My truck is a 2003 F150 Supercrew Lariat 5.4 with about 210k (km) on it. Every now and then it will not upshift and seems to be revving up on its own. Seems to be mostly when it is cold out and with moisture in the air (snow mainly). Eventually it will stop and drive normal. But when it happens if I come to a stop and put it in neutral it will rev up to about 2000 rpm on its own. When it is having this symptom it will hold 2nd or 3rd gear and rev fairly high (2500-4000) without shifting up.

I immediately thought that it would be the IAC so I replaced it with a new Ford one. I thought that it had fixed it but the problem came back the next cold wet day.

Is there something that I am missing? Where do I go with this next. It is so intermittent and usually far from home that it is hard to figure out. The engine makes good vacuum and it is steady so I did not think that it would be a vacuum leak.

Thanks in advance.

 

could be you diablo tuner

 

I thought it may be so I actually removed the tune, it had the same issue with the stock tune so I put the tune back on like I usually have it (set for 87 octane).

 

Any other ideas? I have not had good luck figuring this out with no codes and a sometimes dangerous situation as the truck just wants to drive as if the throttle were open. Not fun when your on the high way in deep traffic with tons of black ice.

 

disconnect you cruise control cable from the throttle body maybe there is a short in there causing it to think you have it set?????

 

Have you checked the throttle position sensor?

 

not yet. would that make the truck rev on its own?

 

no the sensor just says where the throttle is, the other thing you could try is the iac valve but if that is moving then you need to figure out the reason the pcm is telling it to open

 

Sorry, it was late when I posted that. I meant to say IAC valve, but that was replaced as stated in the above post. Guess I better read the posts when I'm not half asleep!