High Idle and high IAC voltage issue
 

I've been able with the help of this site and a subscription to AlldataDIY to solve every problem I've ever had with our 1995 4.0L XLT Extended Aerostar, except for now. We bought it new in 1995 and have had it garage kept since new.

Recently she started idling too low under high loads (head lights, ac, front and rear fans on high) to the point of having the alternator not able to maintain working voltage. Replaced alternator, no improvement. Cleaned IAC, then replaced with new IAC, with not much improvement. Replaced the TPS and things got better but then the high idle issue cropped up.

Searched for vacuum leaks and so far have found none, using sound tracing stethoscope and spraying carb cleaner around intake manifold. Tonight I connected a DC voltmeter to the IAC and got these results: (yes I understand that the IAC control signal is a variable frequency and duty cycle square wave. The DC voltmeter reading is a good approximation of the amount of "on" time of the signal.)
1. KOEO showed approximately 10.4V lead to lead at the IAC.
2. KOER engine idled high for a few seconds then settled to normal idle speed. IAC voltage started around 6v and settled to just under 3v along with engine speed.
3. Engaged A/C clutch and IAC voltage increased to around 6v then settled to around 5v, engine speed followed, increased slightly over normal idle. As clutch cycled IAC voltage also cycled.
4. Placed into reverse and IAC voltage jumped to around 5.5v and engine speed increased slightly over idle.
5. Back to park.
6. Pressed accelerator and IAC voltage jumped to around 6v while engine revved to around 2,500rpm. Released accelerator and engine speed reduced but stayed high ((~1,500-2,000 rpm), IAC voltage stayed at around 5-5.5v.
7. Tapped accelerator quickly, IAC voltage jumped and engine revved, but neither voltage or engine speed reduced.
7. Shut engine off and restarted after just a few seconds, idle went high for a few seconds and then settled to around normal (700-800?). IAC voltage around 2.8-3v.

So in short, just starting the engine with no loads it will idle normally. Tapping the accelerator puts it into a high idle with corresponding high IAC voltage. To say, the idle speed appears to track directly to the IAC voltage. My assumption is that if it were a vacuum leak the IAC voltage would go low trying to overcome the leak that was making it run fast.

While driving it operates normally. Coming to a stop sometimes it will idle normally, sometimes it idles high.

Ideas?

You can test the IAC by pulling its connector when the engine is in one of its high idle mode. If the idle drops, then the IAC is working, and the EEC is telling it to open up for some reason.

Those symptoms sound like a bad TPS, but you said you replaced that already. I once had a similar problem, and it was from bad connections on the TPS plug/socket.

Look for binding restriction in the throttle or the TPS.

What brand of new TPS?

Here is my take on it. Unfortunately the older EEC-IV does not have the live data which could prove helpful in finding this issue.

1. Most likely culprit, vacuum leak. The method you employed of spraying carb clean I have found to be ineffective. Its always worth a try, but i have never been able to find a vacuum leak with that method. Listening with a stethoscope is not likely to produce good results either. If you had an OBD-II system you could look for lean codes or fuel trims that are high, those are good indicators. The best method I have found is with a smoke machine, but those are not exactly commonplace. Even a lot of shops don't have one.

2. Most aftermarket IACs that I have used or dealt with over the years are about equivalent to cleaning an old worn out OEM one. They are absolute garbage and I wouldn't waste my money on them. There are a few brands that work ok. If possible your best option is to buy Motorcraft. I have bought Standard brand, and they always had the Motorcraft numbers ground off and the Standard number, because many of the Standard brand parts from from the OEM factories.

Electrically it sounds like its doing what it should, but your tools are not exactly accurate. If you were using a scope you would get more meaningful results.

Common areas for vacuum leaks? Around the fuel injectors, the upper or lower intake, aftermarket IACs (yes, they can leak and cause unmetered air to get it, check that rubber or plastic cap), breather line, the gas vapor line, and if you have EGR, I have seen leaks at the o-ring where the EGR tube meets the intake.

Xlt4wd90 - I saw one of your other posts about the anti-decel function of the IAC and that would explain why I saw IAC voltage rise on acceleration, thanks.

Actually, the idle speed is directly related to the IAC voltage I measured. What has me puzzled is why the IAC voltage wants to increase from 3v (starting normal idle speed) to 5v after a slight tap on the accelerator, and it stays at 5v. I'd say that the engine is idling at a speed consistent with the voltage provided to the IAC.

KhanTyranitar - my fear is that it IS vacuum related. (And yes, scope would be better but have to use what's available ;-) )

I tested the TPS tonight and the results are:
VREF = 5.1v at all times when running
TP = 1.6v at idle (normal or elevated idle) and 4.3 at WOT. Bumping throttle to cause problem and TP returns to 1.6v. At any idle speed TP stays at 1.6v

The head-scratcher here is that the IAC control signal and the idling speed are directly related as they should be - the signal goes down and the idle goes down, it goes up and the idle goes up. SO, why is it going up to 5v and staying there after a brief tap on the throttle? If it were a vacuum leak in the manifold somewhere why does the problem go away immediately after stopping and restarting the engine? And reappear after a short tap on the accelerator?

Actually that does sound like a bad or incorrectly adjusted TPS. A short blip causes it to increase from idle to low throttle, and the computer is opening the IAC to try to prevent a stall (which is not necessary, but its trying to be prepared). You can try adjusting the TPS if you can. Ford specs on the TPS is ±1%, but most aftermarket parts consider ±3-5% to be acceptable. Could also be the throttle body sticking just a little bit and preventing the TPS from returning to the idle position.

Did the 95 have the plastic intake? egr oring on pipe...I had to use two of them.

So what happened ??

lol, i'd say she was...

Where is the IAC valve located? This will be my first time messing with an IAC valve. Dealing with a similar problem right now. Most of the time the car runs normally, but at least once a day when I go into reverse or drive the van will die if I dont accelerate as soon as I go into gear,also the indactor of when this is going to happen is that the voltage meter flucuates a bit when I start the van up. I have a 90 4.0 V6 4wd btw

Ant time I have a too low idle problem such as stalling, the first thing I do is put some carb cleaner on a shop rag and clean the throttle body and butterfly. 99% of the time that clears up the problem.
One of mine has over 250,000 and the other has over 300,000 miles. All fuel system components are original. Even the fuel filters.
I use only Top Tier gas and put a bottle of injector cleaner in the gas every three months or so.

The general function of the idle air control valve is, as its name implies, to maintain idle speed. It opens and closes rapidly all the time, and its duty cycle determines how much air is bypassed around the throttle plate to maintain idle speed. The closed position of the throttle plate is usually set so that it just opens so it does not jam up in the throttle bore, and to provide a minimal idle speed.

What you're describing is the anti-decel function of the IAC. Carbureted engines used to have an anti-stall dashpot for that. Anti-decel is used in all modern gasoline engines to help control emissions; specifically, to deal with the wall-wetting effect in the wet intake areas. When you open the throttle quickly, the sudden increase in air pressure causes any suspended fuel to condense on the walls of the air intake, causing a sudden lean condition. That's compensated by acceleration enrichment. But when you quickly close the throttle, the sudden decrease in air pressure causes the condensed fuel on the walls to vaporize, causing a sudden rich condition. The problem used to be worse in carbureted engines, with much bigger wet areas. But it still happens in port injected engines, especially in bank-fired types, like many Aerostars. Slowing down the decrease in intake pressure slows down the evaporation of the condensed fuel, making the condition easier to handle.

What kind of o-rings do you have that could withstand the best of your exhaust and be inside your EGR?