So, a week and a half ago, my wife got home from borrowing the Fiero and complained of super-high idle.
Sure enough, 2500 in park. Nice and smooth, but roaring. After reading quite a few high idle threads, I started with the IAC. It buzzed eagerly with the ALDL jumped, so I took it out and cleaned what little dirt was on the pintle. Back in, did the ECM idle reset, and after a little jaunt to learn the new idle, it was roaring and straining against the brakes again. It was obviously getting WAY too much air. No real visible damage to any vac lines, so my next guess based on reading was the EGR system.

EGR diaphragm holds vac perfectly, so I ordered an EGR tube from Rodney Dickman, and dealt with the high idle for my short drives to work for a few days. Tube arrives, I install, and after cutting off what was left of the ratty insulation sleeve on the old one, I find no cracks, but the upper gasket looks ratty. Maybe it didn't seal after the upper intake had been off for a cold start injector gasket a year ago? Reset the ECM again, and lo and behold, we're back to normal. A little higher than the old normal, but only by 1-200 RPM. This was Monday.

BUT THEN... On my drive home Tuesday, I hadn't quite got the battery cable back in tight enough, and wound up making a little parking lot adjustment after an errand to get her started again. Once back on the road, switching from vent back to A/C caused the fan to cut out. After fiddling, I determined that the fan only worked on high speed in vent or htr. I figured I might have faked out some relay with the false start due to the battery thing, so when I got home and parked (normal idle!) I turned the car off then on, and everything worked correctly again. Great!

This morning, everything was still A-OK. Nice gentle A/C breeze on the way to work, normal, smooth idle. On the way home however, the A/C was back to naught, and by the time I got home (~12 min drive), I was back up to 2300 RPM idle in park. Another ECM reset, with a nice long battery unhook, does nothing. Now the engine bay's too hot to monkey around and it's getting dark. Also popped out the blower motor resistors, no visible damage.

TL;DR
-2000-2500 idle in park - super steady, no hunting. ~1500 right after reset, higher after a drive at 45mph for "relearning".
-1 y.o. O2 Sensor
-1 y.o. MAP Sensor
-Cleaned, working IAC
-New EGR tube, EGR valve holds vac
-Sudden onset, sudden re-onset after EGR tube replace.
-Cabin blower intermittently only works in Hi + Vent/Htr
-A/C switch runs trunk and condenser fans normally. FEELS like compressor engages? (slight fluctuation in idle speed when switched on/off)

What vac leak could be this big, but only happen intermittently? I read in the service manual 6E that the ECM controls A/C in addition to engine management, is my ECM fried or is this a weird coincidence?

Pull out the IAC. Put your finger in the hole and cover the vacuum hole way inside. Have someone start the car. See how low you can get the idle with sealing off the IAC passage. If the idle remains high with the IAC vacuum line sealed with your finger, the issue is with a different vacuum leak. If however you can kill the engine or get the idle way below the normal 1,000 rpm, the problem is with the function of the IAC system.

Your finger has to be inside the IAC hole. You need to seal the passage with the tip of your finger. Once started you will feel the vacuum.

[This message has been edited by phonedawgz (edited 06-17-2020).]

The engine started smoothly to 1100 RPM, and stayed there when I covered the IAC inlet inside the throttle body. I'm assuming that means a vacuum leak elsewhere (unless the IAC is lagging open when hot somehow), and THAT is leaking more when hot.
Is the blower motor thing unrelated, there's nothing in the ECM that could be causing both symptoms?

"86 GT High Idle and A/C Shennanigans... Related?"

Maybe.

ECM/PCM has final call to activate AC clutch and can/will "bump the idle" by adding IAC steps, change Timing or both.

You Will need an ECM scan tool to see if/when it commands the AC clutch line to turn on.

------------------
Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)

The Ogre's Fiero Cave

I meant to take out the IAC and cover the hole right there. But your test does about the same. You have a significant air leak.

There is an internal O-ring on the IAC feed tube on the manifold side of the throttle body. This can leak quite a bit of air. Any chance you had the throttle body off lately?

The lower hole has the internal o-ring in it.

I have not had the throttle body off... ever in 17 years, to my knowledge. I don't think the TB was off of the manifold at a shop either. I found Phonedawgz' color-coded vacuum routing diagram in another thread, and I'm trying to figure out where to start. I had taken off the little vacuum line that connects to the upper plenum near the distributor (is that the line in green?) and the idle goes up by 1-200 until I cover the hole with my finger, so it's got to be bigger than that. Thinking of blocking off the brown line to cut the trunk wall stuff out of the vac circuit. The trunk wall lines only power the cc system, right? I'm thinking it's possible the large line toward the firewall side of the TB lost seal when the intake was off for the cold start injector line o-ring.

Also, if the ECM CAN control the idle down to 1100, why does it choose not to SOMETIMES? I mean, I know it's giving me an opportunity to fix the problem, it's just frustrating that it's intermittent.

to get a 2000 rpm idle, you need a big air leak. There are only 2 choices left.

1. EGR tube is broken
2. Brake booster hose is leaking.

EGR tube is 4 days old, and the old one wasn't even cracked when I got it out. I'll have to investigate the brake booster. Will search to see what tests are required.

quote Originally posted by Time Traveller: EDIT: Drive home - Car started up to 850 idle (a little low for her), and by the time I got home 7 mi /13 minutes later, she was at a steady 900.

Well; you can't test for a problem that isn't there. A bad ground or solder connection in the computer may be at fault if it goes back to 2000. The heater problems seem like a separate issue to me.

I did some testing based on what I've found online and in the service manual regarding the blower issue, and it seems like my resistors are too resistant (~8-10 ohm, should be <1, right?), and it seems I need to check the blower motor High Speed Relay. I got 12.4v across the motor with the car off and fan switch in hi, and 11.7 or so in the other settings, with the motor not running. I did not do this with a test load as Ogre's Cave recommends, but are these values too far apart to the point of indicating a shot blower?

That said, if the blower worked in HI with the A/C running, it'd be fine, but it cuts out in any compressor modes. The SM says to conduct the High Speed Relay test in vent mode, somewhat prescient given my circumstances. I'll probably also get in there and clean any fuzz out of the evap this weekend.

DEFINITELY open to any other suggestions on the A/C thing. We topped 97 here today

So, presuming the idle issue IS separate and behaving normally, the A/C blower cutting out on any speed other than HI, and cutting out on HI when the A/C is engaged, remains.
I've conducted all the tests on the blower motor system as outlined in the service manual.
-After some cleaning, the blower resistors are all around 1 ohm when back-probed.
-The A/C Power relay and High Speed Relay seem to function properly.
-Speed switch has continuity per each setting.
-Voltage across the motor is 11.86 on vent/HI with the motor connected or disconnected, and the engine not running. HOWEVER, if the disconnected voltage drops at all (~11.7-11.6 on resistors, 11.42 when AC engaged), the motor seemingly shorts the circuit when connected (dropping the voltage across the motor to like 0.15). I took the motor out and cleaned out a bunch of the carbon with electronics cleaner, let dry overnight, and tried again, same result.

Do I assume correctly that this means the motor is shot? Or am I missing something, and a different component is the cause? Is the voltage drop across the resistors or AC system abnormally high, indicating a different problem?

Tried a new Four Seasons blower motor, symptoms still the same. The armature on the old motor spins about as easily as the brand new one. When I get home I'll do voltage and current tests across both motors, but I feel like I'm missing the real issue. The resistors in these cars don't look like they could have an integrated fuse, which seems to be a common complaint with the newer style resistors, and my motor wire voltage doesn't cut out completely when the resistors are engaged, only when the old motor was connected. The resistances aren't perfect to the factory spec of .5 and .7 ohms or whatever, but I think I read a post somewhere here suggesting that cheap multi-meters aren't going to get an accurate reading on that small of a resistance anyway. Is there a way to be sure on these? I've noticed they're not really an obtainable item.

EDIT:
I was monkeying around with some final tests to both the new and old motors, when it briefly shuddered to life in the low fan setting. One thing I hadn't done yet was swap the A/C Power and High Speed Relays.... Turns out, the relay was shot. A new relay set everything right. I'm not sure how the relay passed the testing I did per the shop manual, but that's that.
Also of note: Since the ECM hasn't experienced the compressor running since the last reset, the idle dropped way off when I first engaged the compressor. A short jaunt with A/C running fixed that. I guess one should use the A/C, if equipped, during the idle re-learning drive.

[This message has been edited by Time Traveller (edited 06-23-2020).]