Engine compression low??

[dcraigbrown]

I did a compression test on my 73 240z, stock engine, and all 6 cylinders read 112 - 117. This appears to be extremely low but I'm confused because all 6 are within a pretty tight tolerance. An oil test on one brought it up 10lbs and slightly less on the others. Does it sound like bad rings that happened to have worn symmetrically? Or am I making too much out of the close readings? Any ideas would be appreciated.

[Oiluj]

Check your gauge out on another engine to verify it's not reading low.

[Nissanman]

If it is an old/worn engine, to me those readings simply show remarkably even wear in each cylinder.

They are low but unless you have any reason to doubt them they may be actual.

When you do a compression test, make sure you remove all spark plugs, jam the throttle wide open, lift the slides up and engine should be at operating temperature.

Crank it over at least 3 times and record the highest reading.

A non-turbo L series in fresh condition should give ~185psi.

A turbo, ~140psi.

Probably a very tired engine and ripe for a rebuild:ermm:

[dcraigbrown]

You're probably right- due for a rebuild :-( However I didn't warm up the car or lift the slides, it was WOT though. I'll give it a try tomorrow- the right way. Thanks for the advice.

[dcraigbrown]

One thing I forgot to mention, or didn't realize it was such a big variable is the altitude I'm at (6400ft). Using a chart that compensates for altitude my compression readings are around 140 psi at sea level. Better but not what I'd like to see

[TRtom]

I have a follow up on this question. I am looking at a 82 ZX, has only 60,000 miles. (non-turbo). Should I be getting around 185 psi? I know the later engines had a lower ration, but not sure what I should be looking for for a strong engine.

Also, may be confused with terms, but what do you mean lift the slides??????

[bigoak]

Ummm...I think 185 is high...

Others can ring in, but I think 160-170 across the board would be a great engine. 140-150 acceptable but coming due on a refresh.

Look for a nice close grouping amongst all cylinders.

I'm used to 240Z's not 280ZX's though. If the #'s are different for the S130's I'm sure someone will chime in...