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Thread: Compression Check

  1. #1
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    Default Compression Check

    I recently completed a compression check on my 1976 280z It's all original from what I was told when I bought the car...

    Engine Has 103xxx Miles on it... and the Last 3xxx I have driven. It hasn't had any major issues in the past that I am aware of, but I did have to reseal the fuel tank and clean out and replace a few fuel Injectors.

    Results:
    Cylinder Pressure
    ..............(Dry).....(Wet)
    1 ... 130 ... 140
    2 ... 120 ... 130
    3 ... 80 ... 90
    4 ... 110 ... 120
    5 ... 120 ... 130
    6 ... 110 ... 120


    I don't Know what the pressure should be but, I'm pretty sure 80/90 on one cylinder isn't right. The car accelerates kind of slow and always has(its an old automatic transmission never really expected anything super amazing). It tops out at about 35/40mph(which isn't right I was getting to 70 on the highways before this issue). And It makes a Metallic clicking sound once it hits 3000-4000rpm.

    Any help or advice would be appreciated.

  2. #2
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    Default

    Run a leak down test and see what the results are. Have you run your valves before (valve lash)?
    1976 280Z
    HLS30288273

  3. #3
    Supporting Member Zedyone_kenobi's Avatar
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    Default

    IT looks more like it is in your valves. Good job on running the wet and dry test. THe rings would seem to indicate fine with your tests. I would definitely look on the top end for a culprit.
    1971 240Z HLS30-38691
    93.9% done and getting better every day
    Now with 100% more DATSUN SPIRIT L28 Power
    1968 Datsun 2000 SRL311-03416

  4. #4
    Z geek at large FastWoman's Avatar
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    Yeah, definitely adjust your valve lash. That could be the problem. FAIW, my 280 was 100% dealer serviced for the 15 years prior to my owning it, and I had a few valves that were too tight. I don't know whether that comes from too infrequently adjusted valve lash (i.e. not keeping up with valve seat wear), or whether it comes from too aggressive adjustments in an ill-motivated attempt to completely silence the clicky tappets.
    My last three sports cars while I still owned all three:

    2001 BMW Z3 Roadster 3.0i soft/hard top (sold)
    1966 Ford Mustang Coupe (sold)
    1978 Datsun 280Z (enjoying very much )

  5. #5
    Registered User madkaw's Avatar
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    Sarah, you did have the throttle pryed wide open during test?( I know I shouldn't ask this with you, but just checking)
    Valve lash is wear I would go first.
    Steve
    71 240z,bw-5sp 2.4-40 over,balanced,e-88,big valves,ported&polished, stage2,header, triple Mikuni's 40's
    3.90 Subaru STI LSD

  6. #6
    Z geek at large FastWoman's Avatar
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    I don't think I tested my compression until well after I adjusted the valve lash. (I had misplaced my gauge.) However, when I did eventually test my compression (approx 185 +/- 5, straight across), it was indeed with the throttle open.

    But this is a valid point. Toti, was your throttle propped open for the test? (A closed throttle is a common error.) If not, your pressures might be reading a bit low, although that certainly wouldn't account for the one cylinder that's off.
    My last three sports cars while I still owned all three:

    2001 BMW Z3 Roadster 3.0i soft/hard top (sold)
    1966 Ford Mustang Coupe (sold)
    1978 Datsun 280Z (enjoying very much )

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