rostdustin

I have a 1988 Ford f150 5.0 efi. The engine has been completely rebuilt. When I start the engine it idles at 1500rpms then comes down to 600-625. When its in park it starts to fluctuate idle from high to low and then will die. Also if the engine is warm sometimes when you start the engine it will stumble idle and die and then if you start it again it will idle better. Someone please help I dont know what else to do.

nrivera04

Any trouble codes or check engine light?

rostdustin

There is no codes

vjsimone

You must have at least 2 codes. 11?

vjsimone

Use this attached guide to set your engine Idle.

Have you measured your TPS signal output ?

Have you cleaned your Air Bypass Valve ?

rostdustin

I have replaced the IAC. I will check the tps signal and the codes and the idle adjustment and then I will get back to you.

first

I fought the same issue. When I first got the truck it had sat for some time so I thought dirty injectors? Nope. Never any codes and would only stumble and stall while in drive while at a stop. After tons of research swapping sensors I found a TSB. Ford made a IAC bypass plate with two adjustment screws. It's been four years and no more stalling but some rough idle from time to time. I found one in a junk yard but you can get them Ford and I think LMR.

rostdustin

Do you have to adjust the screws or does the plate come pre-adjusted? Would the side ports on the throttle body (fuel vapor lines) have any affect on the idle?

first

It has two set screws to adjust it and it won't bother the vapor lines. It just bolts between the iac and plenum

rostdustin

Is there suppose to be a purge valve in the vapor lines?