I've got a 90 F150 4x4 with a manual tranny that I just dropped a 351w into out of a 91 Bronco. I can't seem to get the truck to idle correctly and my gas mileage is horible (7-8 mpg) and it smells rich when it's idling. I don't know if the problems are conected or not. The only code I get when I do an engine running self diagnostic (KOER) test is 332 wich seems to be an EGR issue so I don't see how this is an issue. I have a true dual exaust with an equalizer righ behind the oil pan and I put a new O2 sensor in the equalizer right beside the right bank. I've swapped the IAC with others I have and it seems to make no difference. The truck rarely idles below 1000 rpms and If I don't pop the throttle when I let off it falls very slow and often settles around 1500 rpm. When changing gears the RPMs usually rise then fall slowly like the engine doesn't really want to drop to idle. I checked the timing and noticed that the idle jumps and the timing advances very sudenly when the throttle starts to open wich makes me suspect a bad TPS. If the throttle is released gently the idle stays up and the timing stays advanced. I can't seem to find any vacume leaks yet though it seems to act like it has one. It seems like even with a minor vacume leak the computer would get the idle down to where it is suposed to be. It also seems to miss some and run a little rough when the idle does drop the about 800 rpm wich is as low as I can get it to go. I don't know what else to do. Any help is greatly apreciated, I'm out of money and I need this truck to work because I have to use it every day and while I don't expect to 16 mpg I need better than 7 or 8.

 

Try a KOEO test and see what comes up.

 

I know its been a couple years since you posted this thread but what ended up being the problem, I have a 1990 F-250 and its acting about the same way.

 

years? its only been a month? Anyways, is it idling rough or high?

 

yeah sorry about that i was looking at your join date instead of thread date, well i had a diagnostic test done and it turns out it was the MAP sensor, I had not checked that because the original owner had bought one from summit racing so i figured it was good, i ended it putting the old one back on and it works fine now. Thankyou for replying though. At freeway speeds it ran fine, but at idle it would be very rough and even worse if i gave it some gas, If i came off the freeway and sat at a light, when i would try to take off again it would want to die when i gave it gas. I would have to hold it at 3000 - 4000 rpm for it to not die as i let out the clutch until i was going at a decent speed. It seems to be working fine today.

 

When I was only dealing with the high idle I fixed it by adjusting the TPS a little bit and installing different throttle plates. I'm not sure that I still don't have a vacume leak somewhere but I put some throttle plates on that I had laying around that didn't have any holes in them. I don't know what the purpose of the little holes in the throttle plates is but Ive seen with throttle plates with two different size holes and without holes alltogether. When I put on the plates that didn't have any holes and moved the TPS sensor so that it was slightly retarded in it's sensing of throttle movement. The TPS didn't move much but maybee it was enough.

 

Ill surely keep thatin mind for future problem. The changing of the MAP sensor has seemed to fix the issues that I was having withthe truck. Thank you for the follow up.