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85' f150 runs rough once warmed up.

Phil K

4
0
Help, my 85' f150 5.0 efi runs great when it's cold and warming up. Once engine is warm it will buck and kick and sputter until I let off the gas or hit the gas. At idle the engine speed goes up and down and up and down and if left alone will vary so much the engine will die. Fuel filter replaced, replaced the spark plugs and gapped them correctly, replaced cap and rotor, tested the coil, ICM replaced, new battery. Had emissions work done didn't pass, but after work done, passed with 0 co2. Any ideas? Please help if you can. Thanks.
 
Skandocious

Skandocious

Post Whores Make Me Sick
19,076
655
California
Sounds like maybe the idle air bypass. Here's the process for testing it:

FordFuelInjeciton said:
IAB TESTING

This is really easy to test. First as with all problems you should gather the trouble-codes from the computer. Follow the codes for testing and repair. If you get a code that points to a problem with the IAB start the vehicle and bring the engine up to operating temperature. Allow the engine to idle without any driver input to the throttle or pedal. Go under the hood, and disconnect the electrical connector to the IAB. If the engine begins to stubble or stalls the IAB is functional and does not need to be repaired. If the engine idle does not change you should remove the IAB for inspection.

The IAB can pass and still need repair, or it can fail and not need replacing. The plunger and internal spring can get clogged with dirt and oil. This will slow down the air flow and not allow the IAB to function properly. Remove the IAB and clean it. There are 2 halves to the IAB, and you can not buy just one half, but you can take it apart to clean it. But if the internal solenoid is faulty the IAB needs to be replaced.
Click to expand...

http://www.fordfuelinjection.com/?p=39
 
KdPate

KdPate

After Hours Garage
251
10
Murfreesboro, TN
Does it have EFI or a carb?
 
1985 Ford F-150

1985 Ford F-150

Country Boys Can Survive
7,816
307
Tooele, Utah
Mines got the same problem and mines the computer. It aint all the way bad but its bad enough to where the mechanic couldnt pull any codes from it. I can drive it fine but it has that surge at idle that your talkin about. Were just gonna run it then if it dies convert it to a carb.
 
KdPate

KdPate

After Hours Garage
251
10
Murfreesboro, TN
Skandocious said:
You suckin back some of grandpas old cough medicine? Read the first sentence of his post :rofl2:
Click to expand...

'smiliedoh' YelloThumbUp

x2 on the IAB valve. Also check for any slight vac. leaks around the intake that could cause this.....
 

Phil K

4
0
Thanks

Thanks for all of the good advise, I'll try some things out this weekend. Any other things you can think of will be of great benefit.

Phil
 

Phil K

4
0
Could Computer Have Been Resetting?

The old 85' f150 is running great now. I didn't do a thing with it except drive it a bit and let it sit for about a week. I drove it this morning to work, about 35 minutes of side street and freeway mix and there was no sputtering, or mumbling or odd idling. Could the rough idle been because the battery was disconnected for several days, and the computer was resetting the fuel injection?
 
Skandocious

Skandocious

Post Whores Make Me Sick
19,076
655
California
Yessir. It's a learning computer and if it loses power for long enough to erase the memory then it needs to learn everything all over again. Can take as much as a couple hundred miles, so I've heard.

That being said-- I reset my computer all the time when I'm doing electrical work and I've never noticed a difference in the way that it runs afterward... Whether or not your truck is a different story (perhaps because it's 10 years older) I dunno...
 

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