Ok so I have a 2004 fx4 with the 5.4. I have 92k miles and just replaced spark plugs and two coils. I also just flushed the radiator fluid. My engine used to idle at 500 Rpms while in drive at a stop. It now recently (after coolant flush) started idling weird. When I come to a stop and sit at a light, it will go down to 300rpms or so and the go back to just under 500rpms. I have no misfires and it drives fine when not at stop in gear. I'm getting 16.3 mpg with mixed driving. It is bone stock and I cannot figure this out. Had the tps checked and it said it was within range. The alternator is not making noise and within range. Is there a coolant temp sensor on these trucks? Because it threw an emissions code that said catalyst below threshold at 195 degrees but I was told by a mechanic that it was air pockets in the cooling system that threw it. I had the code shut off and the light hasn't come back on. I'm so frustrated about this because it is a great truck and has not given me probs until now. Anyone?

 

Forgot to add my fan clutch is going out. Not sure how that would affect idle though.

 

Try cleaning your throttle body, and MAF. I've got a similar issue with low idle at 500 rpms, but my truck will randomly stall at stoplights, when stopping hard mostly.

 

You have air in the cooling system.....

 

Will air in the cooling system affect your idle? I think the air should be out by now. We bled it over night and I drove it for 3 days so that my over flow tank would take care of the air. I have hot air and my temp is where it always is.

 

I know on my 08 4.2 the fan clutch was going bad causing the belt to slip and caused random idling and acceleration.

 

Ok we'll hopefully that will help it. I have been running 93 octane and my exhaust smells like fuel but I could be overthinking it because I'm trying to find things. I have also heard that the gm trucks had this and there was a recalibration they had to download to the ecm. Do they have these for fords?