I joined this group because we have tried everything to fix this engine and could use some helpful brainstorming help. It is a 2006 Ford F150 with 5.4 flex fuel engine. Engine was new as of last summer and has only 6000 miles and in the last month 1/2 after it runs for 20-30 min. in warm weather it will act like it wants to cut out when idle or low RPM's. If you rev it after that it clears it up for a little while and will do it again if idling or low RPM's. We have not run any flex fuel through the new engine. Could it have something to do with the fan or fan throttle not working properly? My husband has checked/cleaned/replaced virtually everything on this truck except tranny and cat converter. We are extremely frustrated as the rest of the time the truck drives awesome. He just fixed a leak in the exhaust system and thermostat w/connector.

 

1st thing I'd check is fuel filter then the fuel pump, would also take it and get the codes read when it is acting up.
I personally would stay away from the ethanol if possible.

 

The fuel filter has been replaced, it is brand new. Fuel pump was checked. Also now notice the chugging happens when temps. are higher AND air conditioner on. Yes, we are staying away from flex fuels.

 

Engine was new? New short block or did you get a long block? Have you tried the simple things like clean the maf, and throttle body?

 

If it was "new," whether brand new or rebuilt, it should be under warranty of some kind. If the problem isn't something they replaced, it doesn't cost anything to have them look at it and possibly diagnose it.

 

Replaced fan throttle and fan works more efficiently but started chugging again. It only chugs in idle or really low speeds and when air conditioning is on and the car has been running for 20-30 min. Has anyone replaced an Idle air control valve and/or Throttle positioning sensor and heard that this could cause this problem?

 

You don't have an IAC, 04+ have electronic throttles.

 

Okay. What about the throttle body or having a dealer "flash" the software? Getting all kinds of input from everyone...