Sudden Intermittent High Idle

[DPDISXR4Ti]

I don't have much confidence in the old, used IACs. But if it is working for you then I guess it is fine.

You were right not to have much confidence in a 34 year-old part. I haven't had any high-idle symptoms - in fact, I was noticing that in general the car was idling at a lower RPM. All good, until my wife comes back and tells me that the engine stalled twice. Hmmmmm.... So I drive it yesterday and it stalls at the first stop sign. I'm thinking now that the IAC may have been seized up and not working at all. I figure a high idle is better than a low idle, so back in goes the IAC that was in there before and the process continues.

This does tell me what we've likely already known - that the IAC is being commanded to open and raise the idle. Why that command is being given continues to be the question.

[Ed Lijewski]

I wonder if the Throttle Air Bypass Valve Solenoid diode, or the condition of its contacts, could be involved in the sudden high idle syndrome.
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Screenshot_20240219-171543_Gmail.jpg (800.45 KiB) Viewed 985 times

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.YMMV

[For Dean: this screenshot is exactly 800 KiB.]

[brokencase]

You were right not to have much confidence in a 34 year-old part. I haven't had any high-idle symptoms - in fact, I was noticing that in general the car was idling at a lower RPM. All good, until my wife comes back and tells me that the engine stalled twice. Hmmmmm.... So I drive it yesterday and it stalls at the first stop sign. I'm thinking now that the IAC may have been seized up and not working at all. I figure a high idle is better than a low idle, so back in goes the IAC that was in there before and the process continues.

This does tell me what we've likely already known - that the IAC is being commanded to open and raise the idle. Why that command is being given continues to be the question.

I can't remember if the idle goes up or down when the IAC opens. But there are other things that can kill idle, like a stuck open EGR valve.
It mimics a vacuum leak at idle.

Since EGR only opens when the engine is running at higher rpms, you can check if it is stuck open by feeling the pipe going from the valve to the intake. If it gets hot while the engine is idling then the valve is stuck open. Replace with a quality item. The cheapo ones don't have stainless internal parts.

I would also look for vacuum leaks in general.

[DPDISXR4Ti]

I can't remember if the idle goes up or down when the IAC opens. But there are other things that can kill idle, like a stuck open EGR valve.
It mimics a vacuum leak at idle.

Idle RPM goes up when the IAC opens. Since putting the previous IAC back in the stalling issue has stopped. I'm pretty sure the "test IAC" was completely non-functional.

[Bob Weir]

per Ed's pik
Remove upper dash
Hit Throttle Bypass Valve Solenoid Diode with bf hammer or purse.
Drive with upper dash removed.
Intermittently hit TBVSD with whatever item is handy

[brokencase]

Idle RPM goes up when the IAC opens. Since putting the previous IAC back in the stalling issue has stopped. I'm pretty sure the "test IAC" was completely non-functional.

IACs are pretty simple and I could never figure out why my old one went bad.

If you are correct (and I believe you are) that idle goes up when the valve opens then if the failure is the valve seat leaking then you would expect the idle to rise.

But this is not what happens. There must be some other interaction with the EECIV. It probably detects that it cannot control idle so it stops cycling the IAC and falls back to the throttle stop position on the TB.

If the idle screw on the TB was set on a nice warm day then you would expect stalling in the winter time.

[Ed Lijewski]

If you are correct (and I believe you are) that idle goes up when the valve opens then if the failure is the valve seat leaking then you would expect the idle to rise.

But this is not what happens. There must be some other interaction with the EECIV. It probably detects that it cannot control idle so it stops cycling the IAC and falls back to the throttle stop position on the TB.

I've experienced the SSHIS on my '88 3X. Coming to a slow down and stop the idle would rise close to 2000 rpm. Crud on an IAC's pintle seat alone couldn't result in that. I would shut the engine off and restart it in not more than a few seconds and in each SSHIS event the engine ran normally afterwards.

Subsequent SSHIS events occurred randomly, never consecutively nor close together even in hours.

Clearly the EECIV was in control during those events.

To my knowledge we who experienced SSHIS are three: me, Brad (or wife) and Saul Rivkin with his red '88. Saul reported his experience in a FB MCA post; I didn't see him post again on having identified the cause.

I haven't experienced this issue on my '89.

YMMV

[DPDISXR4Ti]

Clearly the EECIV was in control during those events.

Yes. It is the strategy - based on some input - that is commanding the IAC to open and send the idle high. My direct observation is that the strategy changed by the early 90's and addressed this common Ford problem. I say this because I've run early 90's PCMs on my Lima-4 engines for years now and never again experienced the high idle problem. It will be interesting to see what happens once I get around to running the Ranger 4L PCM on the Scorpio - hopefully this Spring.

[Ed Lijewski]

I say this because I've run early 90's PCMs on my Lima-4 engines for years now and never again experienced the high idle problem.

?I don't recall ever reading/hearing that you experienced the high idle issue on those engines.?

YMMV

[DPDISXR4Ti]

I say this because I've run early 90's PCMs on my Lima-4 engines for years now and never again experienced the high idle problem.

?I don't recall ever reading/hearing that you experienced the high idle issue on those engines.?

Hasn't everyone? I've been running the later 2.3 PCMs for probably 15 years now, so it's nothing something you would have heard from me at all recently.

[Bob Weir]

Do cars with different ignition controllers [EEC, PCM, distributor, etc.] have the sudden high idle condition?

[Ed Lijewski]

Do cars with different ignition controllers [EEC, PCM, distributor, etc.] have the sudden high idle condition?

Yes. There are many posts re this; here's just the first one from a search today:
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.https://www.ford-trucks.com/forums/1348 ... t-tps.html

YMMV

[Ed Lijewski]

I say this because I've run early 90's PCMs on my Lima-4 engines for years now and never again experienced the high idle problem.

?I don't recall ever reading/hearing that you experienced the high idle issue on those engines.?

Hasn't everyone? I've been running the later 2.3 PCMs for probably 15 years now, so it's nothing something you would have heard from me at all recently.

My comment/question was that I hadn't read/heard that with your 2.3 Limas you apparently experienced the high idle issue. So in fact with your 2.3 Limas you did experience the sudden high idle? Once or more? Possibly I missed you posting that. Or maybe that occurred but you never posted about it.

YMMV

[DPDISXR4Ti]

My comment/question was that I hadn't read/heard that with your 2.3 Limas you apparently experienced the high idle issue. So in fact with your 2.3 Limas you did experience the sudden high idle? Once or more?

Yes, several times. But going back 15+ years now when I ran a PF3 (or similar).

[brokencase]

I'm still a firm believer that what you call "Sudden Intermittent High Idle" is simply the EECIV going into limp mode.

Next time it happens Brad, and while the engine is idling high, pull over to the side of the road and turn off the engine, wait a moment and the restart. If the idle goes back to normal then it's definitely limp mode.

It's cause is either some intermittent sensor (or connection to the sensor) such that the EECIV thinks the sensor is bad and so goes into limp mode.
OR
There is a power glitch affecting the EECIV causing it to reset.