Random High IAC counts and idle

[chris989]

Hello All,

everything On my build has been running nice but I still get a random highh IAC count can can not determine why.

1989 v2500 burb, 5.7 TBI.

The truck will idle fine @~650 RPM with 25 IAC. (I have screen shots of ALDL on my shop computer I will post tomorrow).
It will run fine all day, multiple days in a row and then out of nowhere 103 IAC and idling @ ~1400. Nothing is amiss in the flag data except it shows idle cool flag and 1st time idle flag.

I am running a 180 t-stat and the temp was at 174 at the time. If i turn the truck off and immediately restart it, it is back to 650 RPM and will run idle at 650 for the next 5-10 times i drive the truck. Then, A week later back to high idle. Once it has the high idle (103 IAC) it will not go back down until i cycle the key off.

In the bin via Tunerpro i look at idle speed vs coolant and IAC position vs coolant and neither would add much with the engine at 174 degrees.

Any ideas?

Thanks,
Chris

[B52Bombardier1]

I'd check the continuity of the temperature sensor wiring / connector and maybe put in a new temperature sensor if the cost wasn't much.

Rick

[chris989]

I'd check the continuity of the temperature sensor wiring / connector and maybe put in a new temperature sensor if the cost wasn't much.

Rick

Rick,
Thanks for the reply, Are you thinking if it had no connection/fault/short it might default to the -8C setting and through a ton of IAC?

When data logging i have never seen the temp deviate from normal operation.


Pictures are in this order
Park 25
park 103
after restart
flags
25park.PNG103 park.PNGafter restart.PNGflag data.PNG

I just realized Park/neutral flag is not checked. that might account for 25 IAC?

[sturgillbd]

Do you have the park/neutral switch connected to the ecm? Do you also have a speed sensor connected and the ecm able to see the vehicle speed? Either of those signals being absent can cause a high idle after coming to a stop. A engine stop/restart resets the IAC position.

[tayto]

any play in the throttle shaft?

[chris989]

any play in the throttle shaft?

I bought a TBI from this place when I rebuilt the motor/driveline ~5 years ago.

https://www.sprperformance.com/

The injectors that came with it failed after a year. The pedal always feels stiff off idle even after replacing the cable.
In the past I have pushed and pulled on the shaft while the truck was running thinking if there was play i could affect the rpm but it never made a difference.

If the shaft was loose it would just idle high? not add the IAC counts?

[chris989]

I just replaced the park/neutral switch again because i had no reverse lights. It wasn't that old. It has always been hooked up.

The truck is an 89 and came with the 400 turbo. It has the little green box behind the dash. It is no a 4l80 using a MSD atomic controller.

When i drive the truck next I will run ALDL and check for speed data.

[tayto]

Yes I have had throttle bodies where the shaft is loose and causes high or surging idle. i'm 99% sure SPR rebuahes all their units so I wouldn't worry there. FWIW, a '7427 pcm (or '7060) would have controlled your TBI and 4L80E.