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
17196171971719817199

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.

[chris989]

I have the $4F mask. At the time i was not confident about installing and possibly repinning a different ECU. If this continues i might try going with a different unit.

I have a CJ build i can use the MSD with.

[chris989]

I dislike unsolved threads so i wanted to post, It has been a month and the truck has been running perfect and not one instance of high idle. I guess i will leave as is for now.

If I ever figure it out I will post the solution,
Chris

[chris989]

Hello all,
The issue is back today and more consistent. When it idles high i don not have Idle/cool flag
17577

when it idles normal I have the Idle/cool flag.

17579


Any idea what this means? This time at high idle my IAC counts were only 56. but going from reverse to drive the RPM shoots up. Other than re-pinning a better ECU any options?

[chris989]

Possibly solved,

Just in case someone reads this in the future i may have found the culprit. I came home the other day with the high idle, before anything else I grabbed the laptop and data logged.

The TSP data was jumping around from ~.4 to .9 at idle. Resetting the key the ECU starts with a new voltage reading and idles ok. When it starts fluctuating the IAC gets confused? The TPS test ok, It operates smoothly from .54 to 4.7 volts but when it got hot enough it would act up. Not with rhythm, just a blip here and there. Unless you sit and watch the data you would never catch it.

Anyway, tossed another TPS on it and for now appears to be ok.


Chris

[Dieselsj]

Thank you so much for following through with your solution. I have that same issue, though I'm sure I've swapped TPS's since it started. Still, I'll toss in a new one and keep this one as a spare.

[chris989]

Thank you so much for following through with your solution. I have that same issue, though I'm sure I've swapped TPS's since it started. Still, I'll toss in a new one and keep this one as a spare.

The defective TPS came on my SPR throttle body. Maybe it was used/reconditioned,, it was cleaned to appear new. The injectors on the SPR performance TBI failed in one year also.

The TPS that is on my truck now is from the junk yard,, I had it on an old crusty mock-up TBI i was going to use on another build. I was worried it did not have the slots to adjust but it seems ok. It reads .74 volts at idle.

Good luck,

[Dieselsj]

The defective TPS came on my SPR throttle body. Maybe it was used/reconditioned,, it was cleaned to appear new. The injectors on the SPR performance TBI failed in one year also.

The TPS that is on my truck now is from the junk yard,, I had it on an old crusty mock-up TBI i was going to use on another build. I was worried it did not have the slots to adjust but it seems ok. It reads .74 volts at idle.

Good luck,

I have one of their throttle bodies also. I've been through a number of injectors, multiple IAC, and I'm pretty sure several TPS. But I need to get it on the logger when it is acting up. I know I've seen the high IAC counts but haven't paid attention to the TPS % or voltage.