I'm at a loss. I have a 2003 f150 4.2 litter. I at times am having issue with truck running rough idol and for no reason the rpm will start revving up and down from 800 to 1500 rpm and tries to stahl or lunge forward if I don't put it in park. Seems to only happen when the engine is cold. This seems to be a common problem for people from what I've seen on Internet. I did have lean and rich codes. So this is everything I've changed and checked. I have replaced all four o2 sensors, I've replaced the fuel filter, air filter, mass air flow sensor, pcv valve and elbow, the idol air control valve (twice) because I was told by a ford mechanic that he has been seeing bad ones coming from the parts store and he was sure that's what was causing it. But it didn't solve the problem. I am still having the same issues. We've plugged in a reader and did a diagnostic check. Everything seems to be running at all the right levels. The fuel pressure seems to be good. I thought about changing the fuel pressure regulator valve but have been told that those rarely go bad and probably is not the issue. So what could it be. It doesn't happen every time the engine is cold but it is still happening at times and is very annoying. So any suggestions or has anyone had the same thing and finally figured out what was causing it. Thanks Chris

 

Check and clean ur idle air control valve. If that doesn't work then replace it

 

I have replaced it. I'm actually on my second new one because a mechanic told me to replace the first new one cause they felt the first one was junk. But it still runs rough

 

Is it getting power or sending signal back to the ecm

 

In my Nissan Maxima I had the same problem. I know it's not a Ford. There was a temperature sensor that screwed into the block to read the coolant temperature (

 

Prematurely sent whoops........but that sensor would tell the computer the temp so help set the air gas ratio in the fuel injection. Common problem was that the sensor about 3 inches long would crack. Still function but always right. Sometimes for the first 5 minutes or so after start up, the tach would either jump up or drop low or once in a while stall. It was a quick easy fix.
I don't know what sensors or where they are located to gather the info to tell the trucks computers how to set the ratios. I would start there and also the MAF mass air flow sensor. Did you ever use a air filter that you had to oil? If you did and over oiled it, it through off the MAF sensor. A little aerosol spray helps clean it.

 

I'm not sure about the temp sensor Jeff. I'll have to look into that one. I figure it's got to be something stupid causing it. I replaced the Mac last year but maybe it needs cleaned with the rear o2 sensors running bad for a year. Thanks for the info

 

Throttle position sensor?

Also give your vacuum lines a check. If one is cracked it could cause a problem.

Additionally, I had a similar issue with an old jeep that had a loose exhaust manifold. The extra airflow caused the computer to increase fuel and the idle went up like you describe.

 

Pilot pop. Thanks for the info. I'll have to look into that one as well. I'm guessing it's some kind of sensor. Just not sure which one. They all make sense on being cooperate. Thanks

 

If you know how to use a multimeter, try checking the resistance of the various sensors before replacing to save some money. Chiltons and Haynes manuals should have the ohm values for each. If it's a sensor going bad the computer should tell you through a code.