V6 compression too low?

Did a compression test on my 69 V6 just to satisfy my curiosity. PO, who was the original owner, told me the jeep has less than 30K miles on it. I haven't put more than a couple thousand miles on it. The results dry were 105, 94, 101, 110, 97, 107. The results wet were 112, 99, 117, 117, 117, 115.

This seems very low compared to my 66 V6 which has average dry results of 127, measured immediately after doing the compression test on the 69 with the same compression gauge. I live at 5000 feet elevation.

The 69 seems to run strong and smooth and doesn't have any visible smoke coming out the exhaust. Is there something I am missing that could cause the compression to be so low, or is it just a worn out engine that needs a rebuild? Could the speed of the starter make a difference?

 

yes, I had all the plugs out and the throttle open

 

I read somewhere to have the throttle open. I suppose it allows the most air into the intake manifold.

 

IMHO it could possibly be starter related.
My experience shows that CR taken from an idling engine is very low.
Maybe .6% of the normal reading
That implies lower reading as the rpm increases.
Maybe it has low CR pistons ? @ 7.6 CR

 

I don't have anywhere near the expertise of the previous 2 posters, but since you say it runs strong and smooth, I would hook up a vacuum guage and interpret the results, just to start ruling things out. Maybe by process of elimination, you will be able to determine that is from a slow starter or low compression pistons, as oldtime suspects. Just a suggestion. This is a guide I like for reading my vacuum guage: http://www.secondchancegarage.com/public/186.cfm

 

Was the engine warmed up before taking the readings? This can cause erroneous readings if not. Most service manuals in the past specifically directed to make sure the engine was at operating temperature before testing.
I have personally seen readings change from tests done with the throttle held open and throttle closed. Used to be standard of the industry to do compression tests with the throttle plates held wide open. Many fsm's even state to do it this way.


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Yes, the engine was at operating temperature.


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You're fine. The number doesn't matter, as long as you are above 100 or so. Althought I have seen engines in the high seventies that ran. The important thing is that all cylinders are within ten percent of each other. You are good there but might want to keep an eye on number two.

Old, low mileage engines means years of storage. You might have a sticky ring in two. Test it again in June after you have put some miles on it.

I am at 5300 feet. My '66 is pumping around 120. This equals 140ish at sea level.

 

Has this engine been modified, sp. with a cam? The cam timing will change the observed pressure. If you make the cam intake-open duration longer, the effective stroke length of the piston decreases and the compression goes down.

 

Something else to consider is the gauge. Unless you are using a high dollar tool, yours may read high or low. The Craftsman gauge I used read low. A guy I worked with tested with his Snap On and the engine was fine. I bought a new one the next week.

It was then that I caught Snap On fever, an affliction from which I am still recovering. But you should see my 1/4" ratchet collection...

 

I don't know if the PO modified the cam. He did say he had to do some top end work on a cylinder at one time. Nothing surprises me anymore on these old jeeps.

I have tested the compression with two different gauges and come up with similar results, so I feel pretty confident in the results. But, thanks for the warning about the Snap-on addiction! Lately, I have been discovering some pretty cool tools at Harbor Freight. I have hacked my way through removing a lot of oil seals before I found there is a cheap tool made to do this!

I'm guess I'll just keep driving it until there is some indication of a problem.


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I was surprised myself as I didn't believe there would be a difference so a test was performed as a class project when I was in college. We performed a compression test on a vehicle that had been thoroughly warmed up with the throttle plates closed. Then the test was performed with the thriottle plates open. Multiple tests were performed.
Guess my 25 years experience has been different than your 25 years experience.



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As you know standard CR for D-225 was 9.0.
Some later (69-71) smog equipped V-6 CJ's were factrory equipped with 7.6 CR.
That could easily explain the readings.

 

Seems likely, given that the 69 runs good but tested low compared to the 66 model.

 

The engine code is KN H 121; the first digit is a mystery to me; the second digit indicates 67 V6-225; the third digit indicates V6-225 9.0 to 1 C.R.. The vehicle identification tag indicates it is a V6-225 with emission.



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