Sudden Intermittent High Idle

Post by **DPDISXR4Ti** » Fri Apr 05, 2019 10:34 pm

No high idle, but here's today's codes - new ones w/ descriptions

21
24 Intake Air Temperature (IAT) or Vane Air Temperature (VAT) sensor out of range
41
52
67 Park/Neutral circuit fault (Maybe because it was in gear?)

Anyone know if like the XR harness there is a merged ground connection for the sensors buried in the harness? On the XR it's common for that connection to become compromised.

Post by **Ed Lijewski** » Sun Aug 04, 2019 2:09 pm

Time for an update. What have you found or/and done? Issue still randomly occurs, or haven't driven it much?

With only a tad of facetiousness the link below is to a Radiolab podcast on how a gamma ray hitting a computer can alter a command signal (last ~15 minutes focuses on autos, especially sudden acceleration in Toyotas):

https://www.wnycstudios.org/story/bit-flip

YMMV

Post by **DPDISXR4Ti** » Sun Aug 04, 2019 3:44 pm

No real update. The problem hasn't reoccurred the couple times I've driven the car, and I haven't gotten any report from the wife on problems. I'm still suspicious of it being an intermittent wiring issue, but haven't gone hunting for same. The other thing I should check is the PCM itself for any failing/leaking capacitors. This is increasingly a problem with 30+ year-old components. It's plaguing the '80's/90's Ford truck PCM's in particular for some reason.

Post by **brokencase** » Wed Aug 07, 2019 6:49 pm

..and I can confirm bad capacitors on the Scorpio EATC. I think I found about 3 or 4 that were bad ( I have a tester ) I just systematically changed them all.

There are a few electrolytics in the EECIV. I think about 4 or 5. I have not got around to changing them yet....but not a bad idea.

Post by **Ed Lijewski** » Thu Apr 23, 2020 7:16 pm

Resuscitating this cuz it recently occured to me that the issue theoretically could randomly result from air occasionally entering the intake via around the throttle body shaft (I assume there must be seals there but I haven't verified this).

YMMV

Post by **john keefe** » Thu Apr 23, 2020 11:46 pm

So, how did you guys go about delving into those units to investigate? The PCM I can kinda' guess, but the ETAC?

Post by **DPDISXR4Ti** » Fri Apr 24, 2020 8:19 am

Ed Lijewski wrote: ↑Thu Apr 23, 2020 7:16 pm Resuscitating this cuz it recently occured to me that the issue theoretically could randomly result from air occasionally entering the intake via around the throttle body shaft (I assume there must be seals there but I haven't verified this).

I believe there may be o-rings on the TB shaft, but it seems unlikely as the probable cause to me for two reasons.

1) The leak would be very small and unlikely to have a significant effect
2) It would be unlikely to be an intermittent problem. I've only experienced this issue once.

Post by **Ed Lijewski** » Mon Apr 27, 2020 8:05 am

Re: Sudden Intermittent High Idle

Post Fri Mar 29, 2019 4:28 pm
KOEO Error codes pulled today...

21 Engine Coolant Temperature (ECT) sensor out of range
41 System was lean for 15 seconds or more (no HO2S switching)
52 Power Steering Pressure Switch/circuit open

21 and 41 could be related. I guess I'll replace the ECT.

52 I'm not worried about

A vacuum leak can produce a substantial RPM increase.

YMMV

Post by **DPDISXR4Ti** » Fri Jan 21, 2022 6:54 pm

Had another "sudden high idle" symptom with the Scorpio yesterday. FWIW, it happened almost exactly under the same circumstances as last time... Sub-freezing temps - after driving the car for ~15 minutes I noticed at the final stop-light before reaching the highway that the idle hung at near 3000 RPM upon stopping, gradually working it's way down to ~2400 RPM before the light turned green. On the 45 minute ride home I tossed the transmission into neutral a couple times on some downhills and the condition persisted all the way home. Once in my driveway I shut the engine off and re-started the engine before pulling into the garage - the engine started right up and sat at ~1000 RPM like it should.

An additional data-point I gathered this time thanks to the A/F meter.... At all times the PCM kept operations in closed-loop, whether at high-idle or under load. I wouldn't have expected that of "limp mode" operation.

I'll try to remember to pull codes tomorrow. When I grabbed the ones two years ago I may very well have had a loose O2 sensor, so that may have influenced those (at least the 41).

Post by **DPDISXR4Ti** » Sat Jan 22, 2022 7:09 pm

Just pulled codes - 21, 24, & 52 - same as before less the 41 now that my O2 sensor is actually fastened in place.

I replaced the 8GHB with a 8GHC. I was thinking I had a 1989 PCM in there, but the 8GHB is a 1988 while the 8GHC is a 1989 and has a later calibration code of SD102 (vs. SD101 for the 8GHB). It'll be interesting to see now with no other changes what may change.

Post by **Ed Lijewski** » Sun Jan 23, 2022 10:05 am

Seems to me that errant 21s and 24s could be involved in an ECU high idle action. Why haven't you already resolved the reasons for those two codes?

YMMV

Post by **DPDISXR4Ti** » Sun Jan 23, 2022 1:20 pm

Ed Lijewski wrote: ↑Sun Jan 23, 2022 10:05 am Seems to me that errant 21s and 24s could be involved in an ECU high idle action. Why haven't you already resolved the reasons for those two codes?

Darren replaced both sensors and dug into the harness looking for a compromised shared ground. So the easy answers on fixing the reason for those errors have already been explored, without an apparent solution. Since the engine runs fine and closed-loop operation is achieved consistently and predictably, there isn't an outward problem to chase per se.

That said, I'm not comfortable that the errors persist, as does the intermittent high idle. Replacing the PCM is essentially my next diagnostic step.

Post by **DPDISXR4Ti** » Mon Jan 24, 2022 6:45 pm

Drove the Scorpio about 40 miles today. No high idle, but since it is an intermittent issue, that doesn't really prove anything. Checked codes when I got home and only got 52 and & 67, which are non-issues. Notably, 21 and 24 did NOT pop up - that could be significant. Perhaps this later PCM handles deviations from norm a little more "casually", neither setting a code nor activating an error routine (i.e. high idle). Too soon to know - just speculation at this point.

PeterP11c · Post by **PeterP11c** » Thu Jan 27, 2022 12:56 am

Actually, I have had this issue for awhile, sudden raise of RPM on warm engine requiring a shutdown and restart. Took it to a dealer one day, but they quoted me some exorbitant price so I declined. They said they didn't repair anything or so I thought, but the problem went away. For a while at least and then I discovered what a kind soul of a mechanic did as a freebie. He wrapped some insulating tape around wire connectors coming out of Air Bypass Valve. Each time the RPM's would go up, I simply opened the hood and gave them a squeeze using my fingers. That's all and the engine reverted back to normal on idle. Over time I replaced old tape with new and this annoying problem vanished into the thin air. I know it is not a scientific solution, no codes searching was involved and I didn't replace a single item. But sometimes small solutions work just fine.

Post by **Bob Weir** » Thu Jan 27, 2022 11:18 am

Assuming the harness is ok, I think brokencase has an appropriate idea. He mentions "EATC" - maybe he meant "EEC". Regardless, deteriorating capacitors in an "EEC" sounds reasonable.

" Re: Sudden Intermittent High Idle
Post by brokencase » Wed Aug 07, 2019 6:49 pm
..and I can confirm bad capacitors on the Scorpio EATC. I think I found about 3 or 4 that were bad ( I have a tester ) I just systematically changed them all. "

On Sep 22, 2013, caleyfan wrote
" The one 220uf was OK, one of the 100uf was OK but the other 100uF and the four 47uf capacitors were all defective. These do degrade over time,"

The final cure for my Scorpio was changing the EEC. I'm reasonably sure I used a '88 version in a '89.