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Discussion Starter #1
Hey everyone,

I am out of town and limited internet access. Anyways, Bronco was running fine , I drive about 150 miles to my buddy's and my idle is about 1300 all of the time, something has definitely changed. Again this is a 91 with EFI so no carbed stuff.

I was able to get some internet and found Seattles kick ass thread about testing the IAC , I tested it and it is within specs. Mine is about 11 OHMS and I think the range is 7-13 OHMS passes. So the solenoid seems to be checking out.

Can these things fail and still test within specs ? I cleaned the TB and removed the valve half of the IAC and no change. I did not remove the full assembly just the solenoid portion.

any ideas on where to point me ? its idling so fast it kinda slams into gear when shifting so that's obviously concerning.

Edit , no codes for anything either

Thanks gents.
 

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First of all epic quote from the president, Second it is not unlikely that the iac would hang up from time to time giving you high idel. some people say you can soak it to get rid of the junk making it hang up, some people say otherwise on behalf of the electronics.
 

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I'd go ahead and change out the IAC. Its 20+ years old so its worth changing. Also look into your MAP sensor. If its sending bad readings, it could be messing with your air/fuel mix.

Lastly, if you haven't already, open up your PCM and see if you have any bad circuitry. If any of the capacitors leaked onto the board, which is a very common problem these days, that could also be messing with your idle control.
 

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Check vacuum lines, as well. I was screwing with my cruise servo once and the idle was around 1500-2000 rpms bc I had a line hooked up wrong. Look for brittle broken plastic ones and replace with rubber if they're bad.
 

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Discussion Starter #5
Thanks for the replies fellas.

Article - I have cleaned out the IAC and soaked everything and cleaned it up as good as I can.

AF - I swapped out for a spare MAP that I keep in the glovebox , no change.

Ted - been checking over the vacuum lines and cant find a leak , I upgraded all the stock ones to rubber a few years ago and they all look good.

Could this be a TPS issue ? I check that and the reading at idle is 1.01 , with a smooth increase as I open the butterfly.

also when idling , If I unplug the IAC there is zero change . IDK what the hell is going on with it
 

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Discussion Starter #6
making some progress, I used a cigarette to mimic a smoke machine and I have a hige vacuum leak somewhere right in the back of the engine.

The check valve and large hose back on the crossover tube look good. but its coming from somewhere in that general area. IDK what the hell is going on , its like the intake has a hole in it or something ? ?? ? ?

any other items back there that can fail ? and make a massive vacuum leak
 

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First of all, is that leak sucking in or blowing out. I don't think you'd be able to use smoke to find a vacuum leak, since a vacuum leak would pull air IN, not blow it out. If you're seeing smoke get blown around back there, its probably your EGR tube leading down to your muffler. I have the exact same setup with engine and transmission.

This picture shows the rubber EGR lines, but that bottom line goes down to a steel flex tube that leads to the muffler and those tend to get rust rot pretty bad.


Here is how mine looked before I replaced my exhaust pipes. I ended up replaced the steel flex tube with 3/4 inch rubber hose.

It was a real pain to fix, but it did help in the end.

With all that being said, I don't know how that would affect your idle speed.

You have checked your timing right? What is it currently set at?

Also, if its slamming into gear, I'd seriously look at the PCM as I mentioned. I had to do a PCM swap and the guys I bought my new one from sent me the wrong PCM at first and it was slamming into gear because of that and it would stall out the motor. The PCM also controls your E4OD transmission. The "E" stands for Electronic control and it definitely sounds like your PCM is working in "failure mode" which was built into the PCM to allow you to get home NO MATTER WHAT, which is something I really like about these older PCMs. Its probably the capacitors leaking onto the board and failing. You should be able to fix that though with a soldering iron and a quick trip to a radio shack or similar electronics store, so long as they sell replacement capacitors.
 

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Discussion Starter #10
Hey fellas, I thought I would update this as I have definately found the problem and it seems like one of those problems that shouldnt happen. I would have bet the farm it had soemthing to do with the crossover tube but I was wrong.

The damn gasket between the upper and lower manifold must have rotted away and probably at least a linear inch is completely missing of the gasket = huge vacuum leak . Still scratching my head as to how that can happen. I could literally hear the hole whistling when driving at different RPMs

Its hard to tell but it feels like the entire gasket section between those 2 rear bolts of the upper is gone (Seattles first pic shows the area pretty well) . Its right in the very back about the same height as the fuel rail.

my IAC and everything was fine , somehow a piece of that manifold gasket decided that it wanted to break off (That couldnt have been good for one of my pistons) . I have a temporary band aid fix on it now with some quiksteel to plug the hole up.

Anyways, I am working on getting everything lined up to fix that. I think I am going to do the new explorer style injectors and a new motorcraft TPS while I have the upper off. as far as I know the injectors have never been serviced.

AF - This was a vacuum leak , it wasnt blowing . You do the smoke trick while the engine is off , basically plow a bunch of cigarette smoke through a vacuum line and you should be able to see smoke leaking out where the leak is. Worked like a charm in finding this leak, I just needed a small mirror to confirm exactly where the smoke was piling out.

thanks again everyone
 

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Discussion Starter #12

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I've been having issue of extremely high idle after replacing fuel injectors in 1991 Bronco 5.8. Before replacing engine ran rough and slow takeoff now revs high and wants to jump off line. In park after startup it is over 3000RPM and while in gear around 2000.*I replaced manifold gaskets, IAC, MAP, and TPS, but still not fixed. I have let it run for several minutes, disconnected battery, and driven down the road a couple times, but it gets hot quick. Any other ideas or adjustments need to be made?
 

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Should start a new thread, but whadya mean it gets hot quick? Coolant full? Gotta fix that first.
 
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