When my truck is in gear and stopped, it idles very rough. If its in park, it idles very smooth. Didnt think it was the iac, but cleaned it anyway and didnt help. Tried sea foam, no go. I think its a vacuum leak somewhere, but where/what else could be causing this? 1998 f150, 4.2 w/4 speed auto.

 

Start with the basics;when was the fuel filter changed?Plugs and wires?Throttle body cleaning?MAF sensor cleaning?Are you having to add coolant?Any CEL codes? PCV valve changed last? Answer as many of these as you can;we might be able to narrow it down to something simple.

 

Keep with the vaccum lines, they are the cheapest thing to check first and probably the culprit as it was on mine.

The line directly behind the upper manifold was the one killing my truck in gear but acting normal when I slipped it in neutral and even better in park.

 

i did the pcv valve, oil change, air filter, cleaned the iac, fuel filter and it got better. Still there though, i think im going to do the vacuum lines even though they dont look like its going to be fun. I think cleaning the maf wont hurt either. I think my trans is on its way out, since its shifting hard into first sometimes.

 

Check the MAF, Mine did that and felt like it wanted to sputter at idle, in park and drive, did you light trip? i had the code ran and it said Bank 2, mix to lean, replaced the MAF the first time and was good for awhile, but did it again recently, not sure if bank 2 is passed the MAF though, into the Throttle body? Anyone know??? Its getting annoying, it clears up when i clear the code for about a half of tank of gas or so, then back again.

 

yea a while back i got the bank two o2 sensor, the one behind the cat on the right. Replaced the o2 and code hasnt been back on since.