IAC control input

[Fierofool]

The IAC isn't really a sensor in and of itself though it's function output acts as a sensor.  What other sensor causes the IAC to react?

I have set the TPS to exactly 0.50 volts at idle throttle plate position.  The CTS was replaced shortly after the 3.1 engine was installed.  I may have even replaced the MAP sensor and I think the O-2 sensor has also been replaced.  The IAC was replaced with a new one but didn't resolve the problem.  All these were replaced at least a year ago. 

The 87 engine that was in the car would immediately start and idle smoothly any time it was started, whether hot or cold.  Everything from the old engine was transferred to the current engine but it will not idle at cold start.  The idle fluctuates up then down and stalls.  The accelerator must be held at high idle until it does go into closed loop and even then it will waver until it reaches near full operating temperature.  Once it does, it idles perfectly. 

Anyone have any ideas what might be telling the IAC to constantly adjust when it's cold?

[Raydar]

I'm not thinking that there should be much of a difference between how the 2.8 ran, and how the 3.1 runs. So either I'm wrong, or something changed with the swap.
I'm guessing that you kept the "original" injectors. Just to test, try unplugging the fuel regulator, before you start it. Cap off the hose or fitting that supplies the vacuum. (From memory, I believe it's a "T" fitting that also goes to the MAP sensor.) This will allow full fuel pressure to get to the injectors. If it's running lean, that could possibly help it. 
I think I remember scanning it, but I don't remember what the fuel trim looked like. But it's not going to show any fuel trim response in open loop, anyway. The O2 sensor has no effect when the engine is cold. It sits at "midrange", and the ECM plugs in a baseline value. It has to start switching before the ECM goes closed loop and starts responding to it.
Also, in closed loop, it's already mostly warmed up, so it won't require as much fuel.

Something that occurred to me... The 85 V6s had a documented idle issue, especially with the A/C on. I experienced it with my 85 GT, back in the day. Ron Robinson experienced it with his 85 SE.
In Ron's case, the fix was a new ECM. I ended up trading the car before it was fixed.
If you've got a spare (any year V6) ECM with a manual chip, it might be worth a try. 

I remember when Ron Scantlin tweaked his 3.4 chip for the infamous hunting idle. Seems like his fix had to do with supply voltages, and how the injectors responded at different values.
Did you also keep the same alternator that was on the 2.8? It might be worth watching the 12v source to the injectors. (If you can get to the connector, it will be either of the pink wires. Or you can measure it at either of the TBI fuses in the fuse block. There is one for each bank. Or at the EGR solenoid, which is fed by one of those fuses.)
If the voltage is going "too high", the ECM could be restricting the injectors pulse. This will cause the engine to slow down, in turn causing the voltage to drop. Then the ECM will try to correct for THAT, by opening the IAC AND pulsing the injectors longer.

It also occurs to me that any dis-continuity between the alternator and the battery (positive OR ground side) might contribute to this. The battery will act like a buffer - or a filter - connected to the alternator. Will help to smooth out voltage spikes and sags. Check your grounds between the engine and the frame, and the battery. There is also a fusible link between the alternator and the terminal strip next to the battery. Might be worth a look. Of course, that doesn't explain why it only does it when cold.

For what it's worth, my 4.9 does the same thing, but only when it's very hot, and I get off the highway with the A/C running, and come to a stop. I still haven't fixed that.
All of this is pure conjecture, though. YMMV.

[Fierofool]

The car sat for about 2 years at Scott's before it got the new engine.  When we first started it, the injectors had gone bad, so I had them refurbished at Injector Connection.  On the day I started home, I made it about 10 miles and those injectors crapped out on me.  I bought new FJ9 injectors from a company in Florida and that's what's in it now.

I have an 85 ECM and I also have the ECM from my 87 after the 7730 conversion, plus another 87 ECM.  When the weather permits I'll try your suggestion of changing out the ECM. 

A curious thing about this one is that since that battery cable melted and the main cable came loose from the terminal, leaving the chassis power lead to the C500, any time the ECM loses its power, restarting the engine results in something that sounds almost like it's 180 out of time and is difficult to keep running until the computer starts to pick up some sensor information as the engine warms up.  I've never had this result on any Fiero after clearing codes to the ECM. 

You may be onto something with the ECM being the issue.  I'll let you know.  May be a couple of days.   

[The Art Doctor]

You guys know so much anything I throw out there would be a wild guess.  That said, the worst running situation with a fluctuating idle where I had to hold down the accelerator to make it run/start at all was when the coolant temperature sensor went bad.  Apparently it was telling the ECM it was -40*F outside and dumping in tons of fuel.  Scott helped me diagnose it after I messed with the IAC/distributor/TPS trying to figure it out on my own.

[Fierofool]

My CTS is relatively new, but it could still be bad.  I once had a CTS totally fail to the cold side while I was sitting at a traffic light on Ga. 316.  Looked like a diesel truck when they make it blow the black smoke. The ECM was telling it that it needed a quart of fuel per minute per injector, the way it was smoking.  I actually unplugged it and drove from the garage to the auto parts store to get a new sensor,  It never threw a code until I unplugged it, and it actually ran pretty good, compared.   

My car doesn't run rich, it just drops so low that it stalls.  Once it's warmed up, it's great, so maybe you're onto something.  I've never checked the Ohms when it was cold to see what temp it's indicating.  I'm going to change out the ECM first (no cost to do that) before I get another CTS. 

[Fierofool]

I purchased a new CTS with plans to install it tomorrow if the ECM swap doesn't correct the problem.  As I was leaving the parts store, the Service Engine Soon light briefly popped on.  Car was running lousy all the way to and from the parts store.  When I got home, I checked the code, expecting it to be the EGR code 32 since it came on and went off.  I was surprised to see 44 pop up.  o2 sensor.  I'll pick up one tomorrow when I return the CTS and we'll see what goes from there.  Still may swap in the 87 ECM just to get rid of the SHIFT light. 

[Fierofool]

I replaced the ecm, o2 sensor, and CTS, one at a time in that order. They didn't solve the problem, but I haven't driven it on the highway, yet.

[Fierofool]

I drove the car Tuesday afternoon and the highway drive smoothed out and returned to normal after the 30 minute highway drive.  Warm idle is normal. 

This morning, Wednesday, I started it and the hunting idle and stalling is still there when cold.  I have one more thing to replace, so I will pull a MAP sensor off the shelf and see if that's the cure.  After that, there are no more sensors to replace that would possibly affect the idle except to go back and replace another IAC and TPS sensor. 

[Fierofool]

The car sat for about 4 hours so I felt it was cool enough that it would be in open loop.  I replaced the MAP sensor but the hunting and stalling was still there.  The good news is that even though I had to hold the throttle at high idle, it very quickly settled down, like within a minute or two.  Tomorrow morning will be the true indicator as it will sit overnight before I head out for an appointment. 

[Fierofool]

It appears the MAP sensor controls the IAC. After sitting all night, the car started and idled smoothly this morning.

I have read some reports that say the MAP sensor can be damaged if the engine backfires or coughs back through the intake. A Pennocks thread once said that it could also blow the vacuum line off the bottom. I found that was the answer on the 88 Coupe I once owned.

So if you're having a hunting isle condition, check out the MAP sensor first. Mine never set a trouble code.

[Raydar]

After sitting all night, the car started and idled smoothly this morning.

Good info! I'm happy that you got it sorted.
I would be curious if the MAP sensor was leaking vacuum, or if it was just "bad", and giving an erroneous reading.

[Fierofool]

Per your reference, I checked the MAP vacuum connections and didn't find anything loose but I didn't disconnect it to do the test because I've expended all my vacuum caps.  There were never any trouble codes set until the Code 44 briefly triggered the light.