sudburyman

Can anyone help me, I have a '98 f-150 at idle it revs up and the revs normal and contunues to do so while have the heat/air **** is on, turn it off and its completely normal.
Please reply, thank you in advance
Paul

harm63

one of two things comes to my mind as I have that issue right now...first, I would either clean out your IAC Valve or just replace it.... how many miles on your truck and what engine do you have in it?

I replaced mine as it was completely black inside and with 156,000 miles on my truck when i got it 3 weeks ago..i didn't want to waste my time cleaning it in vain...

replacing that didn't fix my issue, so my next "to do" is to replace the TPS (Throttle Position Sensor).... reason being... i had an explorer that did that when i was sitting still and replacing the TPS fixed the issue...

Hope that helps ya.

Austin97

It could be a vacuum line that is connected to the heating/cooling system. pull the trim around the radio off and unbolt the HVAC and there should be a connector with a bunch of vacuum lines. One of them or one of the lines under the hood may be leaking.

Also is it only in AC or defrost? Because if that's the case there's a good chance it's your AC compressor locking up. Sometimes they lock up so bad they actually stop the motor from turning. If you want to check that take the belt off and run the truck. Don't run it for long because the alternator wont be charging the battery and more importantly the water pump wont be moving coolant.