I recently purchased a 1988 Duke with an Isuzu 5-speed. It runs well most of the time, but after it gets up to full temp (driving it at non-highway speed for 10-15 minutes) and sits for a few minutes and/or we drive it in stop-and-go traffic for 15+ minutes, it begins to idle erratically and subsequently die.

It will consistently start up and idle fine again after it sits for long enough time for it to fully cool down again (at least an hour), so it seems like the issue is either 1) a sensor acting up when hot or 2) a valve of some kind in the ignition plumbing getting stuck when hot.

After reading multiple posts on this forum, I replaced what seemed to be potential candidates (all of which had some age to them), none of which fixed the problem. Instead of throwing more parts at the problem, I wanted to reach out to all of you folks in Fieroland, hoping someone can help me diagnose the issue with a little more precision.

I should also note that a) the car seems to be running very rich, based on what’s coming out of the exhaust and what kind of buildup I’m seeing inside the intake (lots of black soot), but b) the only diagnostic code that I have been getting is 44 – running lean at the exhaust manifold. The old O2 sensor I pulled out looked all burned up/dried out, similar to what a spark plug looks like after extended lean conditions, but everything on the front end of the TBI seems like it’s getting too much fuel and/or not enough air, which seems like a strange combination.

Below are the parts I’ve replaced in recent weeks:
- EGR Valve
- O2 Sensor
- Idle Air Control Valve
- Throttle Position Sensor
- MAT/Intake Manifold Air Temp Sensor

Couple of final observations: the MAP sensor seems to be working fine, and I don’t seem to have any vacuum leaks. Also, the Duke consistently “telegraphs” to us that it’s starting to have issues because it goes through stages of i) mild idle issues and then ii) very erratic idle issues prior to iii) shutting down/dying altogether. Could the coolant temperature sensor be creating these issues?

Below are a photos of a few of the parts I replaced, in case that helps someone with a more skilled mechanical eye diagnose the situation better. Any thoughts or suggestions would be much appreciated, so thanks in advance for offering feedback to someone who’s brand new to all things Fiero!

IAC Valve:




Manifold Air Intake Temp Sensor:



EGR Valve Seat:



O2 Sensor:



Video of Erratic Idle:

Video of Erratic Idle


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1988 2.5L 5-Speed

[This message has been edited by 1988Duke (edited 10-29-2023).]

quote Originally posted by 1988Duke: Any thoughts or suggestions would be much appreciated...

Use the Upload Media tab at the lower left of the message box to embed your images in your post. If you click the EDIT tab on your opening post, you'll be able to take care of the images.

It seems as if the video link came through just fine but the Image upload feature didn't work, so edited the original post using the Upload Media feature instead.

[This message has been edited by 1988Duke (edited 10-29-2023).]

EGR ports will look like that.

IAC valve should have "wear marks" all the way around the tip. Can be damage IAC to dirt/damage in TBI. But likely not your problem killing idle.

DTC 44 "Running Lean" means ECM is Adding Fuel & still O2 sensor stay lean.
You think has no vac leak often mean you're wrong. see https://web.archive.org/web...cast.net/~fierocave/ vac leak

You Must have ECM scanner to see anything that ECM sees.
see https://web.archive.org/web...cast.net/~fierocave/ ecm section.

also https://web.archive.org/web...~fierocave/gmdis.htm as maybe problem w/ iffy coil(s) ICM/PIM etc.
Iffy plugs &or plug wires can make problem or destroy the coil(s).

Also take apart every "ground" in engine bay to start, clean/fix & coat w/ Silicon or Permatex Green label Brake Grease.
see cave, wire service page.

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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

[This message has been edited by theogre (edited 10-29-2023).]

That version of MAT sensor can have problems because of uncoated NTC thermistor used.
But often if true, often causes DTC 45 running Rich & @ idle the MIL light comes On.

Insulating Coating on the sensor element also has metal screen not plastic over it.
Uncoated element lets dirt, carbon, water, & more to change resistance & that error affects high heat reading more. Air Temp gets hotter @ idle because low flow.

Type/Version Matters more w/ 4 cyl because of location. V6 often doesn't care which is used for same reason in the air cleaner box.
Before you think will move it... Don't. ECM programing expect high heat @ idle & causes problems if moved "upstream" to TBI.

Thanks for the quick reply, Ogre. And fair enough on the vacuum leak potential - I haven't exhausted my search for potential leaks, so will check to make sure the TBI is seated properly since that seems to be a common problem. And I didn't think of looking at the vapor recovery canister (also didn't know those things had filters that needed to be changed...the more you know!), so will check all of that out as well.

Also, related to your ECM Scanner suggestion, the only scanner and app combo I could find that isn't discontinued or out of stock consists of the following. Seems like these should work for my '88, but I wanted to see if anyone on here had any experience with this setup:

1320 Electronics OBDI Scanner w/ BT: https://www.1320electronics...pin_ALDL_BT_MK2.html
ALDLDroid: https://play.google.com/sto...om.sgiroux.aldldroid

And just a quick clarification on your quote below: Were you suggesting not to use the MAT sensor with the plastic/uncoated element, or were you just noting that the plastic/uncoated ones tend to have problems due to build-up so that I would be aware to check that mine is clean/free of debris from time to time? I'm guessing the latter because your Servicing GM DIS page said to stay away from the metal caged ones and only use the plastic caged type, but just wanted to double check that I was reading it properly.

quote Originally posted by theogre: That version of MAT sensor can have problems because of uncoated NTC thermistor used. But often if true, often causes DTC 45 running Rich & @ idle the MIL light comes On. Insulating Coating on the sensor element also has metal screen not plastic over it. Uncoated element lets dirt, carbon, water, & more to change resistance & that error affects high heat reading more. Air Temp gets hotter @ idle because low flow.

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1988 2.5L 5-Speed

Which type of MAT doesn't matter a lot as far as ECM reading them.
Plastic one is a bit faster read but Metal one w/ insulation helps keep crap off the actual sensor.

Can try cleaning the one you have w/ plastic safe electronic cleaners that don't leave oil behind. After cleaning check the flat sides of sensor & look for flaking metal or other damage. If the plastic cage looks melted/fried, likely burned backfiring & uncoated sensor likely cooked too.

I've use both types since writing that & Should have updated that page but can't edit the archive page.

Have you scanned the ECM?

That's the first thing you should do before attempting any repairs on a computer controlled car.

quote Originally posted by olejoedad: Have you scanned the ECM? That's the first thing you should do before attempting any repairs on a computer controlled car.

Many think that but doesn't help a lot of times.
Because Iffy connector, "grounds," & even sensors often causes "bad data" & scanners often give no clue what's wrong. (& not just a problem w/ OBD1 systems.)

Scanners May help w/ DTC 44 & 45 because you see O2 is "Pegged" but Pegging is often side effect of vac leaks or fuel system problems.
Example: 45 is often injector(s) leaking, return line plugged, iffy MAT for DIS dukes. 44 is often 1 or more Vac Leaks & many are in the cave page.
So scanners are "just" Check Engine Light to say ECM has a problem but you have to find the Vac Leak or maybe crack exhaust manifold because that can let exhaust out & suck in air depending exactly where the crack is.

Or if DIS brick is missing the guard between coils & ICM etc cause engine dying, The scanner often is useless.
Example: Iffy Plugs, plug wires, or coils, worse "performance" coils, can drive some, coils &or ICM/PIM get hot, dies, cool off, drive again, repeat.
If Crank Pick-up coils get hot & dies, can do same but often wait longer to cool because engine block & oil keeps it hotter longer.

Can clean/fix "ground," check some sensors, etc w/o a scanner & cost near nothing to do that as first step.
Again, w/ DTC 44 look everywhere for vac leaks. Brake Booster & very long line can have hidden leaks very hard to find so block that @ the engine end & don't drive w/ booster disabled. Just plug EVAP lines @ TBI end that also disable Cruise when have that.

This Does Not help w/ 44 either...

The valve "cone" is not sealing w/ TBI hole/seat inside. But ECM likely can't trip IAC codes because barely under whatever limit to say IAC problems.
I don't see anything wrong w/ IAC motor in above pic's but possible. If the shaft is "bent" for any reason may not center w/ the seat & make above wear mark.
But before buying a IAC... I would look into TBI holes to look for crap or damage in the bottom when the cone hits. Many cameras can work w/o taking TBI off.

quote Originally posted by theogre: But before buying a IAC... I would look into TBI holes to look for crap or damage in the bottom when the cone hits. Many cameras can work w/o taking TBI off.

Thanks again, Ogre. I already replaced the IAC, but good call on using a camera to check that out. I did try to clean it out as best I could blindly using several q-tips until it came out clean (it was pretty full of carbon build-up where the cone is supposed to seat when extended), but I have an endoscope that I can use to look in there and see if anything else is going on before I pull off the TBI this weekend to check for any leaks at the base of it.

I'll report back once I get some time to work on it - very curious to figure out what the root cause actually is.

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1988 2.5L 5-Speed

After that...
take IAC out, "Paint" the cone w/ marker, install then drive.
after drive for a day to week, pull IAC again to look @ the cone.

the "Wear mark" should be an even circle around the cone.
I don't have a pic handle to post.

The TBI intake port for IAC can let crap, rust from air cleaner, etc & block the cone to seat in there.

TBI should have a gasket for air cleaner to sit on. Often is missing.
TBI Fuel Reg often leaks a little w/ engine running.
If there true... cheaper to get a "rebuild" kit w/ all gaskets + reg part. More so since have or will remove the TBI for intake & have that in the kit too.

When you turn the key to accessory do you hear the fuel pump prime? I'm wondering if the main fuel relay is not functioning properly and your oil pressure sensor is flaky at temperature.

quote Originally posted by skywurz: When you turn the key to accessory do you hear the fuel pump prime? I'm wondering if the main fuel relay is not functioning properly and your oil pressure sensor is flaky at temperature.

That will cause the engine to cranks way longer to have enough oil pressure to close the F-pump switch.

quote Originally posted by theogre: [QUOTE]Originally posted by skywurz: When you turn the key to accessory do you hear the fuel pump prime? I'm wondering if the main fuel relay is not functioning properly and your oil pressure sensor is flaky at temperature.

That will cause the engine to cranks way longer to have enough oil pressure to close the F-pump switch.[/QUOTE]

Right and it can cause the fuel pump to lose power if the oil pressure sensor is flaky

My experience with a '88 Duke included rought idle and stalls.
Started by looking at the ignition and fuel pressure, also looking for a bad sensor. The last item replaced was the throttle body fuel injector.

I replaced the injector after stalling on the freeway. I was able to get it running again by removing the fuel injector and tapping it. There was fuel getting to the throttle body. Had the right size torx driver with me.