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

[Ed Lijewski]

After three weeks of daily driving haven't had any sudden/high idle episodes. Idle currently is good in all situations.

The change of TPS unit made the difference.

YMMV

[my8950]

Interesting read....

[dimswits]

Some years ago I inadvertently stumbled on the root cause of this while datalogging my turbo scorpio. I didn't have a bad TPS, rather, it was faulty wiring power distribution wiring, but the symptoms are exactly the same: The car drives fine for a while, then as if by magic, when coming to a stop during the drive, the idle revs up to 1500 rpm or so, and stays there. Cycling the car off and on fixes the issue for a short time, but it always returns.

The EEC-IV registers closed throttle as the lowest voltage reported by the TPS that is also greater than 0.49 V. Part throttle is a variable value that is 0.04 V higher than the closed throttle value. The closed throttle value is a continuous ratchet, meaning it's always the lowest value seen by the EEC for the current driving session.

What I was seeing in the datalog is my TPS would be at .85v closed. I'd drive and warm up the car. On decelerating to a stop the engine RPM would dip momentarily, causing an overall voltage drop in the whole car, and the TPS would correspondingly report just over 0.55v. Just for an instant. That 0.55v becomes the new closed throttle, and the normal closed TPS setting of 0.85 is 6 times the difference needed to trigger part throttle. Since the closed throttle voltage cannot increase, the car can never drop from part throttle mode down to closed throttle, and so the timing stays advanced and the IAC stays open.

[Ed Lijewski]

That seems to suggest an EEC failure of dashpot mode, which shouldn't occur, but randomly does. But why?

YMMV

[Bob Weir]

Hi Ed

You already have Dimitri, but maybe you can contact Ken Kizer [http://www.aphenos.net/scorpio.htm] and Darren Needham for help. I have phone & email if needed.

[my8950]

Hi Ed

You already have Dimitri, but maybe you can contact Ken Kizer [http://www.aphenos.net/scorpio.htm] and Darren Needham for help. I have phone & email if needed.

The is 404'd....

[DPDISXR4Ti]

Some years ago I inadvertently stumbled on the root cause of this while datalogging my turbo scorpio. I didn't have a bad TPS, rather, it was faulty wiring power distribution wiring, but the symptoms are exactly the same: The car drives fine for a while, then as if by magic, when coming to a stop during the drive, the idle revs up to 1500 rpm or so, and stays there. Cycling the car off and on fixes the issue for a short time, but it always returns.

The EEC-IV registers closed throttle as the lowest voltage reported by the TPS that is also greater than 0.49 V. Part throttle is a variable value that is 0.04 V higher than the closed throttle value. The closed throttle value is a continuous ratchet, meaning it's always the lowest value seen by the EEC for the current driving session.

What I was seeing in the datalog is my TPS would be at .85v closed. I'd drive and warm up the car. On decelerating to a stop the engine RPM would dip momentarily, causing an overall voltage drop in the whole car, and the TPS would correspondingly report just over 0.55v. Just for an instant. That 0.55v becomes the new closed throttle, and the normal closed TPS setting of 0.85 is 6 times the difference needed to trigger part throttle. Since the closed throttle voltage cannot increase, the car can never drop from part throttle mode down to closed throttle, and so the timing stays advanced and the IAC stays open.

I guess I missed this ^^^ when first posted. I've now read it four times and it's the most in-depth description I've read of this problem. Were you able to fix it D'?

[Bob Weir]

I’m reading this post about Sudden Random High Idle aka “SrHi” = New Acronym for a Scorpio PIA!
Obviously, you guys all know so much more than me, but this is my story and I’m sticking to it.

Memories of a similar situation I had with my 88 Burgundy Scorpio.

It was the engine harness, specifically that 1/3 portion that lays on top of the engine and connects almost every engine management sensor and the fuel injectors. The other 2/3 connects to it and goes through the firewall to the EEC.

What really caught my attention was a post by dimswits of a decrease of voltage of TPS from .85 to .55v when decelerating in his Turbo Scorpio. Regardless, if he turbo’d a Scorpio v6 or swapped in a XR L4 is not important. What is important, are the similar if not identical sensors and the numerical volt variance from spec.

I referenced my Scorpio Workshop Manual. In the back is the EEC-IV Quick/Pinpoint Tests. In “short” [pun intended] the most prevalent volts are <4 or >6.

When I was diagnosing my 88 Scorpio, I rarely saw any numbers close to <4 or >6, let alone .85! My readings were ridiculous. So much so, I bought a new es380 multimeter. Same absurd numbers. I also bought Dan Sullivan’s book with the meter. His primary principle is “it’s the wiring preventing correct voltage flow, not the sensor”.

I also remembered installing Ford’s “improved” TPS connector with GOLD contacts including quality hot melt connectors. Apparently Ford was concerned with voltage when they upgraded the TPS connector.

Enough testing, I cut open the engine FI / Sensors Harness. Obvious! Missing insulation, strands of wire! I’m amazed the Scorpio even started. No wonder voltage readings were absurd.

Why random? Sitting idleing in the driveway with minimal engine shake, drive it down the street hit a few bumps and the harness moved = SHORT causing base idle shift = high idle. Shut engine off, computer resets at restart = runs fine - mostly.

Why only some Merkurs? I suspect different wiring insulation. Maybe the wiring was disturbed enough to break the insulation apart causing a short? We drove our 89 Silver Scorpio for 5 years, interstates to & through Philadelphia and it never misbehaved! Herson drove it to Chicago!

Related story. I had a 94 Benz Wagon, ran great for >33k miles until it starting going into “limp home mode”. In ’86, Benz starting having their WIRING HARNESS made with a new insulation. It took 10+ years until the insulation deteriorated! Much the same as in my Scorpio. Luckily it’s harness $1,000 had been replaced, but unluckily, not the throttle body $455 to rewire internals! The Benzes reacted the same way as our Scorpios; runs great, hitting the smallest bump disturbs the harness; the Benz goes into limp home mode, the Scorpio goes to hi idle. Computer resets at Restart - runs fine - for how long?

Suggestion: Open up the wiring harness and EEC! If I’m wrong, I owe you some electrical tape and plastic wrap. Note: If you buy another harness, pay attention to the connector to the harness portion that goes through the firewall. There are two different types of connectors. Maybe the connectors are the tell tale difference for why only some Merkurs? And you might as well open the EEC. My wiring was so bad, it toasted the EEC circuit board.

Bob
PS Thanks to all of you for helping me keep my Merkurs driving down the road and not sitting in the driveway.

[andyofcolumbusmerkur]

Ya good stuff. Also think about how impedance (resistance) changes with temperature. And "all of a sudden for no reason" car starts acting up.

[DPDISXR4Ti]

A small data-point I'll add... I had this issue on my 2.3 when I was using a stock 2.3T (PF3 or LA3) PCM, but when I changed to a later, early 90's Mustang PCM (D1L1), the problem disappeared and never returned. Perhaps the factory calibration took into account this potential common problem and figured out a means to better handle it?

[petenhxr4ti895spd]

Did you change anything on the D1L1? I installed a D1L1 and my car work not run. Thanks in advance pete

[DPDISXR4Ti]

Did you change anything on the D1L1? I installed a D1L1 and my car work not run. Thanks in advance pete

Nothing on the D1L1 but OMG, a huge amount of stuff on the engine. A D1L1 will NOT run a stock 2.3Turbo.

[petenhxr4ti895spd]

Brad, I found that out. Thanks for the reply

[thesameguy]

I lost track of this conversation, but what dimwits is saying makes absolutely perfect sense - and is very consistent with my experience for whatever that's worth.

When I did the last whatever it was on my motor, I also found insulation of the ECM "subharness" in terrible shape, and fixed everything I found. That's when I found the wrecked diode. I did see an improvement to the high idle, but not 100% success.

I wonder if this problem is a result of crap voltage regulators inside the ECM... whether the input to the ECM is 10v or 15v the whole system should not be affected... there should be regulators or converters or whatever in place to provide steady voltage both to the ECM itself and to sensors. Ford would hardly be the the first company to provide insufficient or unreliable voltage regulation in their system.

Mercedes for many years used a "voltage regulator relay" (bad name) to ensure everything important in the car got consistent voltages. I wonder if one could do something similar for EEC, and just provide it regulated voltage. I have some "probably adequate" DC-DC converters ... I may plop one down in front of EEC and see what happens.

[Ed Lijewski]

My experience with Scorpio HIS was that it appeared randomly when the car had ~120K onward. Pulling up to stop at a semaphore, e.g., with no hiccough of any kind in engine rpm or performance after stopping the idle rose to 1500+.

For non-related reasons I swapped that engine for one from my parts Scorpio with 100K some years ago.

I haven't put many miles on this Scorp since (I have two XRs and another Scorp, '89, just under 100K, which hasn't displayed the SHIS.), but on every outing since then it hasn't displayed SHIS.

Years ago in discussions about SHIS there was speculation that distributor bushing wear might play a random role in screwing up data used by the ECU to maintain idle rpm. Dunno. I do know that the engine now in my '88 Scorp is a great performer in every respect including a rock solid idle in every on-road circumstance.

YMMV