Cylinder misfire

It seems to me that the 4.0L is prone to problems with cylinder misfires. I know in my case I've had 3 of them with the problem. I've tried everything I could think of to eradicate the problem. Until recently to no avail. I tried plugs, wires (on the 1998), a new coil, different coil packs (on the 2001), numerous different plugs, new injectors, etc.

I've had the problem with my 1998 and also 2 2001's. I happened to find a TSB related to cylinder misfires that stated the problem was with carbon buildup on the back of the exhaust valve and the problem would be intermittent. sounded to me like it fit the bill.

A few weeks ago I bit the bullet and pulled the head from my wife's 2001. Oddly I didn't find and smoking gun like a cracked valve, or carbon build up. As a matter of fact everything look pretty decent. The exhaust valves and seats were pretty pitted but that seems to be somewhat normal on late model motors. What I did find later though is that when I ground the valve seats in the head several of the exhaust seats were off center of the valve guides. I believe that between the pitting, the offset of the exhaust seats, and loss of spring pressure from time, the exhaust valves were not sealing all of the time.

I got everything back together a few days ago and so far it is running much better. No more misfire, and no more check engine light. I'm thinking that like me many people out there with these 4.0L's are living with the misfire problem due to looking everywhere but where the problem is. Obviously there are other things to look at before pulling the head and doing a valve job, but when you've eliminated everything you can think of it's a good thing to consider. I'm guessing it's a very common problem.

HTH,
B-loose

The 0331 head cracks between 3 and 4. im not sure what code you have.also sometimes the valvesprings become weak

I know about the head problems because this motor is a replacement because the last motor had the head crack. The symptoms of the cracked head were different in that I kept losing coolant but couldn't figure out where it was going. I figured out that it was going into the crankcase too late and had to replace the motor.

The first motor also had a misfire problem and I believe it started even before the head cracked. I didn't replace the head on the current motor, I only gave it a valve job and it runs great, no longer is missing, and of course is not throwing a code. After many years of trying to track this down and coming up short, I think I've finally solved it. Because of this I thought I would share this so someone else having the same problem would have another thing to consider.

B

oh gotcha my bad thought you were looking for advise. guess i should of read the whole post

That's cool. Thanks.

There is a TSB that recommends occassional operation at high RPM so the valve rotators operate. I assume this laps the valves in and prevents what you described. What are your driving habits?

The 2001 doesn't have valve rotators. They use beehive style springs with their smaller lighter retainers and no rotator hardware. I don't know what they ran on earlier heads though. Rotators as far as I know don't actually lap the valves, they only keep the valves rotating so the mating surfaces don't keep touching in the same spot. By doing this theoretically the wear surfaces should last longer and the wear should be more consistent throughout. The problem is this advantage is mostly theoretical and doesn't translate into much more longevity. It does create a heavier and more costly retainer assembly though.

Once again, the valve seats being off center of the valve guides is most likely a manufacturing error and not something that was caused by anything after machining.

B