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EFI 302 Wont idle down in park/nuetral

J

jonward786

Member
Im posting this on behalf of my dad, and while its not mustang related, its Ford related. hes been having an issue recently with his 1990 f-150 idling very high in park and drive. he says when he shifts into reverse or drive it idles down as it should but idles VERY high in park/neutral.

were it a carbeurated engine i could solve the problem in 30 seconds, but i dont know the first thing about FI. im going home this weekend to visit and wanted to try to get it handled for him so he doesnt have to take it to the mechanic.

and advice?
 
This is a sorta wild guess, but I'm going to guess a sticky idle air control motor. Should be bolted to the top of the throttle body, if memory serves.

Take it off and scrub it with carb cleaner and an old toothbrush, clean the gasket surface, RTV it, and reinstall. A gunked up IAC is a fairly famous cause of idle problems in older Fords.
 
Here's something that will blow your mind. I've had it happen to me and I was floored. I'm not saying that's what it is, I'm just saying don't be surprised if it's something you would never expect. An alternator. Seriously. I had a car that this happened to. Something about instead of losing a charge it was overcharging. When in gear, the rpm's were pulled down. Out of a drive gear, it would race way up. When I got it to the shop I could not believe what they told me.
 
Has anything been done to the motor? heads, cam, intake?? Those things can do lots to the stock programming. The most common is the surging idle.

A vacuum leak can wreak havok with the idle. A major one will make it run like crap...don't ask.

Check the idle air bypass. Unplug it and see if there are any changes. It might be stuck open. you can disassemble it and clean it if there's any carbon build up. Where's the timing at? unplug the Spout -- that locks the timing at what should be 10° and not allow the computer to swing it. How far open it the throttle blade at idle. It should be 1 to 1 1/2 turns from fully closed. TPS voltage?

Yes, I can believe that a bad alternator will cause problems. At different output voltages, there different constants that get multiplied in the computer. But at around 10-10.5 volts the computer shuts everything down. Again, don't ask how I know this....

Do you have a code reader? it might be throwing a code, but not the MIL light.

Just a couple of things to check off the top of my head.
 
"Sluggo" said:
Vacuum leak?

Rick, wasn't you truck doing that?
Click to expand...

Yeah, mine was, before I sold it. I know I replaced the throttle position sensor, but I cant recall if it was for that or the check engine code it was throwing out.
 
im still trying to figure out whats going on with this truck.

with the engine running and idling about 1500 when warm, i pulled the wire off the IAC and the truck died. does that mean anything?

ive already pulled the IAC off and sprayed it really good with carb cleaner and reassembled. guess ill pull the TPS off now and try to clean it maybe.
 
Try spraying carb cleaner around the intake manifold. If you can hear the idle change you will know if it is leaking.
 
i sprayed carb cleaner around intake and all vacuum lines, no change. the idle is very sportatic now it seems, going up and down by itself
 
The TPS needs adjusted to .99 Volts on a 5.0 mustang... if it is not adjusted properly it will do what you are describing. I am sure a F150 uses the same principal. Look up TPS adjustment, it will show how to do it.
 
"jonward786" said:
i sprayed carb cleaner around intake and all vacuum lines, no change. the idle is very sportatic now it seems, going up and down by itself
Click to expand...

If the engine is running too rich, so that the computer can't compensate, the result is a surging idle.

For the TPS adjustment...it's the green wire that you want to test. A common trick is to use a thumb tack to pierce the wire, then touch the volt meter to the head of the tack.

Have you cleaned the MAF meter? It's common especially when using a oiled filter like a K&N to get the wire oily and throwoff the airflow.
 
"gotstang" said:
This is a sorta wild guess, but I'm going to guess a sticky idle air control motor. Should be bolted to the top of the throttle body, if memory serves.
Click to expand...

+1
 
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