Ok I have researched this for days on here and google as well as took my truck to a mechanic. I have an 89 F150 302 and it has a flucuating idle in park that goes from 900rpm to 1500rpm cold or hot but only in park. Once it stops doing that it idles right at 750rpm but has a horrible shake or miss? I have ran a check for codes with my innova 3145 and get an 11 pass code on regular an continuous memory and no codes during KOER. Also I have checked for vacuums leaks all over and nothing. I have recently replaced the upper plenum gasket, rerouted the PCV to the center of the plenum so it dont get the #8 piston failure, new motorcraft IAC and TPS. New motorcraft plugs and plug wires(which ohm out fine and routed to prevent crossfire), wix fuel filter, cap and rotor as well as an o2 sensor and MAP sensor and timing is set at 14*. Prior to all the replacement parts it had the same issue. In putting everything back on I clean the entire upper plenum and throttle body.I've noticed it runs really rich, so rich as in I cant leave it idling in my garage for long or my wife will throw a fit. I have a new TFI module and fuel pressure regulator coming and after those I will be at a loss of what could be causing the loped idle and stumble at idle. Any words of advice would be great, I am to the point of getting rid of this thing right here \/

 

have you ever replaced the fuel filter? also, take apart the throttle body and attack it with some carb cleaner and a tooth brush. mine was filthy the first time i did it

 

oops forgot to add that.. new wix fuel filter and the whole upper plenum and plenum attachments were all thoroughly cleaned in a chemical bath.

 

Pull the vacuum line off of the regulator and check for fuel.

 

Have you tried setting the idle?

 

You got an injector stuck open?

How about cam wear, does the truck have cats? If it was running rich for a long time, disconnect those and see how it runs.

 

I will try that, what exactly am I looking for? And do it while running?

Non-adjustable idle. Messing with the stop screw is a hillbilly fix and my tps has been calibrated multiple times.

I havent checked injectors as my theory on that would be a frequent problem in any gear as well not only in park. Cam wear should be ok truck has never been driven hard as well as cam wear would be more consistant and I have power through the full range.

Truck has no cats, my exhaust system consists of Pacesetter shorty headers, 2.25" tubing from collectors to an off-road H-pipe and then into a magnaflow Y-pipe and 2.25" tubing to a magnaflow muffler and dumped out the back.

 

Check this out:


I had to follow this when I installed my 351w bored .040 over with a small cam.

 

I've pretty much done all that all the down to the idle re-learn prcedure and adjusting the TPS. No work. Truck will idle at 600rpm with IAC unplugged but once plugged back in it will flucuate. Leads me to believe something is either feeding to much fuel or the EEC isnt recognizing the idle.

 

or the motorcraft IAC which is brand new was a dud.

 

very possible. i had a dud iac do crazy things with my idle, however it was a standard part, not motorcraft. do you still have the old iac?

 

I don't know how long this has been doing this but could it be bad gas? (not the kind you get after a pot luck dinner). I that had a similar problem on my old 95 f150, I changed the fuel filter it helped for about 5 min and started to do it again. long story short and 3 fuel filters later the gas tank had rotted out on the inside and rust flakes kept clogging the filter. when you drove or gave it throttle it would run poor then snap out of it and run fine not exactly sure why but when the rpm's dropped it would start to run like crap again. after replacing the tank and another filter it was fine.

 

I do still have the old IAC however it is a dura-CRAP and I doubt thats any better.

Definately not bad gas, it has a half a tank of 87 octane and when changing the previous filter nothing dirty came out. I was rather surprised myself

 

Check the purge solenoid. Start engine and unplug the electrical connector on top of it. Then pull the vacum line off of it going to the charcoal canister. If its pulling vacum the solenoid is bad thus giving meaning to the old term of "sucking fumes".

 

I have been curious of this myself. But where is the solenoid? I looked everywhere and can't find one where they say it is