Newbe here, Thanks for welcoming me to this forum. Looking forward to using and learning about my truck(1986 f150 5.0 Efi “N”) from this site.
Problem: very high idle (over 2000rpm) when first started warm or cold. Newer iac (dies when unplugged) newer tps (.86vdc idle 4.5vdc wot). CTS check,s out cold. No apparent vac. leaks. Timing 10 btc. No dtc,s. Preformed base idle setting (iac unplugged, set screw to where it will barely idle. Disconnected bat. 10 min. Same fast idle warm or cold. This was dad,s truck had a long time. Backup truck. Can’t remember if it ever idled right. Retired now just decided to tackle this idle issue. Any help appreciated. Stumped? Thanks for listening.

 

Goose, welcome to FTE.
I am not up on the FI trucks like others on here but you are on the right track as you posted what the truck has for a motor and year and the other things you did.

Being you unplug the IAC and the idle drops would tell me something is telling it to stay open I would think but what?
I did not see where you pulled codes, did you and if so what did you get?
The other thing is the EGR, don't know if that would tell the IAC to open or not but might be something to look at also.
Dave ----

 

Thanks Dave. I did list no dtc,s ( diagnostic codes). That’s a good idea on the egr. I’ll put a vacuum gauge inline to egr and see what’s happening. I do know the egr is getting vac. to it, maybe wrong times. Thanks again

 

My 1986 F150 5.0L EFI has idled high when started, hot or cold, since new. Was told at Ford dealer that the idle was normal. 34 years later, this month, it still does the high idle. Never liked it idling so high but made peace with it decades ago.

 

What code did you get? You should get at least one.

 

11 dtc l, believe it is a normal or pass

 

How are you checking and setting the timing? Another test question.

Just trying to see if you missed anything,

 

I had a 86 F-150 that was idling high when I bought it, my friend at the Merc- Lincoln dealership told me that it was the gasket under the plenum. That straightened it out. Also a good time to replace the valve cover gaskets.

 

Timing with light and pulled spout.
Yes I’ve already have the gaskets to do the job and check everything else under that big hunk of aluminum. It’s weird it idles good accept on over night cold starts and restarts throughout the day. Overnight cold starts it really races(bearings knock until op,s come up), restarts not as much. Used a ton of carb cleaner and start fluid searching for vac. leaks. Thanks guys for the replies!!!

 

You are passing all the test questions. I get a hint from the above quote that it only high idles during a cold start and for a very short period on a hot restart? I think this was mentioned previously, this is normal. Now if it idles high all the time, coming up to a stop light and other times like that, then that is not normal.

If you want to replace the manifold gaskets go ahead, it can't hurt. But I wonder since it dies when you unplug the IAC, if there is really a large air leak anywhere. If there was, it would use that air and keep idling. But possibly it's a small leak and it will drop your rpms a couple hundred if it is leaking a little bit.

 

hey Franklin, thanks for your reply. Yes you are correct on the high idles. Let me change the word IDLES high during cold starts to REALLY RACING high during cold starts. I bet that suckers above 2000 rpm,s. Could live with the hot restarts idle rpm,s I wondering if a small vacuum leak is magnified when cold. Im not so sure I don’t have a very small vac leak changing the rpm,s 150-200 rpm. I’ll know after going through everything.PS what’s with passing all the test ? Thanks again.