4.0L Compression Test

[WillCherokee]

So, I had a compression test done on my 91 XJ after an engine rebuild, the results were (cyl 1-6) 50, 20, 25, 20, 75, 110. Everything was tightened to spec, new head and valve cover gaskets but I did reuse my head bolts. What can be problem(s)?

 

[rookies]

Maybe valve adjustment to tight? With that kind of compression the engine probable would not run. Back the lifters off a half a turn and try it.

 

[RWKHausSupply]

Few things I found when I last went threw a comp test.

Make sure the battery is up to charge and if you can (highly recommended) put it on a charger while doing test.

Get the engine up to operating temp.

Remove a plug and then the coil wire and screw in comp tester as fast as you can.

Turn engine over at least 4 full revolutions.

Take reading.

Put plug back in and coil wire and start it for a moment to get the oil and heat back.

Move to next cyl and repeat above.

You can also skip the restarting of it each cyl, but this is the best way to get accurate readings that are true to each cyl. The oil splashing on the cyl wall is also key to getting a true reading.

I went from almost a cold just remove all plugs and test each cyl. to the above and I went from about 70psi in a few holes up to 125-140psi. I am running 9.7:1 though.

I would try the above before I went doing repair or "fixing" of anything.

 

[old_man]

So, I had a compression test done on my 91 XJ after an engine rebuild, the results were (cyl 1-6) 50, 20, 25, 20, 75, 110. Everything was tightened to spec, new head and valve cover gaskets but I did reuse my head bolts. What can be problem(s)?

With readings that low, I would double check that the compression tester is sealing. Does it have an O-ring?

Personally, I pull all the plugs at once and do the test. It allows the engine to spin more freely.

Do the test again and then squirt a couple of tablespoons of ATF in each cylinder and try again.

 

[WillCherokee]

So I removed the valve cover to find out the lifters were bad SO I replaced them in addition to new head bolts, push rods, FELPRO valve cover gasket, FELPRO head gasket, valve seals, scrubbed and cleaned the head, lapped the valves and seats and the the head should be ready to go back in. What do you guys think?

 

[old_man]

Dumb question, how did you determine your lifters were bad?

 

[WillCherokee]

Never a dumb question, just a jeep rookie taking a guess because they were loose and Im not really getting alot of help on here wasnt a final analysis tho, I had some new ones so I swapped them in.

Quick question tho:
So I hear and read multiple things: When I'm setting the ignition coil, is it pointing directly at the #1 spot on the distributor cap or a lil bit past it going clockwise?

 

[md21722]

This is the easiest way and follows the FSM procedure:

https://www.youtube.com/watch?v=xRHW0i3V6qM

Takes the guess work of being a tooth off, etc. Done it this way in on my own and for another person at the campground at Colorado Fest.

There are other ways but I've found this to be the easiest.

I usually take my heads to the machine shop. I have used a few over the years and straight six heads always seem to need some truing up to make them flat. I know others check them with glass and lay them back down, but I've always had good luck going to the machine shop. Last one cost me $135 for all cleaning, valve working, and machine. I did bring them new valve seals.