Hanging idle

JonS

SCCoA Member
What would happen if I just blocked off my IAC with a solid gasket? She idles better with the IAC plug off than in.
 
Idles better...?

It should barely idle in that case, I think. What year/mods?

Garage link in my signature.

My question is more general tho. What does the car do without the IAC? or, what does the IAC add? I think the TB passes enough air to maintain idle. Idle at 750 unhooked, 900 hook up, and then the is that hanging idle from throttle for 7 to 9 seconds.
 
I see - '95 w/aftermarket TB. Can't help diagnose that combo for iAC, sorry.

In general, tho, the IAC is under computer control, adding air when starting (usually rich otherwise, depending on), catching the idle when going off throttle quickly, stepping it down gracefully, and helping to increase idle under load, such as when A/C is on, steering load w/low pump output, etc.

But the idle should hang for a few seconds, at times. Jump on start, then settle down, as an example.
 
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It's a massively common issue. I've got it to where it is very drivable. Read every thread here and in Mustang forums. It's not vacuum, TPS is perfect, throttle plate exact, etc. Shifts great now at least, but that hang is about 4 second too long at idle. It may be I just don't drive her enough for ECU to learn. SPRING WILL FIX THAT.
 
It's a massively common issue. I've got it to where it is very drivable. Read every thread here and in Mustang forums. It's not vacuum, TPS is perfect, throttle plate exact, etc. Shifts great now at least, but that hang is about 4 second too long at idle. It may be I just don't drive her enough for ECU to learn. SPRING WILL FIX THAT.

You've applied a band aide for a bigger problem. Granted, that might be easier than actually fixing the issue, but it's a band aide nonetheless. And just FYI, the EEC will never learn idle without the IAC functional, so don't hold your breath on that.
 
You've applied a band aide for a bigger problem. Granted, that might be easier than actually fixing the issue, but it's a band aide nonetheless. And just FYI, the EEC will never learn idle without the IAC functional, so don't hold your breath on that.

EEC, right! Band aide? It's a new IAC and still connected to harness. No intention of running disconnected, just asking what would happen for that reason. It you're implying tune ....
 
EEC, right! Band aide? It's a new IAC and still connected to harness. No intention of running disconnected, just asking what would happen for that reason. It you're implying tune ....
I'm not implying anything. If your idle is hanging then you have excessive air entering the system. Fix that problem and the rest of it will also correct at the same time. You have a mechanical problem which is why I said band aide. Disconnecting the IAC does not solve the original problem and the EEC won't learn around a disconnected IAC. That's all I said and it's exactly what I meant.
 
I'm not implying anything. If your idle is hanging then you have excessive air entering the system. Fix that problem and the rest of it will also correct at the same time. You have a mechanical problem which is why I said band aide. Disconnecting the IAC does not solve the original problem and the EEC won't learn around a disconnected IAC. That's all I said and it's exactly what I meant.

Gotcha. It's the throttle body I'm sure that is the'problem'. Not sure if it's around the blade or what. If a diconnected IAC has an idle of 750, and connected is 900 and also hangs, it is trying to do something that the TB is messing up. And a closed IAC doesn't pass air right? The OEM TB idles fine so the location of 'problem' isn't a mystery, but the mechanics of it eludes me. I'm just trying to understand the basic function of the IAC.

I really do appreciate the feedback and your experience, that's why I ask
 
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The aftermarket throttle body passes too much air around it's blade when closed. The blade can't close all the way because if it did then it would stick, so it's cocked slightly open. All TB's are like this. But the OE throttle body was a very precise fit between the blade and the bore of the TB and the EEC knows exactly how much air this is. The IAC values and this preset amount of "leakage" are programmed in. The IAC (when it's working) has the ability to learn a certain amount. Ford calibrated this in so that if the stock TB wears, the vehicle will still idle properly. It takes a few cycles of hot idle for it to optimize these settings. But it can only learn so much and most likely it will never learn enough to compensate for a TB with poor tolerances. You can try to set the blade lower but ultimately you'll be limited when the blade starts to stick after a quick throttle snap. Leakage through the shaft bearings aggravates this situation.

The reason the idle hangs, then drops suddenly to the correct speed or close to it has nothing to do with the IAC. The IAC only reacts slowly. The quick drop in idle speed is due to "option B" on the part of the EEC. If idle speed is not reached within a short time using the preset IAC values for idle, the EEC will use ignition timing to pull idle into line. Spark control is instantaneous hence the quick drop in idle speed. The problem here is that if a large drop in idle speed is being observed due to spark control, that means most likely that parameter is getting maxed out too. If spark is allowed to pull idle speed down by a large factor then your idle spark is going to be very retarded which means excessive heat into the exhaust (if you have headers, they can actually start glowing red at idle) which is very bad for catalytic converters and exhaust valves. So even if spark control is able to band aide the problem, it really should be fixed properly.

A tune can help because a tune can tell the EEC to command less air than the adaptive strategy will allow, but it may still not be enough, and in any case it's always better to fix the problem than to band aide it. Stock TB's are best, BBK TB's are usually decent but not always, and MP throttle bodies are generally good, but Pro Prod throttle bodies are pretty much always poor in this department.
 
Excellent explanation

The aftermarket throttle body passes too much air around it's blade when closed. ... Stock TB's are best, BBK TB's are usually decent but not always, and MP throttle bodies are generally good, but Pro Prod throttle bodies are pretty much always poor in this department.

Mint this response because it is pure gold. Thanks, Dave.
 
Idle problem solved

I haven't found too many "high idle" threads that end in solved, so I want to close this one out.

First, XR7 Dave was exactly right. He said too much air, and I've been chasing vacuum on this car forever. I smoke tested it. I made a propane sniffer. I tore things down and built them back up assuming I had caused the problem.

But the idle only got worse with time and eventually it affected drive-ability. And finally I started getting CELs and pulling codes that made little sense to me given I was sure it was vacuum.

So I took her in and asked the mechanic to diagnose it for me. (It's a hobby car so I resisted.)

Final answer: It was the IC return plenum to intake manifold gasket. I never touched that and didn't look very hard there.

But why the CEL and codes? The MAF sensor failed (see WTB thread recently). Fixed those two things and she idles and drives like new.

I don't believe in coincidences, but the two failures were just that, but if the MAF hadn't failed I might not have found the leak.

So all those threads on PP TBs being terrible, idle problems, etc. that never reported being solved ... in my case it was a vacuum leak totally unrelated to any mods or maintenance issues I was involved in.
 
Every aftermarket TB I played with leaks massive amounts of air past the bearings on the throttle shaft. Have to modify them to add shaft seals to get the leakage to a point the IAC can do it’s job.
 
Every aftermarket TB I played with leaks massive amounts of air past the bearings on the throttle shaft. Have to modify them to add shaft seals to get the leakage to a point the IAC can do it’s job.

In my case I have a BBK/Edelbrock 75mm, so I wouldn't know
 
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