My '98 Sport 4.0 liter is misfiring randomly. Starts fine and can rev rpms no problem while sitting still, but will misfire at random while traveling and can be pretty severe at times. New plugs, cap, button, and crank and throttle position sensors. Still misfiring. Any suggestions on what to try next? Fuel pump/filter assembly? Help?

 

I would check my fuel pressure, then I would look for vacuum leaks.

 

i would stop trying things, and start testing things. Firing the parts cannon is expensive and frustrating.

 

Any codes? Cleaning your fuel injectors is free (if you don't F up the O rings), and that fixed my misfire, but I had a code for the fuel injector.

 

Also check the coil to make sure it is putting out a good spark .

 

Codes were for the crank/throttle sensors which were replaced first. Ran OK for a couple of trips and then started misfiring at speed again. No problems starting. Seems to be random misfire at speed, and then can really start to run rough at times. Vacuum leak may be a good idea.

 

Yes, vacuum leak, and also check the connection to the coil. If it's loose it will cause that.

 

So I took it out for a spin yesterday . . . about 5 minutes in started misfiring. The check engine light came on so I went ahead and kept going (coasting in neutral seems to extend the smoot running when I shift back into gear) and was able to get to the autoparts store . . . code was "crankshaft position sensor circuit". Same code as a few weeks ago and now with a new sensor. Thoughts on what would foul up a new sensor? Wire short, oil/coolant leak?

 

What brand of sensor did you install? How is the wiring?

 

O2 Sensor

 

didn't replace the plug wires?

 

Did not replace the wires when I did the plugs. The sensor was a mopar / from local Jeep dealer in Nashville. I'm gonna start it up and see if check engine light comes back on. If it does, perhaps a bad sensor. If it does not, perhaps bad wiring.

 

Replace the plug wires.......

 

What EXACTLY was the code?