I have an 01' 4.o swapped into my cj, it has developed a misfire. I get check engine codes p0303 for a misfire in the #3 cylinder and p0300 for a multiple cylinder misfire. it comes and goes, at odd intervals.Have you had this happen? Any ideas on cause?

Does it do this after a hot soak. If the heat sheild between the manifolds is missing the #3 injector is known to 'vapor lock' causeing a miss when first started. Other causes may be as simple as a bad plug or dirty injector. Check for carbon tracking down the plug.

It doesn't really matter the temp of the engine, and it doesn't happen only on start. it stumbles while running too. Injectors are only about a year old o2 sensors are about 6 months old. when this happens while i'm driving, it feels rough and the check engine light flashes. I tried revving it to see if it would clear out and it will climb to 3k rpm and fall off.

google search yielded
* Faulty spark plugs I have bosch double platinum (3-4 years old I will check them)
* Faulty coil (pack)
* Faulty oxygen sensor(s) (6months old)
* Faulty fuel injector(s) (1 year old)
* Burned exhaust valve
* Faulty catalytic converter(s)
* Stuck/blocked EGR valve
/ passages
* Faulty camshaft position sensor
* Defective computer
Which of these seams most likely?

i had this happen to me on a four cylinder. and the plug wires were bad and it was grounding out on the head.

the motor is out of an 01' so its got 3 coil packs monunted to the plugs. I'm wondering if the coil packs are dead? I figure I'll start with looking at the plugs and go from there.

My dad had a similar issue years ago on his 1989 jeep pioneer. After 140,000miles the first distributor went on it. Somehow it would cause the engine to run on 5 cylinders. After it was replaced, and new wires were installed it ran great for another 30,000miles. I think the oil pump was supposed to be changed, and the mechanic installed something wrong, and destroyed the engine. The second engine resides, with over 197,000miles on it. The 4.0 is quite a resilient motor, not good on fuel economy, but you know that already

Chris

The 4.0 is quite a resilient motor, not good on fuel economy, but you know that already

Chris
actually its not bad, with a little care i can get 19mpg +-2 with a heavy foot it drops to about 13

cam lobe wear has been an issue at times with the 4.0's

Threw a set of the plugs in this morning, so far so good :-) the old number 3 had a serious crack in the isolator in the combustion chamber . Will post back up in a day or two with an update. Thanks for the help.