compression test -results

[m2buell4me]

Did some recent compression tests of my newly acquired 91 4L cherokee. Needle on tester was jumping around while cranking between 120-150. Anyone know why that happens? I've done other compression tests on other engines where the number reaches 120 say, and then stays there while cranking.
FYI, I crank 4-5 seconds for each cyclinder, engine at 150 temperature.

[JDPascal]

Hi M2...

There is a schrader type valve (like a tire valve core) in the compression tester that is leaking. Some of them are the same as the tire valve core and some can be unique to the tester brand.

Try taking it out and cleaning it and put it back in. It will be in the end of the tube or hose that goes into the spark plug hole. Some have a second valve on the side of the tube that can be the source of the problem too.

JD

[m2buell4me]

Thanks JD. Never thought of that.

[Saudade]

Sometimes, there is also a bleed valve near the gage to release the pressure trapped in the hose (I think this is what JD was referring to). Check that too, if you have one.

[m2buell4me]

Hey folks,
Replaced the valve on the compression tester. That stopped it from jumping around. Though I noticed the gauge doesn't zero. Should it?

Here are results from #6 to 1; 150, 120,150, 120,120, 135. Anyone think these results appear to be a head gasket problem?

[m2buell4me]

Additonal note here; I drained the oil at suggestion of a friend to look for rad coolant in oil. Found none so I am assuming no chance of head gasket leak?

[Saudade]

No coolant in the oil (or vice versa) only means that there is no leak into the water jacket. There *could* still be a leak. However, your compressions readings taken at face value are OK (barely, but OK). So I would rule out a headgasket leak. You do have a fairly large pressure differential between adjacent cylinders. Still in spec according to my source but on the border.

Yes, your gage should return to zero. How far off is it? This could affect the readings.

[m2buell4me]

Thanks. It returns to about 30. never to zero. Was told today by a friend that I should crank longer than 2-3 seconds(apx) and try adding oil to each cylinder for a second test. 150 is supposed to be good?
Obviously a Haynes manual would help here, but haven't bought one yet.

[JDPascal]

Originally Posted by m2buell4me

Thanks. It returns to about 30. never to zero. Was told today by a friend that I should crank longer than 2-3 seconds(apx) and try adding oil to each cylinder for a second test. 150 is supposed to be good?
Obviously a Haynes manual would help here, but haven't bought one yet.

I always do a 5 -6 second cranking when compression testing. Also, engine is at operating temp.

Look at the first & second jumps of the needle on the gauge. The higher the jump, the better the cylinder is.

The test with oil added is to differentiate between a ring problem and a gasket/valve problem... Big jump after oil is added likely means the rings are a major cause of the problem.

Best is to have less than 10%difference from highest to lowest but some will go with 20%.

All this is assuming that the battery will support extended cranking and the starter is in good shape.

JD

[m2buell4me]

I have had a friend crank for me while watching the gauge. Anyone know where a remote starter woudl connect on the solenoid to make it a one person task? I think the solenoid is near the distributor?

[Saudade]

I don't have the FSM for your year but my '88 FSM says the range of compression is 120-150 with no more than a 30psi difference. The '93 FSM has the same range with only a 5 psi difference.

JD always has good info on these matters.

However, I'm concerned with your gage being so far off zero. If you're sure you've bled all pressure out of the gage and it reads 30 psi then you may have quite lower compression than the reading. Any chance you could try another gage?

[m2buell4me]

Good point. I will try another gauge as this one is free of pressure before each test and rests at 30. Will do the longer crank too.