I've got a 99 4.2 throwing a code of P0303 (#3 misfire). This things runs fine until warmed up, then misses at light throttle. Under moderate/heavy acceleration it runs fine. I've changed plugs, plug wires, coil pack, and coolant temp sensor to no avail.
During troubleshooting I stubled upon something -- When I disconnected the temp sensor (at back of driver's side head) the miss went away and the truck ran beautifully. I suppose this made the ECM think the engine is cold. This has to be a significant discovery, but I'm not sure what it means. I can rule out mechanical issues? I'm looking to the EGR system next. Has anyone had similar issues? What fixed it?
Mike Allred

 

Try changing your coolant? Just a guess.

 

Update: EGR valve is off schedule.. Took a spin with EGR valve vacuum line disconnected and the thing runs great. I then checked the DPFE sensor and found it faulty - when backprobing the "signal" wire from sensor, it was stuck at approximately 1.10 volts. Properly working sensor is supposed to be .2-.7 volts with cold engine and 4.0-6.0 vdc when at operating temp. Guess I'll try a DPFE sensor unless anyone has a better idea. I'll keep everyone updated.

 

Now that I think back... When my DPFE went bad, I had terrible spark knock/pinging. After the new one was put on, no more spark knock. Sounds like you're on the right path.

 

I didn't have any problems when my DPFE went bad, but I am having a missing problem like you metioned. I have a bit of a rought idle, but once it warms up it is fine. However, I do have a bit of a miss/slip under light acceleration around 45-60 MPH, but I thought it was from the transmission.

 

Sounds like your IAC is sticking. Try cleaning it out with throttle body cleaner, and then spray a few shots of WD-40 in it.

 

Hello I am new to this forum. I have a 97 F 150 with the 4.2 and I am having an ongoing problem that sounds like the DPFE sensor but am not getting any codes. I thought it might be fuel but sounds more like what you are talking about. Could I get this miss without a code? Seems to be in at lower speeds on acceleration, light or heavy doesnt seem to matter. Just a quick miss then it goes away. Also doesnt matter if warm or not. Thanks for the info SAC

 

SAC , wait for the code, The quickest way to get a light is to push the motor harder. Babying it gives the computer time to adjust within it's default parameters. If you push it , you don't give it the necessary time , your type of miss will be hard to detect for the comp, you have to push it - It's called forcing a light.

 

Update: DPFE sensor changed and didn't fix the problem. Still pinging, missing, etc. I pulled the vacuum line off the EGR valve and it's smooth as a kitten. Danf EGR valve's still off schedule. What else has direct control of the EGR? Beat me... The DPFE sensor I tried was an Autozone piece.... hmmmmm..

 

I had the same problem with my 4.2, then when i went to get the running diagnostic done, it cleared up by itself. I think the injectors were causing my problem. just FYI..