Cylinder 4 misfire help

[MO5D3F]

This is a 97 ranger 4.0 ohv. It had blown heads, both were cracked. I put new ones on. fixed a few gremlins that popped up but am now stuck on this problem.

symptoms:

cold starts quick and idles nice, but if within 60 seconds of starting I try to drive away it struggles like there is now power. after about one minute that goes away and I can peel out. normal? related? idk

main issue: only seems to happen on the highway after I get her going up hill on full throttle, cylinder misfire and significant loss of power. feels like spark is escaping somewhere but cables all look good and so does new plug. I had swapped cables and plugs, problem still on cyl 4, changed coil pack, still cyl 4, new injector on 4, still cyl 4, checked fuel pressure while parked and throttling, about 34psi and ramps up to about 40psi while throttling, compression tested hits 120 psi no problem, same with rest of cylinders.


really hoping for another direction to look for in trouble shooting this one.

thx

 

[Dirtman]

Were the heads rebuilt or replaced with new? Reman? Kinda sounds like a valve in cylinder 4 is floating or sticking under heavy load.

 

[MO5D3F]

new as far as i could tell. I did have to reuse the original pushrods and rocker arms though. They looked good, I have pics of those.

 

[MO5D3F]

to be clear machine shop told me they were new, and they looked brand spanking new

 

[MO5D3F]

well I guess I'm going to try the exhaust and sheet of paper test tomorrow to see if maybe there is a sticky valve i can identify. I'm not sure if I will be able to notice it if im not running under load though. But gotta try something.

Any other tips would be helpful ..

thx dirtman

 

[MO5D3F]

I did the paper exhaust test today. Its my first time doing this trick and I just seen it done on youtube. I'm not entirely sure what it means but what I'm thinking is when the paper gets sucked in it means one of the exhaust valves isn't closing all the way before the intake stroke begins. video of test


any thoughts?

 

[MO5D3F]

I think I got it. No more misfires. The answer: Miracle mystery oil in the motor oil.

I thought about what you said, and saw what looked like confirmation with the paper test. The heads are new, but the lifters and bottom end are not. When I did the heads I didn't think to pull out the lifters and inspect. They must have been all gummed up. I replaced a quart of motor oil with MMO and ran it up and down the highway again, a few recorded misfires but the gross misfire went away promptly. Changed the oil about 50 miles later no more check engine, no more misfires and passed smog.

Thanks again Dirtman