I have a '99, 4.0L, 4X4, Auto with 107k miles. The engine is missing, surging and hesitating at all speeds. Sometimes it is so bad it barely runs, at other times, it runs fine. It is particularly bad when accelerating. Started with an occassional miss. Changed spark plugs (replaced with Autolite platinum) which had about 60k on them. Didn't help. Removed, checked gap, replaced and put on new wires. Have checked most all sensors (MAP, IAC, throttle position) which show correct voltage and resistance or pass other tests and changed PVC and air filter as part of routine maintenance. Seems to be getting worse. I am getting no trouble codes other than an occassional #6 cylinder missfire code that only displays when it really misses. Have checked #6 cylinder several times, replaced plug, etc. Have tried to change the fuel filter but can't get it off (have other post on that). Does it when the engine is cold sometimes but worse after it warms. At other times, no miss at all and runs fine. I wonder if the O2 sensors could be going bad (had a slow response code once after I ran injector cleaner) but no other codes on them. Any thoughts?

 

I would get the fuel filter replaced.

 

Have you monitored the values while it's missing?

 

I would change the filter ( I think you need the special for that). Think the injector cleaner may kill the O2's, but it also sounds like your #6 injector may be dead. Or maybe EGR dumping too much ( but that should set a code).

 

You could have a bad coil pack........

 

I went back to my original post on this to say I have apparently fixed my problem - but don't know what it was. To summarize, my truck started with an occassional misfire. I did a tune-up with plugs, filters, etc. to no avail and it actually got worse. At about that time I started setting a "#6 cylinder misfire" code as well. After checking all the sensors and re-checking everything else, I replaced my number six injector last week. The problem seemed to get even worse. By now, my truck was barely running.

Two nights ago I checked my fuel pressure. It was fine, 65 psi and steady (spec says 55-70) and I decided I was just going to go back thru everything and re-check it. I checked to make sure each plug wire was properly connected and checked the continuity of each with no problems. I pulled each plug and checked the gap and re-installed them with no problems found. While I was at it I also decided to switch out the #6 and #4 plugs just to see if my misfire was plug related. Maybe a cracked insulator or something I couldn't see. I was wanting to see if the misfire moved to the #4 cylinder. By then it was late and I quit.

Since then , not one misfire in two days. Now what happened I don't know. All I really did was switch two plugs around that were in different holes than when I started.

I know the obvious thought would be a plug wire not on tight or a gap off but they all were fine when I removed them and I did not change the gap on any plugs. Guys, I cannot figure it out.

I do have a bit of rough idle but it seems like a loose vacuum hose or something which I will check out but other than that, it runs fine.

My only thought is that my core problem was the need for a tune-up. My plugs had about 60k on them and the wires were original. In the process I cleaned my MAP and throttle body and they were a bit dirty but not extreme. Perhaps my #6 injector was also a bit dirty and the plug that ended up in the #6 cylinder was weak or something. Maybe I didn't have the plug wire on real tight or something but a bunch of little problems escalated into one dead cylinder.