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
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