I am trying to help a friend out.
What would typical compression readings be on a decent running 4.0, 1990 Renix engine?

 

150 +/- 10lbs after 5 or so revolutions. the higher the better but all must be within 10 or so lbs to be considered "acceptable"

 

thanks Agent Orange,
I loaned him my compression gauge, and I'll let him know what he should be looking for.

His is definately not in decent running condition........It's an ex Air Force XJ......with somewhere around 150,000 miles on it.
It blows a ton of oil vapor into the air filter box, and overheats quickly...........
I told him to get a block tester to check for combustion gasses in the coolant, and do a compression check to start with...........

 

Your numbers might be higher or lower, but the relative difference is what really matters. (in one sense you are testing your starter, battery and cables as well). Nice if they are within 15% of each-other. I'd rather the engine was at least warm, but no big deal there.

He can write down one set of numbers, then add a couple tablespoons of oil and do a second set for a wet/dry test. I use a piece of clear venal tube to blow the oil in. The oil will cut down on what gets past the rings, so if a low one comes up, suspect the rings, if it doesn't, suspect the HG or valves.

Postm' up! We love that.

 

Update on said Renix XJ:

This past week, he used the block tester to check his coolant......He said he did the test 3 or so times to verify, and no color change in the fluid.......So no combustion leak into cooling system.

He brought it over today, and we pulled all 6 spark plugs......They didn't look too bad, or oil soaked...........However, when we did the compression check, this is what we found.

Starting at the front of the engine with number 1 cylinder, and working toward the back, we recorded:

*102 psi
*100 psi
*115 psi
*110 psi
*55 psi
*65 psi

We double checked our results, just to make sure we didn't mess it up.....
So rear two cylinders dead.........That kinda explains the excessive blow-by oil vapor being pushed out of the breather tube......
Now he is trying to decide to re-ring this engine, or source another engine..

 

Denver's a mile high. Your compression readings are gonna be somewhat lower. 5 and 6, next to each other? Head gasket? What happens if you squirt oil in the cylinders and do a wet test?

 

X2^ two low together, suggest a breach in the HG between the two. I'll even wager if you do the "wet test" you will get a similar result.

OR! the rear runs hotter, and cracked/failed exhaust valves do happen. I might pull the head, flop it on its side and put some diesel or kerosene in the ports and see even, slight seepage, put a straight edge on it and see less than .004 space anywhere, and try a new gasket. AND/OR, go ahead and rent a valve spring compressor, pull the valves and lap them with a drill and section of fuel line, and install the new valve-stem seals that come in the head gasket set. But I'm pretty cheap, just that's what I myself might do.

 

Yeah, I told him should probably pull the head first to check for a compromised head gasket..........The excessive crankcase pressure leads me to believe it's not valves
When I say it pushes oil vapor out of the breather tube...........I mean it looks like one of those smoke machines that you use to check for vacuum/emission leaks.
I'll let you all know what we find once the head comes off

 

I guess the WET/dry compression test doesn't take long. Rings can do that.