God I hate being an idiot - so to unplug the radio circuit do I just disconnect radio head from wiring harness? Or unplug the fuse for radio (I'm sure I'll be able to find it somewhere) and see if it still has power?

Also, how would read just the manifold vacuum?

I did the cap all the vacuum ports and ran only vacuum lines to the FPR and MAP, unplugged the IAC and on cold start up it was running rough for about 30 sec then jumped up, not super high, around 1000 or 1100 or so. Plugged vacuum lines in one by one and didn't change anything. Plugged IAC back in and heard a suction sound and idle kind of went up a little, but not much at all.

Reset the test, although engine was a little warm now. And it was still idling high and turning on A/C would drop it a little.

I sprayed some cleaner around upper manifold gasket and it idle went up a little. This was definite. If it's a vacuum leak, wouldn't the idle go down though? I put new o-rings on the injectors... And the more I think about it, I'm not sure a new upper gasket came with the head gasket kit...

Thanks for all the help guys, more than I could've expected. Unfortunately I'm out of town for the next few days for work so wont get to it more until next week.

 

You are not an idiot, just on a steep learning curve.

I a$$-u-me that the radio chassis is not OEM FORD and is an aftermarket. The warning display is telling you the power source is not correct. If the power is supplied off a circuit tied into the EEC, it may be affecting the engine control circuitry.

Simply unplug the power supply to the radio chassis for now for engine diagnosis and worry about it later.

 

There you go, a vacuum leak (unless massive) will increase idle speed.

To check manifold vacuum, attach to direct manifold vacuum source.

 

Your tests show the vacuum leak is on the engine.

A vacuum leak causes a high idle RPM since unmetered air is making it into the engine.

 

Well, got back after being out of town and blocked off all the vacuum lines except MAF and fuel pressure, and unplugged IAC. Then I sprayed some fluid and found that when I went around the plenum gasket it dropped the idle! Thought I had it there. Put a new gasket on - nothing. So went and got another gasket and put some RTV on both sides just in case. Let it set for a day. Still had high idle. Defeated, i let it sit for a few days. Then went back to blocking all vacuum lines, leaving only MAF and fuel, unplugged IAC and started spraying again, seems like idle would drop around #6 injector and intake bolt. So today, I got in there planning on taking intake out and replacing gaskets and take it to machine shop to make sure it wasn't warped or cracked. Well, when I got the plenum off I noticed a strong gas smell and it looked quite wet in there.

Is there something from fuel controls that could be causing high idle only in park and neutral? I will say that I feel like I might feel a miss when stopped in drive (remember in gear the idle is fine). Any help on this would be appreciated. Really don't want to take the lower intake off and replace the gasket, as this was just done around 4k miles ago...

 

Interesting, I have a problem with a high idle with my '91 F250 5.8W. Mine starts at around 900 rpm when cold but after driving for 10-20 minutes idle goes up to 1400. Giving the throttle a quick stab or switching off motor then restarting brings idle back to 1000-1100. Running on LPG so not a injector issue. Am thinking IAC or ecu issue maybe.

 

gmw67, if you are asking a question, I recommend you start your own new thread.

 

The PO had tried to fix the high idle. The two spots on the butterfly valves are holes that have been J/B welded or something else. I know because I did the same thing and it improved my idle some. I am not sure about your motor but why is only one (freeze plug) looking thing in the throttle body? I would think there would/should be one on the other side but VJ can probably tell you. Sandy

 

You have already proved that engine has an unmetered air source(s) in Post# 10.

The result is the computer thinks you have a lean condition so the fuel trims are set to dump in more fuel. That is probably why there is a lot of gas in the intake.

 

Well, decided to just crack in there. Soon as I got a good view of the intake I noticed the gasket bubbled out from the head on each side. Took lower intake off and saw that it was split at one spot on each side! Glad I found it. But, why would this happen? Around 4k miles and lower intake gasket splits? Right at the same spot on each side, above middle of passageway. Let me know what anyone thinks, I hate to put it together just to have it happen again... thanks in advance.