I have a 92 f150 5.0L 302. We just got the truck maybe 3 weeks ago and replaced the plugs, wires, rotor, distributor cap, and air filter.

When I start it up it warms up at a higher rpm (~1200) and then slows down to a steady 500-550, which I at the time thought was a little low, but now figure is pretty normal (is that true?). I read that a disfunctional IAC could be the problem so I took it out, cleaned it, and put a little bit of wd-40 in it to help. After I reinstalled it, I started up the truck again and it sounded a ton better when it was warming up but then it abruptly died, which I figured was the computer re-adjusting to the IAC again.

I started it again and it stayed at 1000 rpm for a second, then dropped down to the normal 550, it sounded better, so I gave it some gas and after it peaked it went straight down to 300 rpm and sounded like it was going to stall, and did this the next 5 times I revved it. Could it just be the computer re-adjusting still? Or is there something else off?

Sorry for the long post...

 

You should pull the battery for half an hour or so to reset the computer. Then it'll switch to defaults and if it is a learning problem, it should clear it up.

Is this an auto or a manual? I don't think I've ever seen my 5.0/AOD idle lower than 700.

 

Its an automatic, I'll go pull the battery now.

Yeah I figured that it was idling low but my dad kept telling me otherwise.

 

Iv'e certainally have learned that everytime I replace componants on my truck (89'f250
efi 460) that you have to remove the battery cable when working on it.

 

Yeah disconnecting the battery didn't help at all.

It still starts strong, Idles at around 1000 rpm for about 5 seconds before it abruptly drops down to 500, and it sounds like its going to stall after it drops, it catches itself and then steadies itself at about 500, but whenever I rev it, it just drops really low again and sounds like its going to stall, then gets ahold of itself and steadies at around 500.

It wasn't doing this earlier today / yesterday, so I don't know.

The truck isn't throwing any codes, but the Rear ABS light is on, but I seriously doubt that has anything to do with it.

EDIT: The exhaust from the cat back is severely screwed up, I'm getting a new flowmaster cat back system tomorrow, but for some reason I doubt that will help.

 

the idle speed can be adjusted on most whith snappon scanner

 

To be honest I don't think that's the problem. Would a low idle like that cause it to almost stall out every time I rev the engine?

 

Actually, the truck is OBD1, the idle can't be adjusted with a scan tool. In fact the best most tools can give you is the 2 or 3 digit error codes from the ECC.

Although there is an idle adjusting screw, you should NOT touch it.

I'd suspect a faulty IAC, vac leak or possibly the TPS.

Hit fordfuelinjection.com for detailed testing information and make sure you follow the instructions to pull your stored codes. Just because the light isn't on doesn't mean it hasn't been storing a code for you.

Pull those, post them back. Since you cleared the ECC it might not have any in it anymore, but it's worth a shot.

 

Okay, it still sounds like the IAC valve, like me cleaning the thing didn't do a thing, but once i replaced it, with the batt diconnected, It ran like it should. In my manual (chiltons) it does mention that just cleaning may not work and that the IAC is just a R & R part.

 

Yhea, in most cases it's just a part you replace with new.

But, like anyone who likes to save a buck, I think a good cleaning is worth the hour or so you spend.

I would still suspect the IAC first, and if confirmed good, I'd go from there.

 

But if the solinoid is bad, all the cleaning you do wont do a thing to help it, I just sucked it up and spent the money, for me it was 58 bucks

 

This is a table from fordfuelinjection.com it explains which sensors are used when in the control strategy. Based on when you are having the problem I would say it is during warm idle, so the sensors involved are the RPM / TPS / ECT / ACT / HEGO (could still be in warm up though). If you want to check these out the same website has good info in the "sensors" section or you can try disconnecting a sensor and running your truck. By disconnecting it it will go into failure mode. So if a senosr is faulty if will probably run better with it disconnected.
<TABLE style="FONT-SIZE: 80%; TEXT-ALIGN: justify" width=744 align=center bgColor=#e8e8e8 border=1 table><TBODY><TR><TD width=135>Strategy
</TD><TD width=150>Fuel Control
</TD><TD width=150>Spark Control
</TD><TD width=125>Emissions
</TD><TD width=150>Sensors in Use
</TD></TR><TR><TD width=135></TD><TD width=150></TD><TD width=150></TD><TD width=125>None
</TD><TD width=150></TD></TR><TR><TD width=135></TD><TD width=150></TD><TD width=150></TD><TD width=125></TD><TD width=150></TD></TR><TR><TD width=135></TD><TD width=150></TD><TD width=150></TD><TD width=125></TD><TD width=150></TD></TR><TR><TD width=135></TD><TD width=150></TD><TD width=150></TD><TD width=125></TD><TD width=150></TD></TR><TR><TD width=135>Warm Cruise
</TD><TD width=150>Closed loop: 14.7:1
</TD><TD width=150>RPM vs Load Multiplier
ECT Multiplier
EGR Multiplier
</TD><TD width=125>EGR
Canister Purge
Air to CAT
</TD><TD width=150>RPM / MAF or MAP / TPS / ECT / ACT / EGR / HEGO
</TD></TR><TR><TD width=135></TD><TD width=150></TD><TD width=150></TD><TD width=125></TD><TD width=150></TD></TR><TR><TD width=135></TD><TD width=150></TD><TD width=150></TD><TD width=125>None
</TD><TD width=150></TD></TR><TR><TD width=135></TD><TD width=150></TD><TD width=150></TD><TD width=125></TD><TD width=150></TD></TR></TBODY></TABLE>

 

I had the same problem and it went away with the installation of a new IAC

 

I did that this morning, cleaned it out with carb cleaner and let it dry out for like 2 hours, it didn't help. Thats what led me to believe it was something else.

But if it is faulty, it would run the exact same if i disconnected the IAC? I remember reading something to that effect, but totally spaced it out and didn't check while I was looking at those sensors. (which are fine by the way, thank you for the diagram / link striker)

Worse comes to worst I'll spend the last of my birthday money on a new IAC.

 

rarely cleaning the iac helps.

buy a quality replacement.