Can I manually manipulate my idle speed? (Page 1/3)
Kitskaboodle APR 15, 12:17 PM
Until I can resolve my high idle issue, I was wondering if there is any way I can manually keep the idle down during the initial start up / cold start sequence? And yes, I realize the computer controls the idle speed.
Here’s the problem:
85 GT- when cold it will very quickly go into a fast idle (as it should) but by fast idle I mean 2800 rpm! It will stay this way for 4-5 minutes and then will eventually settle down to about 1400 at idle and 1250 in Drive. I could live with it but I know it’s not right and I don’t like the engine idling so dang fast when it’s cold, the oil is thick and the engine is not up to normal operating temperature. 🙁
I’ve looked hard and long for vacuum leaks but can’t find any. Which leads me to my next point. The engine runs great, no roughness, no hesitation, missing, etc.
I’m certainly open to ideas about the root cause but until it’s resolved, I’m curious if I can somehow override the computer and FORCE the engine to idle lower when cold. Can it be done? How can I do it?
Thanks, Kit

[This message has been edited by Kitskaboodle (edited 04-15-2025).]

1985 Fiero GT APR 15, 12:37 PM
Short answer, no, you need to find the root cause. A diagnostic step, you can have the car running (doesn't matter what rpm), and cover the throttle body with your hand out something that seals, it needs to seal it 100%, if the car dies immediately, no vacuum leak, the issue is with the computer control IAC system somewhere (could be coolant temp sensor or other things) if it continues to run, even if it's significantly slower, then you have a decent vacuum leak somewhere.

Putting the ECM into diag mode will target 1000 rpm idle (as well as fixing the timing to base), but that is a workaround and not a solution, if you have a vacuum leak, it might not even be able to go that low. What is the rpm when hot?
1985 Fiero GT APR 15, 12:42 PM
You can also just unplug the iac valve at your desired idle speed, let it warm up until your idle is acceptable (not to low or you'll never run cold), then unplug the iac and it's position will not change from that, you can drive, start, etc. and it will not move. But that means the idle won't compensate for AC, putting car in gear(auto) or creeping forward(manual), so it might be easier to stall.
Vintage-Nut APR 15, 01:13 PM
Did You Replace the Engine Coolant Temperature Sensor for ECM?

GM 25036979

Many members didn't get a 14/15 code and reported a "high idle issue" that was a 'mystery'. By replacing this sensor, their idle problems were solved...

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Original Owner of a Silver '88 GT
Under 'Production Refurbishment' @ 136k Miles

[This message has been edited by Vintage-Nut (edited 04-15-2025).]

Dennis LaGrua APR 15, 01:19 PM
High idle could most likely be a vacuum leak or a defective sensor or IAC valve. The ECM sets the idle speed higher from sensor inputs when cold and it cuts the speed down as the engine reaches operating temperature. Check the IAC, coolant and air temperature sensors,

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" THE BLACK PARALYZER" -87GT 3800SC Series III engine, custom ZZP /Frozen Boost Intercooler setup, 3.4" Pulley, Northstar TB, LS1 MAF, 3" Spintech/Hedman Exhaust, P-log Manifold, Autolite 104's, MSD wires, Custom CAI, 4T65eHD w. custom axles, Champion Radiator, S10 Brake Booster, HP Tuners VCM Suite.
"THE COLUSSUS"
87GT - ALL OUT 3.4L Turbocharged engine, Garrett Hybrid Turbo, MSD ign., modified TH125H
" ON THE LOOSE WITHOUT THE JUICE "

Kitskaboodle APR 15, 02:25 PM
Thanks for all the input guys. 😊
Ok, I will remove rubber snorkel
at throttle body, start car, cover throttle body with damp towel / rag and see how she responds.
Coolant sensor? I replaced that with a Rodney low temp one about 60-70K ago. As for the iac, I replaced it about 6-8K ago. Air temp sensor? Isn’t that the one that screws in to the side of the air cleaner housing?
As far as confirming the coolant sensor and air temp sensor, can I test & confirm these with a multimeter? What should be the reading for both?
Kit
Dennis LaGrua APR 15, 03:03 PM

quote
Originally posted by Kitskaboodle:

Thanks for all the input guys. 😊
Ok, I will remove rubber snorkel
at throttle body, start car, cover throttle body with damp towel / rag and see how she responds.
Coolant sensor? I replaced that with a Rodney low temp one about 60-70K ago. As for the iac, I replaced it about 6-8K ago. Air temp sensor? Isn’t that the one that screws in to the side of the air cleaner housing?
As far as confirming the coolant sensor and air temp sensor, can I test & confirm these with a multimeter? What should be the reading for both?
Kit



The coolant temperature sensor and the cooling fan switch are in the same area on a 2.8L. Rodney sells a low temperature coolant fan switch. If you put the cooling fan switch in the location for the CTS that may cause problems. The ambient air temp sensor is the one that sits on the air filter box. You can test sensors by measuring the resistance with a multimeter but you should have some fairly hot water available to take sensor readings at both ends of the spectrum.

------------------
" THE BLACK PARALYZER" -87GT 3800SC Series III engine, custom ZZP /Frozen Boost Intercooler setup, 3.4" Pulley, Northstar TB, LS1 MAF, 3" Spintech/Hedman Exhaust, P-log Manifold, Autolite 104's, MSD wires, Custom CAI, 4T65eHD w. custom axles, Champion Radiator, S10 Brake Booster, HP Tuners VCM Suite.
"THE COLUSSUS"
87GT - ALL OUT 3.4L Turbocharged engine, Garrett Hybrid Turbo, MSD ign., modified TH125H
" ON THE LOOSE WITHOUT THE JUICE "

Patrick APR 15, 03:12 PM

quote
Originally posted by Kitskaboodle:

Coolant sensor? I replaced that with a Rodney low temp one about 60-70K ago.



That makes no sense... as Dennis has addressed above.

If either the CTS or the IAT are reporting low (actual or incorrect) temperatures to the ECU, a high idle speed will result. Either test the resistance of these sensors as mentioned above... or use WinALDL (or a scanner) to see in real time what the temperature readings are. When I've tested mine, I've done it first thing in the morning after the engine has sat all night. With the ignition key turned to RUN and the engine not running, the temperature readings from both sensors should be within a few degrees of each other, and basically the same as the ambient air temperature around the car.
Vintage-Nut APR 15, 03:28 PM

quote
Coolant sensor? I replaced that with a Rodney low temp one about 60-70K ago.


You're confused as Rodney unit is a low temp fan switch...

You need to check the sensor which sends a signal to the Engine Control Module {ECM}


If you want to try to test than replace...


BTW - Members have reported that their "High Idle" problem was solved by replacing this sensor.


quote
ketstang:
I replaced the engine coolant temperature {sensor} for the ECM and initially didn't think it solved my high idle problem, but after driving about 100 miles with the new sensor the idle has settled down to 800-900rpm. I am so happy to have finally found a solution. Thanks

[This message has been edited by Vintage-Nut (edited 04-15-2025).]

Kitskaboodle APR 15, 05:31 PM
You guys are a huge help!
Thanks for the coolant sensor correction. Actually, I knew you guys were saying coolant sensor but in my head I was thinking “fan switch”. (sorry, I’m getting old 🙁) Anyways, that GM resistance: ohm chart is a keeper for future testing. 😌 I will let you guys know what I come up with.
In the meantime, about an hour ago I did go out and do the first test we discussed above. As an fyi, there’s a right way and a wrong way to starve the engine of air. I initially took about 4 shop towels, folded them into a square and tried to seal off the throttle body but it was still breathing right through that wad of shop towels. (even wetting them didn’t help) So, I got a small, flat piece of wood + 1 folded shop towel, then sealed off the throttle body opening and the engine died immediately. So, as said above, that confirms I don’t have any vacuum leaks, correct?
Kit