engine and drivetrain Low but equal compression... Causes?

[PabloM]

So just before my friend and I started pulling out my new engine out of his car we did a compression check and the numbers were: 120, 118, 119 ,116. He tested the engine exactly a year ago and his numbers then were between 155 and 163. My question is, what could drop the compression so low in such a short time? The engine has 18,000 miles on a complete rebuild. Some theories so far are the valve timing is 1 tooth off allowing pressure to escape or that the tensioner is loose, retarding the cam a bit and also allowing pressure to escape. Any ideas?

EDIT: I should also add that since the 160 numbers the head has been milled twice(increasing compression) but a thicker headgasket has been put in(.020" over I think) what would this do to the compression? The tests were both dry and cold engine.

[c.d.iesel]

.........test again hot with a different test gauge

do nothing else untill you do that.

(be sure to hold the throttle WIDE OPEN while cranking)

[PabloM]

Should the injectors be spraying while testing? We unhooked them when we did it.

[PabloM]

Ok now I'm really confused. I've been doing more research and literally 50% says the throttle does have to be wide open and the other 50% says it doesn't matter. What's the reason the throttle has to be open?

[allbim]

If you don't let any air in, there is none to compress. Open the throttle.

[TobyB]

Lash your valves and try again...

then I'd do a leakdown, because compression on its own leaves lots of

questions...

and yeah, hot and prop the throttle open. As long as it's not closed,

that's good enough. Fully closed can mess up your readings.

And I disagree with CD on this one- you should use the SAME

gauges over time for the most consistent readings. Even if they're off,

they'll show you trends. Same's true of any gauge, really-

the relative change is usually more important than the numbers.

t

[aa/fd]

Valve lash, cam timing, throttle opening, battery and starter, and condition of you compression gauge. All will effect your readings to a point.

Compression gauges faults are mostly with the Schroeder valve in the end of the hose, they get dirty and leak.

Retarded cam timing will generate less compression, we're looking at two teeth too loose this much and should have no power down low.

Throttle opening will effect a test result, but I doubt you will loose 40psi. If being closed choked off that much air it would not start or idle.

Simply put if it doesn't spin as fast as it did, the readings will be a little less.

If valve lash is so loose to lose 40psi you should hear the clatter coming down the street. If so tight it drops, it should miss, noticeably.

So start with the easiest.

A-B test the compression gauge with a known good unit.

Cylinder leakage test would be my next move seeking the source of the loss if any. If I saw under 10% leak I'd stop there and check the valve timing.

This is just how I would approach the problem and solve it

[dubois]

Pablo, equal is good. You really have to take these compression numbers with a grain of salt. Unless you are comparing apples to apples, ie all conditions the same, or you use the same gage with the same motor and plot a trend, nothing is absolute. I had 200psi motors that could not outperform 145 psi motors, why? beats the hell out of me!!!

[nbristow01]

Maybe the guage is bad