Sorry Cant find my original post.
166000 miles. V6 4.0
I had misfire cyl. 3 through coding.
Oil on plug.
Injector clicking. Plug has spark.
Went for inspection now won't pass emissions.
Compression test cyl. at 130 .
Valve seals or ring problem ?
Is it worth pulling down? Or change motor ?
Thanks in advance

 

130 is low but meaningless without at least 2 other cylinder's compression numbers
You need to do at least 3 cylinders at the same time to take out battery condition(crank speed), gauge "issues" and general test conditions.

Other spark plugs look good?

Oil can come from intake Valve Guide seal but won't lower compression.
Lower compression and oil is most likely piston rings.
But the oil has a tendency to seal the rings during compression tests, so higher number than should be, lol.
Which is why testing other cylinders is good.

If possible test another cylinder dry, then add a teaspoon of oil and retest, number will always go up with oil added.
Do the same with #3, if rings are the problem it won't go up as much because oil was already sealing the rings.
But that could also mean burnt valve issue, exhaust valve not sealing fully, since adding oil wouldn't help seal it, but burnt valve also won't show oil on spark plug.

I am over 300k on my '94 4.0l rings, and 4.0l OHV is not known for ring or lower end problems at all, it can of course happen to any engine, just not at all common on the 4.0l OHV.

I just had to replace a spark plug because of cracked electrode, #6, that does happen with the 4.0l OHV, they eat spark plugs.

 

Thanks for your reply.
Its my daily driver so i got to get to work.
I have not had time to do alternate cylinder compression test.(getting cold in New York )
So my question now is.
After testing surrounding cylinders and if I find better numbers?
Take it apart and have heads checked?
If there is ring damage do you think I will find scoring or telltale sign on wall?
I bought a temporary dodge Dakota I have work history with, intend to lay up ranger.
The truck has cost me no money other than brakes and oil.
Got it with 85 thousand i Put another 88 thousand on it.
Thoughts?
Thanks again for your time.
Keith

 

Do a "dry compression test" on 3 cylinders and write down the numbers

Then do "wet test" on the same 3 cylinders

Wet test is when you add a teaspoon of oil via spark plug hole to the cylinder, just dip a straw into oil bottle and put finger over the end to hold the oil in straw then transfer to spark plug hole, you can do dry test and then wet test on same cylinder and then move to the next one.

Compression numbers will always go up on wet test, the amount they go up tells you if problem is rings or valves.
If cylinder is 130 dry and then 150 wet the rings are leaking too much
If 130 only goes to 140 then valves are leaking
This is assuming "good cylinders" are 160+psi

All these parts are metal on metal, so are not air tight and are not suppose to be air tight.
Which is why the speed of the piston compressing the air is a factor, the slower the piston the more time air has to leak out so the lower the numbers.

If you test with all spark plugs removed then you will have best crank RPM so best piston speed compressing the air.
Same for battery condition, low battery means slower starter motor RPM and crank RPM.
But these are "common" to the test conditions at that time.
This is why you need to test more than 1 cylinder.
130psi could be good for the "conditions" of the test
i.e. battery condition, starter motor, gauge and altitude above sea level, higher altitude shows lower numbers.

But if you have at least 3 cylinder tested under the same "conditions" you have a small but reasonable baseline to compare to see if a cylinder is lower than the others