Compression Check

[Toti]

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.

[Pomorza]

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

[Zedyone_kenobi]

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.

[FastWoman]

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.

[madkaw]

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.

[FastWoman]

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.