I just performed a compression test on my KJ2.

Readings:

Cylinder 1, 151 psi dry, 162psi wet
Cylinder 2, 151 psi dry, 179psi wet
Cylinder 3, 150 psi dry, 174psi wet
Cylinder 4, 150 psi dry, 181psi wet
Cylinder 5, 151 psi dry, 190psi wet
Cylinder 6, 150 psi dry, 180psi wet


Can someone tell me:
1. Is 150psi dry too low?
2. Dry tests are surprisingly even, but wet tests are surprisingly uneven, what does this mean?

1. No, it's quite good
2. The dry test is all you needed to do. If dry tests are okay and well balanced between cylinders (which they definitely are), then i wouldn't bother with a wet test

Normally you do a wet test if you have inconsistencies between cylinders. This helps to diagnose if the problem is with either rings or valves.

Did you meaure identical amounts of oil into each cylinder?


If so how much?


I have some info (can't remember where I got it) that the 3g74 has dished pistons with 22.5 ml dish volume and 50.3 ml combustion chamber volume


So oil sitting in the dish could easily alter the final compressed volume and maybe not even wet the walls to provide the enhanced wall/ring/piston seal being sought by wet test to eliminate rings as the problem


I agree with flyboy - ignore the wet test the dry is fine!

I used 7-9ml of oil per cylinder. I used a mostly filled (some bubbles etc) 10ml syringe to squirt oil down there

I used 7-9ml of oil per cylinder. I used a mostly filled (some bubbles etc) 10ml syringe to squirt oil down there


That would reduce Vc to 72.8 - 9 = 63.8 and indicated test pressure would rise to 151*72.8/64.8 ~ 173 psi (or 167psi for 9 ml) if all the oil remained in the dish. Maybe some did some didn't depending on how hard u injected and in which direction.


Did u test for interest or to diagnose symptoms? I don't have a 3g74 workshop manual - what is specified as acceptable by Mits?

I didn't have a hose so I used a thin tube which means the oil would have trickled down onto the piston from the spark plug hole.

I did a compression test as I recently did some maintenance (including valve stem seals) and the car is now running horribly, which made me believe I bent a valve, though I hope these results exclude that possibility

which made me believe I bent a valve, though I hope these results exclude that possibility


I agree - dry test being even seems that all is OK


Maybe ignition timing or plugs / leads / dizzy

Removing each lead makes it run worse. I double checked and I believe the leads are not mixed up.
Unplugging injector #4 though makes no difference to how the car idles. I've replaced the injector but still same problem

Guys just to revisit my old thread, I've found out the issue with the poor running (faulty brand new OEM ignition lead of all things) however still curious as to whether my engine is really worn out?

It feels smooth and strong, it's quiet and fuel economical, however the dry compression test netted some 20% lower compression that expected.
The only thing I can think that may have contributed to a false read would be that the engine was not at operating temp, or particularly warm at all. I switched it on for a couple of minutes and then started taking the manifold off etc.

I got the impression that these motors had a really long life in terms of piston rings etc. Is it possible/probable that the 11 year old motor at 197,000km and a good service history is worn out?

With dry compression readings as you have quoted, I would say that the rings are OK and at this stage, so are the valves. The main thing with a compression test is to check that the readings are close to the same as each other. The absolute value is not as important because things such as the gauge you use, the cranking speed and the elevation you are testing at all play a part in the readings. As long as you are not down into the low 100's, I would say that your compressions are good. Injecting oil into the cylinders only temporarily seals up worn rings. It will not help burnt valves.

The other thing to ask is "Is the engine using oil?" Even if it is, with readings like yours I would tend to say that oil is cheap, rings and fitting them are not. And if it is using oil, is it burning it oir leaking it?

From what I have read, I would say your engine is OK. Given regular oil changes and reasonable running (not too many short start/stops etc), modern engines should go easily to 400000 km before needing rings.

My engine was (is?) using oil but I have just replaced the valve stem seals which I hope would have fixed that issue. The rate was nearly 1L per 2,000km or so.

Another question out of curiosity, if one motor is 20% down on compression to an identical motor, does this equate to 20% less power as well?

. . .my engine is really worn out?

It feels smooth and strong, it's quiet and fuel economical . . . .



You have answered your own question, really.
A worn engine with leaky valves or rings will be down on power and use more fuel than normal.

Trust me, I've had a few dead-dog engines in the past!
(Mostly early Mitsus - blocks made of soft metal that wear their bores oval, and rings that lose tension, blocks that crack, etc - all at less than 180,000 km. )

You should be good to go to double the current km.

My engine was (is?) using oil but I have just replaced the valve stem seals which I hope would have fixed that issue. The rate was nearly 1L per 2,000km or so.

Another question out of curiosity, if one motor is 20% down on compression to an identical motor, does this equate to 20% less power as well?

No, differences in compression gauges can account for that 20%.
As stated before, being even is more important.

Most engines leak past the rings at low revs, turning an engine over by hand will give you a "hiss" as the compressed gases sneak past the rings. But with some revs on, this leakage is insignificant.

Thanks Max,
I know that a worn engine can be "felt" but I also considered that 3.5L with a 5 speed transmission may be hiding/masking issues, seeing as even if it was down on power there would still be plenty in reserve.

How would the wet readings be explained though? On one cylinder reaching 190psi, and on one just 162psi?

How would the wet readings be explained though? On one cylinder reaching 190psi, and on one just 162psi?

I'd be guessing, and I get told off for guessing on this forum :iough:

Seems strange though.

I bet it blew a lot of smoke when you started it! lol :eek2:

The only thing I can think that may have contributed to a false read would be that the engine was not at operating temp, or particularly warm at all. I switched it on for a couple of minutes and then started taking the manifold off etc.



Was all the testing done with the upper section of the inlet manifold off? If not and you had the throttle in the normal (closed) position you would get a lower pressure in the cylinder as it would not fill with as much air.

No all the testing was done with the intake manifold off and all spark plugs out.

The oil usage seems to have stopped. I say seems as it no longer blows smoke after idling but only time/km will tell