Low compresson

[v4vAnimalMother]

I have 74 260z. Last fall just before I was about to put her into storage it blew the water pump. Anyways I put her into storage early with plans on replacing it after Xmas and a new roof.

Anyways, I did a compression test last night and came up with these numbers:


1# - 100
2# - 100
3# - 105
4# - 107
5# - 105
6# - 100

Is it just me or are these really low? I'm not a mechanic, just starting to learn as much as I can. I do have a backyard mechanic helping me though that said the numbers were good because they were close to the same but after reading through other posts I think they should be around 150. If so what should be my next move?

[Diseazd]

First thing.....adjust your valves to .008 intake .010 exhaust. Then do another test.....if you don't get an increase in compression, you have somewhat of a problem. True, no cylinder should be 15% off of another, but 100 lbs. is too low. Probably needs rings. Should be reading 160 or so. Good luck!

[ChrisZ]

Your backyard mechanic is somehow right, the wear on your engine is equal on all cylinders, try to add a squirt of oil in to the cylinders and make a new compression test, if the numbers goes up, you have worn rings, there is much more in to this, more than i will try to explain in another native than my own, others might want to chime in.

Chris

[jmortensen]

Compression testers (especially cheap ones) shouldn't be used to measure the pressure in a cylinder accurately. They really should be used to compare one cylinder to another as your friend said. What you have there looks good to me, if there aren't other problems I'd drive it, and if there are I'd suggest a leakdown test.

[carrott]

Good advise, get a good compression tester and only do a "running Temp" test ,ie warm engine up to operating temp, remove all plugs , ground coil lead ,hold throttle fully open (2nd person) and crank engine.Full pressure should be achieved or close to it in 3 compressions ,if it is relatively even ,all good.If they are all even but low add a little oil, wind over engine to seal against cylinder wall , if test shows a marked increase then rings would be problem, if on the other hand only one cylinder is low adjust valve to spec if no improvement , then possible valve . Again it is critical to warm engine up, as you said engine has been sitting so there could be some rust under the valves which will eventually wear off.
Good luck

[d240zx2]

Don't forget to replace the water pump....!

[rjk629]

deffinatly do a wet test with oil in the cylinders if it goes up you have ring trouble. funy story i couldn't get my 240z to run right and with different sets of carbs the same cylinders would not run. so i did a compression check my highest was at about 95 the next two were at about 60 and then the worst were at 30!!! god damn it sure explained why the damn thing would not run. anyways new motor now. totally rebuild l2 with 4 screw round top su's. nothing like a rebuilt motor