The IAC isn't really a sensor in and of itself though its 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 would 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?
IAC responds to outputs from the ECM for RPM vs. Target RPM.
Target RPM is different based on Cooltant Temp, A/C status, selected gear (auto) and vehicle speed to name a few.
If the setup you adjusted was on a 2.8 engine and then installed it on a 3.1 engine, then it will need readjusted. 3.1 engine will need more airflow at idle. If the two engines had different camshafts, then again it will need to be adjusted.
[This message has been edited by fieroguru (edited 03-08-2025).]
The IAC isn't really a sensor in and of itself though its 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 would 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?
The iac is a simple target rpm mainly based on temperature, when fully warmed up it targets about 900 or so rpm, when cold it is higher, 2000 maybe, the ECM is always adjusting it to the target rpm, whatever that is. Check for vacuum leaks, clean out the whole throttle body/area where the IAC is, and it's passages.
[This message has been edited by 1985 Fiero GT (edited 03-08-2025).]
Fieroguru, the problem was on the 3.1 when first installed. All adjustments were on the 3.1 to try to correct the stalling. Actually, the only adjustments were the TPS voltage and the IAC when the new ones was installed. Nothing else can be adjusted.
When I installed the 3.4 in my other car, everything was swapped over from the 2.8 just as it was and there was never an issue or adjustment needed.
The only scan tool I have is the one that came with the Fieros. The AC Delco GM Code Coin. I have had a scanner on it at one time and it showed that it was constantly adding and trimming fuel, which would be expected as the IAC rapidly adjusts idle speed. It had been sitting for a couple of hours at the time of the test so we could duplicate the issue.
The ECM, the o2 sensor, the CTS sensor, and the MAP sensor have all been replaced, in that order and tested between each. After sitting for about 4 hours after I swapped in another MAP sensor, the hunting and stalling were still there, and I had to hold the throttle at a high idle. It settled down within about a minute with only slight fluctuations in idle speed. I'll know the result tomorrow morning after sitting all night.
Have you driven the car? In order for the ECM to establish a known position for the idle air controller, the engine needs to be at operating temperature and driven at speeds above 40 MPH for a few minutes. Until that happens, the idle may be very erratic.
------------------ Rod Schneider, Ball Ground, Ga. "You can't have too many toys!" 1988 Fiero GT 1988 Porsche 928S4 1987 Corvette 2016 Mustang EcoBoost Van's RV-6 airplane
[This message has been edited by Rodrv6 (edited 03-13-2025).]
I did drive the car after each part replacement. Raydar pointed me toward the MAP sensor, which would have been and was my last target. Hunting and stalling was there until the MAP sensor was replaced. On initial start, it was still there, but I drove it for about 60 miles yesterday. 30 miles, stop for an hour, then 30 miles back. This morning I started it and it immediately stalled. Restarted it and it settled down and idled like it should.
