I have a 97 F150 and it was idleing way down when i started it and then would rev way up. So I replaced the IAC, solved that problem. Now whenever I'm sit at a stop light or Taco Bell drive thru, the truck acts like it wants to die. Would this be a MAF issue. I've cleaned it recently. Now issue #2. About 6 months ago I ran some seafoam thru my truck and now my truck knocks when it's in park and under heavy acceleration. I doesn't do it when it's just setting in gear, only in park. Any ideas on either? Thanks in advance!

 

Well, I'm just guessing here but I assume the knock sounds similar in both situations and it is not spark knock because it occurs at idle. Generally a knock when the motor is not under a load like idle or coasting indicates a damaged wrist pin (holds the piston on the rod). A knock under acceleration or load indicates a damaged rod bearing (holds the rod on the crankshaft). It is possible that the seafoam dislodged some metalic crud in your engine and it has now damaged the bearings. You should drain your oil immediately and look for metal flakes. Not a good sign. Another possibility is you collapsed a lifter because some crud blocked the oil passage. They make a lighter clicking sound at idle but dissapear at higher rpm because of higher oil pressure but then to reappear under heavy load....I don't know....maybe.

 

The sound is more of a ticking than a knock. When I change the oil should I put any additives in it?

 

When I was using Penzoil I also added Slick50. Two years ago I switched to Mobil-1 full synthetic and use no extras. The increase in smoothness and quicker acceleration was noticeable. It's up to you..

 

Back to the idle issue. Last week I replaced my IAC and my truck still idles rough. What else could be causing this problem???

 

MAF or Vacuum. You said you cleaned your MAF recently but I would clean it again first. What did you clean it with? I use MAF cleaner but carb cleaner works well too I've heard. It's probably the same thing in a different bottle anyway.

 

Also could be fuel delivery, since you asked what could cause it. Just went with the usual suspects first...

 

I just chased down a "rough Idle" problem on my 97 150 5.4.....After cleaning the MAF, replacing IAC,plug boots,throttle position sensor(or something), battery, coils and egr, My buddy(far better mechanic than me) got under the hood for bout 1 minute and found a vacume leak on the Back of the PVC hose right where it connects with to the intake.
Boom!!!! $30 from the dealer and im back in buisness!!!