up and they seem run in upwards of $150. I love to buy tools I'll need "for the long term", but that's a little rich me right now. I will call around today and see if I can rent one.

    [ QUOTE ]
    The PCV hose needs to go to a vaccum source on the carb, not filtered. The PCV needs vaccum to work right. The PCV would have been in the Driver side cover.

    [/ QUOTE ]
    OK, my PCV should be fine. I know that "actual" PCV valve is on the driver's side, but I thought the hose on the passenger side was more important than it is.

    [ QUOTE ]
    That being said I dont think your problem is due to high oil pressure.

    [/ QUOTE ]
    That's kind of what I figured, I thought I'd ask.

    [ QUOTE ]
    Again, me distrusting shops, I'd still check the carb.

    [/ QUOTE ]
    Once I am done with the compression test tonite, I will pull the carb and check the plugs. I do have the factory service manual and the Rochester rebuild book, so those combined with your instructions, I should be ok.

    Thanks a whole lot for the help.
     

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