I have a 2000 Ford F-150 4.2 ltr v6 2wd. It was throwing codes p0171 and p0174. I changed the upstream o2 sensors and the MAF. Took it back to Autozone to check the codes one more time and they said it was an IAC valve (idle air control) code so I replaced that. After I cleaned the throttle body, it started running at an average 1250 RPM’s instead of the usual 850.
It only seems to do it when it’s at operating temperature. I’m thinking it could be the TPS now. I’ve checked for vacuum leaks but sometimes it idles at 1250- 1400 rpms and other times at 1000. It would seem to me if it was a vacuum leak somewhere wouldn’t it idle in a consistent manner?
I’m at a loss here.
The more I research the possibilities the more threads I read and learn it could be perhaps the EVAP or EGR. Maybe it’s the temperature sensor telling the PCM it’s cold all the time. The only time I really notice the issue is while I’m coming to a stop. I see and can hear the rpms acting funny as in revving back up to 1500 rpms then slowly drop to 1100-1350. So maybe its some kind of transmission sensor going out. Any ideas, suggestions?
Usually after cleaning the throttle body, you need a scanner to perform an idle relearn procedure. Once you remove the dirt and carbon build up from the throttle opening, you’re basically letting in more air now than the PCM has become accustomed to. It does not know how to react, other than raising the idle speed.
Sometimes, depending on the vehicle, a long drive and then coming to idle and sitting at idle for 2-3 minutes, shutting truck off for 1 minute, then restarting will normally reset the idle speed, but other times it has to be reset with a scan tool. Worst case scenario, the PCM will need be reprogrammed.
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