Here's a little back info on the build.


- 1996 4.0L, 160k miles, auto trans

The vehicle was purchased mainly for the drive train to go in a different build. It was "running" when purchased. First thing I did was go through and do some general maintenance - cap, rotor, wires plugs, injectors, & and timing set. It was running very rough so I went through and did a compression test and it was low on several cylinders.


I went ahead and pulled the cylinder head to check everything internally. A couple pistons has some wear on the ring landings and a few rings were stuck compressed. I went ahead and pulled all the pistons and rods, honed all cylinder bores, checked for straightness, and squareness. Installed all new pistons with new rings - as well as new rod bearings since I was in there. I reinstalled the cylinder head and reassembled everything. It still wasn't running right and had 0 compression on a few cylinders. I did a leak down and it was coming out the valves. Turns out a few of the lifters were stuck up preloading the valves because of sitting. I ran some marvel mystery oil through and got the lifters loosened up and finally ended up firing on all cylinders, but it was still running rough.


Another compression test yielded 50psi on 1, 2, and 3 and 150psi on 4, 5, and 6. Now the weird part is the the leak down test following this yielded 0% leak down on all cylinders.

I went ahead and checked over everything from cam timing to valve lift, duration ect.

Ultimately I ended up pulling the head again and found that there was a few small cracks that were overlooked before. I sourced another head and had it checked, installed new seals and lapped all valves. I went ahead and put a new cam and new lifters in while I had the head off.


Reassembled everything again ​​​. STILL the same problem. Did a double check on all my bases and ended up doing another compression test. The result was the exact same numbers to the T. 50psi on 1-3 and 150 on 4-6. Pulled valve cover and all the valves are opening the same. I am planning to do leak down again, but I'm expecting a similar result as before.


Has anyone ever experienced something like this. I've been working over it in my head and spit balling with my boss and we're both a little stumped. Just wanting to see if anyone has any insight before I go tearing everything apart again.


Thanks

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Its hard to make sense of that

With the engine running, what happens when you pull a spark plug on each cylinder..if its rough, the bad ones and good one you should be able to tell apart, confirming your measurements

Assuming your measurements are correct, I can think of no other thing than some slight wear or fault on the associated camshaft lobes or lifters

I am inexpert in these parts, others may offer a way to check the pushrod clearance is correct, or some way you could check for this fault

To be truthful, even that doesnt match if the leakdown test is a pass, its is a head scratcher

 

Bent push rods? Not allowing a full opening of the intake valve, resulting in a less than full amount to be compressed? Just guessing. It would be easy to check....

 

Very puzzling indeed!! The camshaft is new, although not broken in (because the motor isn't running well enough to proper break-in). I don't think it could've flattened all of the lobes that quickly, and it's even more interesting that it's the same 3 cylinders as before.

 

Unfortunately not, I've checked all for straightness and correct length. I'm thinking the heads coming back off this weekend before I pull all of my hair out

 

You said you checked valve lift and duration, I'm assuming on each cylinder. That eliminated a lot of possibilities
I know you replaced pistons and rings but just for grins it would be interesting to do a wet compression test.
Only other thing I can think of is that lifters are still somewhat gummed up, mouse milk can only do so much.