IAC Issues, Scorp and XR, Maybe the "EEC-IV CHIEF" Knows How To Resolve?

[Ed Lijewski]

Possible strange, unlikely but possible, failure modes : could any such explain the high idling described here? Electrical matters are his wheelhouse.

YMMV

[Mike McCreight]

Non scientific, I know, but after 20+ years of trial and error, I think it's PIP related. Or something in the distributor.
Something something, the distributor throws a glitch, and the EEC goes into cold start/high idle.
I have "solved" high idle issues on 8-10 cars (all XR4Ti) with a distributor swap after all other efforts failed.

Assuming you have a parts car or decent parts stash (gawd, I've tossed a dumpster full of spares this year...) it's a 1/2 hour experiment.

[Ed Lijewski]

My XR current distributor was a new unit (not remanned) installed ~15/20 K ago. sold what was the good original former distributor (know it was good because no high idle occurred with it).

Interestingly, on my Scorpio after he high idle occured a few times I wondered about the distributor being the source of it; IIRC Ken Kizer pointed to possble shaft play as a suspect). For unrelated reasons I later swapped out the 150K 2.9 for one with just over 100K keeping that one's distributor in place. Haven't put a lot of miles on it since (my '88 is mynfull time DD-er cuz its heat and a/c are THE BEST) but in 4-5-6 years since then it hasn't had a sudden high idle episode. Hmm....

YMMV

[Ed Lijewski]

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Screenshot_2018-07-11-09-39-24-1-445x196.png (35.11 KiB) Viewed 9104 times

YMMV

[thesameguy]

Yep, that's typical of vintage American idle valves. Very simple and IMHO not very good. High idle (from the valve) would be created with an inappropriately high duty cycle.

The diode in the circuit is there to protect the ECM. Because the IAC is a electro-mechanical, every time it cycles a small electrical pulse is generated and the diode is there to shield the ECM from that backwards-travelling voltage. Without the diode, the driver for the IAC could be damaged. I doubt there would be any technical information for side-effects of a failed diode because those results would be unpredictable. Most newer valves of this style (like, into the '90s, not actually "new") have integrated diodes, which is why you don't see an external diode hanging around in the harness.

Edit: So, that makes me wonder about something... I see on rockauto that these Merkur IAVs were used on cars into the '90s. I wonder if these replacement valves have integrated diodes for the later cars, and keeping that second external one is creating a problem. I don't know how to really test to see if the IAC has an internal diode... but it would take me one second to eliminate my diode (it's an ATM-format replaceable diode). Maybe worth a shot.

Edit 2: Maybe another approach (for me) is just buying a '90s valve that I know has an internal diode and then eliminating mine. I think the last Fox Mustangs have an electrically-compatible idle valve, and I know they don't have external diodes.

[Ed Lijewski]

I knew.

But....Brokencase works with electronics or that's his avocation, either way both his experience/expertise and of course his contacts with The EEC-IV Chief could advance our discussion. Or put it to rest

YMMV

[andyofcolumbusmerkur]

What always solved high idle, wandering idle for me was to disconnect a battery terminal for 5 seconds then let the car sit overnight. I could then drive all week and it would drive like a new car. As soon as I dogged it a little (holding it at high boost and blowing off boost to atmosphere ) it would start running bad again. I know this isn't what you guys are really talking about here, it's just the simple computer not knowing whats going on. But could something else (like a diode) cause a glitch that results in the same state of confusion?

[Ed Lijewski]

I'd say yes. When a random high (very...) idle occurred with my Scorpio, before I swapped its engine for one with lower miles, as soon as I could safely do it I shut off the engine and restarted it without waiting. Most times the high idle disappeared; a handful of times it reappeared; I'd then shut it off and wait a bit before restarting and move on normally.

YMMV

[Ed Lijewski]

I'd say yes, with my Scorpio, but no with my XR. When a random high (very...) idle occurred with mybScorpio, before I swapped its engine for one,with lower miles, as soon as I could safely do it I shut off the engine and restarted it without waiting. Most times the high idle disappeared; a handful of times it reappeared; I'd then shut it off and wait a bit before restarting and move on normally.

On my XR, after I shut it down even waiting a while doesn't clear the high idle, only unplugging the IAC results in the engine running normally at idle.

YMMV

[thesameguy]

The diode is easy to test - put your multimeter in diode test mode and test it. There doesn't need to be any mystery here.

The flyback diode in the AC circuit is unlikely to cause non-permanent problems. If it's not there, every time the IAC is working little voltage spikes are hammering whatever drives the IAC. Turning off the car and turning it back on or disconnecting the battery won't do anything... next time you start the car and the valve is energized the same thing happens again. The diode is there to protect the ECM, without it the ECM may get damaged long term. The diode is not part of any logic function, not having it wouldn't cause problems sometimes, and not problems that a key cycle could fix. The damage is being done all the time the diode is nonfunctional, and the damage caused is permanent.*

FWIW, I experienced the "high boost messes up the idle" problem - that was a sticky PCV valve. I don't think "conventional PCV valve" is up to the task, Motorcraft or otherwise.

* Edit - Although I have no idea how long mine was broken, and there isn't any outward sign of a problem having been caused, unless my current random idle issue is that problem. Replacing the diode did nothing either way.

[Ed Lijewski]

I run only Motorcraft PCVs (and most other service items).

The diode and wiring to/from it look good. I could test it, but why, since you replaced a clearly degraded one but it changed nothing. Conceptually I agrèe it's not likely at all to be where the solution lies.

One possibility is a less than optimal, intermittent, ground for the IAC. Per the EVTM that ground is on the RH lower cowl near the ECU. As the ECU actuates the IAC by grounding pulses (see above) if that ground is not continuously good the ECU might be constantly trying to set the IAC valve opening to where signals from it and other sensors (IDM, ECT, BAR...) received by the ECU indicate it should be.

As I posted, on my Scorpio with former engine, shutting down and restarting did result in the high idle disappearing on that (not long) drive. From experience I know that the ECU can/will start but later suddenly shut down if its ground wire grounding is faulty, then restart and run normally later. It's reasonable to think that the IAC ground, if not optimally reliable, would result in the same syndrome.

It's been too hot and humid to look into that. Maybe Sacto weather is more favorable for tinkering.

YMMV

[brokencase]

Sorry for chiming in late...just saw this. I usually am only watching the Scorpio forum...

This high idle condition you speak of sounds like the ECC-IV going into limp mode.
The only time I have run into this is when my A/C diode failed. The A/C clutch EMF would glitch the ECC-IV
and it would go into limp mode and the idle would go to around 1200-1500.
I would pull over and shut the car down and restart and the idle would then be fine.

That is the only time I have seen the behavior you described. I have owned two Scorpios
and my present one is my daily driver for the past two years.

If your IAC solenoid does not have the diode in it then I would put one in. I would also put a new A/C
diode in if you have not already done so. Make sure they are soldered in.

I will speak to the "EEC-IV Chief", but his knowledge is more of a general nature and not
to any specific vehicle.

[Ed Lijewski]

Serendipitously just found this:

https://eectuning.org/forums/viewtopic.php?t=20208

Why would TPS voltage dropping to ~0.80 produce the high idle? Why would the EEC read that signal as calling for greater IAC voltage/duty cycle?

I'll try a different TPS (hopefully the other used one I have is good in this regard).

YMMV

[brokencase]

From your link he states...

"turned off engine and restarted and everything is back to normal... until driven for a few minutes then idle goes up on its own again."

From recent experience I can proclaim that this behavior is most definitely the EEC-IV going into "limp home mode" as a result of an electrical surge. This is the exact symptom of a failed A/C clutch diode that happened to me. But it could be A/C clutch diode, IAC diode, or some alternator issue.

Ed, I've already explained to you that there was a service bulletin regarding the A/C diode.

There is also one regarding a change in the IACS (see below). An open/failed diode in the IAC results in the same "glitching" of the EEC-IV as I experienced with the A/C clutch diode.

We shall not burden "The EEC-IV Chief" on trivial things we can diagnose ourselves!

Everyone concerned - do yourselves a big favor. Take your Merkur down to your local Auto Electric repair shop and tell him you want an new A/C clutch diode SOLDERED in. Won't cost much and will save you much headache.

Other than that make sure your IACS has a good diode installed correctly as described below.

[Ed Lijewski]

From the penultimate paragraph of the link I shared above:

"Re: Idle air control issue
Post Sat Oct 20, 2012 2:50 am

"Update: Seems as if issues are gone! It was a bad TPS sensor.

"I started by cleaning the 10-pin connectors, and pinched the females a little bit so the bullets on the male ends would have a tighter fit. After about 10 or 15 minutes of idling, it started again. So looking at the datalogs, I noticed the TPS had dropped its initial voltage from cold start to warm idle around 1/10th of a volt - from .91v to .80v. Thats pretty significant. So I went and bought BWD TPS sensor and logged the voltage fluctuations. Upon returning to idle, the voltage was dead on, consistent, every single time."

My Update:! I swapped in a different (used) TPS sensor. Idle control now operates exactly as it should, on start-up, after start-up, on dashpot, with alternator load from cooling fan on at idle, on restart. Through three separate 10 minute typical local routes.

Will this last? Dunno. But the other poster's experience and mine suggest it may.

I didn't replace diode(s) because thinking a lot about that they didn't seem likely to be the source of the high/sudden high issue.

I don't know what in the former TPS was wrong or how that caused the issue; I did the voltmeter sweep of it several times earlier and also today and nothing erratic registered. But replacing that unit produced the dramatic resolution.

So, any other XR owner with persistent erratic or high idle issues should consider replacing the TPS.

YMMV