I am new to this forum and I am lost for solutions to my situation. I have a 1998 E250 with a 4.2L V6 and 200k. No matter what I do I keep getting a p303 for a misfire on cylinder 3. I have installed new plugs, new wires, a new coil - and triple checked everything to make sure all is ok.

Compression on all cylinders is good.

If I reset the code I can let the van run in the driveway for an hour and not get a misfire. If I drive it lightly everything is fine. It seems as soon as I go over about 40mph the check engine light starts blinking and I get the same error code again for a cylinder 3 misfire.

If I drive the van on local streets it feels fine, on the highway the misfire is very noticeable.

Any ideas what this might be? It is only cylinder 3 and I get no other codes. Thanks for any input!

 

Being it has 200K on the engine, have you run any type of fuel injector cleaner thru the system? A dirty injector will not spray properly and can cause mis-fires under load due to poor Air/Fuel mixing. I'd pop a double dose of Techron concentrated fuel cleaner to a tank of fuel and run the tank down.

 

I just added two cans of seafoam to a full gas tank on saturday, not much better but I've only used about 1/3 of the tank. Should I have gone a little stronger on the seafoam? Or is the other additive better?

 

The Seafoam treatment may resolve your problem. I personally use AMSOIL, but Off-The-Shelf, I recommend the Techron. Seafoam is an oil based solvent which is why it smokes when used for cleaning intakes. I would run the tank down and try the Techron. It's still easier to thry this than pull your hair out.

You may just have a bad injector that no cleaning will resolve. We bought a new 2000 Mustang that ended up with the same symptoms when it had about 5K miles on it. The mechanic said that it was from not driving it and storing it during the winter months. WRONG...When the injector was pulled under warranty, the injector looked brand new. It just wasn't pulsing all the time, just plain bad.

 

If the previous suggestions have not located or eliminated the problem , here are a few more to check. Check the harness for burn thru around the EGR pipe. It could be blocking the injector pulse to that injector when the engine is accelerated slightly. Try tapping on the PCM. Sometimes this will help find a flakey injector driver in the PCM. Try disconnecting the EGR vacuum line from the EGR valve and see if there is any difference when you can create the misfire.

 

I unplugged the battery for an hour while I wrestled with the fuel filter. Have driven the van all day today to jobs, including highway and no check engine & the misfire seems to be gone.

I wouldn't think a fuel filter would cause only one cylinder to misfire. Maybe the PCM reset did it? I erased the codes with my scan tool after I performed each action, does unplugging the battery do more?

Anyways all seems good and for 206000 miles it's running great. Thanks for the input!

 

Aahhhh...The wonderful world of electronic gremlins...Well, a marginal flowing injector with not enough fuel flowing to it due to a dirty filter reducing flow under load could cause a misfire.