1992 F150 5.0 XLT Auto
Good afternoon y'all, fixing up my ol' high school truck for my son who is now driving. When cold, the truck will rev high for about 30 seconds and then start "hunting" for idle. It will start to idle down, then rev back up, then down again, then back up, over and over until it finally settles into a constant idle after 2 or so minutes. Sometimes it even dies, multiple times, when hitting one of the low spots low in its hunting process.
Once warm and settled into a constant idle it runs great, I dare say perfect. It's just the cold start that I'm having an issue with.

I've replaced:
-most of the vacuum hoses
-EGR valve
-Idle control valve
-plugs
-plug wires
-dist. cap
-fuel pump
-fuel filter
-PSOM
-timing chain and gears
-MAP sensor
-cleaned the throttle body
-Sea Foamed the intake

Any help you can offer would be greatly appreciated.

 

When you cleaned the throttle body, did you also take off and clean the idle air control (IAC) solenoid on the side, straddling the throttle butterfly?

Mine would cold-start idle as you described, cleaning the IAC usually resolved it, had to do this once every couple-three years or so.

Your problem could be other things, but cleaning the IAC is relatively easy and cheap - if nothing else, to cross that off the list of suspects.

Good luck, and keep us posted.

 

I replaced it with a new unit. I really thought that would help but it made no difference whatsoever.

 

You will save a lot of time and money if you start diagnosing each component instead of just blindly replacing. Check that your fuel pressure and timing are all within spec. i had a similar issue a few years back that ended up being the fuel pressure regulator.

 

My 87 is similar. does your truck idle too high in general ? if you put it in gear does it move forward without giving it gas ? i suspect mine has a vacuum leak. the plenum is always suspect on these trucks.

if you could have someone perform a "smoke test" for vacuum leaks, i think you would find some issues. i need to do that on mine also

 

No, in fact I have code 13 which is too low idle once it gets warmed up. I have bumped up the timing to 12 or 13° instead of 10 to try and help with that. Replacing the timing chain and gears, plus the distributor cap and rotor did not help in that area.
I did a smoke test months ago, and that’s when I started replacing the vacuum lines. The only lines and hoses on the engine I have not replaced are the large hard/metal ones that run across the back of the block.

 

Fuel pressure regulator has been replaced with a new unit - no change. The truck still dies on startup most days as it transitions from high idle down to low/static idle. I'm going to try smoke testing the vacuum lines again - this time with a cigar instead of a party-style smoke machine.

In regards to replacing so many components, they all were not done in hopes of fixing this particular issue. The truck had other problems that needed resolving, plus, most of the parts were original and I didn't want them to go out while my son is away at college starting in the Fall. He will be too far away for me to go down, diagnose, and fix whatever problems arise. And he will be in a dorm, so he won't have access to all the tools he would need to do it himself. I'm trying my best to make the truck like new for him so he will have years of trouble-free driving ahead of him.

 

Adjusting the engine out-of-spec does not help with problems - it might temporarily mask one problem, but it can create several others. You goal should be to return it to stock condition since it obviously ran perfectly when it left the assembly line. After that, if you feel the need to modify it, you'll at least know that you're modifying something that works right, and how to return it to that condition if you don't like your mods.

Read this page and then start TESTING each sensor, disturbing as LITTLE as possible in the process:

(click this text)

 

Saw you replaced the MAP sensor, so have you looked at all the vacuum lines?

My issues were tight wheel turns at idle would surge engine and cut it off. I replaced all vacuum lines with silicon, a new vacuum reservoir, then replaced IAC and MAP sensors. Truck runs like a champ now. Total parts? Maybe, $120.

 

Checked for vacuum leaks with a cigar today - smoke was coming out of the holes in the bottom of the EGR valve.
Should that be happening?