high IAC position at warm start

[Craig 94 TA GT]

Everytime I start the car when it has been run within 4-5 hours previously, the idle shoots up to ~1500, then very slowly decreases to normal. However, on cold starts letting it sit overnight, it starts perfectly. So I did a scan of the IAC position during a "warm" start...and here are the results:
RPM: 50
IAC: 109

RPM: 1375 (start)
IAC: 87

RPM: 1375
IAC 86

(RPM the same for 3-4 more intervals, and IAC decreases by 1 each time)

RPM: 1350
IAC: 83

RPM: 1325
IAC: 82

Continues the same pattern, IAC slowly going down 1 integer at a time
.....

All the way to normal idle:

RPM: 875
IAC: 62

I've recently thoroughly inspected and cleaned the IAC, and it seems to be in pretty decent shape. I'm also going to replace the EGR valve, but I don't know if that has anything to do with this
Thanks for any advice.

[Rob B (shoebox)]

There are times my engine will do that. It is after being driven and then engine stopped for 10-15 minutes (like you went into a store). Come back out and it will idle high on initial start and then be back to normal idle within about 10 seconds. It does not do it every time, but always under those circumstances. I have not been able to figure out the trigger. I can't say it is the IAC, because the values seen there may be the result of something else and not the cause. Maybe it has something to do with values present when the engine is shut off.
It's not frequent enough to be a real pain or to make it easy to troubleshoot. Some day I might be able figure it out.

[Craig 94 TA GT]

Hmmm...that's really strange. Steve's 93Z also does it too sometimes. The scan shows...before actually starting the car with the key on, the IAC position is way up there, 109. So you're right, it has to be something else that keeps it in that position when shutting it down.

This used to occur to me once in awhile like yours, and only up to an hour after shut down. However, it now occurs every single time at start up within 4-5 hours of a previous run. Sometimes it will shoot up to 2 grand and sit there for a bit longer too

I wish I could find out the cause (if it is potentially harmful) because it's getting worse over time.

[z28_xxx]

Have you unplugged the MAF like I suggested to run in speed density mode?

Hmmm...that's really strange. Steve's 93Z also does it too sometimes. The scan shows...before actually starting the car with the key on, the IAC position is way up there, 109. So you're right, it has to be something else that keeps it in that position when shutting it down.

This used to occur to me once in awhile like yours, and only up to an hour after shut down. However, it now occurs every single time at start up within 4-5 hours of a previous run. Sometimes it will shoot up to 2 grand and sit there for a bit longer too

I wish I could find out the cause (if it is potentially harmful) because it's getting worse over time.

[Craig 94 TA GT]

Yes, and I don't really see any kind of connection with the IAC position and MAF? IMHO, this is more of a problem dealing with something dependent on temperature reading(s), where if the car has been warmed previously, it will do this. Even with the MAF out it shoots up to 1800 RPMs and sits there for 5-10 seconds at startup just as usual.
And as far as troubleshooting for a low end stumble with the MAF off, it's hard to diagnose...the PCM kicks into fail-safe mode, and kicks the fans on high speed, so the temp never exceeds 180. Since the low end stumble mostly occurs at temps around 190+, the problem will be masked. But the BLMs and other readings were similar as with the MAF on.
I'm going to replace the EGR valve just because I've got some miles, and see what it does. Probably nothing, but mostly everything else already has been replaced. All I know is that it's enabled at 90*F coolant temp, when the IAT reads > 59*, else enable at 180*F. It's not supposed to work in park/idle though (which would prove it has nothing to do with this), but I'll just see what happens.

[z28_xxx]

It was in response to your other post, I didn't know if you
saw the suggestion to unplug the MAF.

Your right, EGR valve should do nothing at idle.

You must have a vacuum leak some where.

Yes, and I don't really see any kind of connection with the IAC position and MAF?

[Rob B (shoebox)]

As long as you feel confident that the EGR is closing completely, why not just temporarily cap the vacuum supply going to it, so it cannot open at all. Do some test runs and see if the problem repeats.

[Injuneer]

Coolant temp comes to mind, since the idle RPM is a fucntion of coolant temp. But yours is sort of working backwards. I guess if the coolant temp was indicating lower than actual, it might put you into the levels of the idle vs coolant temp table that increases idle speed. Seems like yours is backwards, though. Check the coolant temp before a cold start, and also before an "after 4-5 hours" start.

Also, check the TPS sensor voltage at closed throttle under both start conditions. If the TPS voltage is above 1.0, it starts to affect the idle speed, at least mine did when I had a wiring problem with the TPS.

[Craig 94 TA GT]

Thanks for your guys' help and suggestions.

Rob, I will test the EGR as you said. Also, I actually ordered a new EGR valve through gmpartsdirect about a week ago...and am still for it to ship

Fred, The TPS reading at startup is consistently 0.55 volts, and 0 in % (or whatever the other TPS value stands for). That's when the problem occurs and when it doesn't...so I guess that's ruled out.

Here are 2 cases of when the problem occured: the coolant temp was 44* and 42* C (I guess that's around ~110* F?). Coolant sensor volts read 2.2 and 2.16 respectively. After the drive, the coolant sensor voltage was at 3.2 or 83* C.
When it didn't do this problem (7.5 hour interval between runs), the coolant sensor voltage was at 1.69, and temp was 24.5 C (I guess around 75 F or so).

I can't really find anything out of the ordinary

[Craig 94 TA GT]

I think I found and cured the problem. I don't know if this makes sense but here goes...

Ever since I installed the Vortech elbow, the IAT sensor has been heavily pushing on one of the coolant lines.


Obviously that line gets really hot, so it soaks the IAT sensor, which is probably reading an insanely high temp at start up. Why it would kick the idle that high, I don't know But as the air comes rushing in, slowly cooling the sensor, the idle slowly comes back down to normal.

So I relocated mine a few inches above the filter, even though I hated drilling the CAI. Well, after a day of driving, under the same conditions as before, it started up perfectly everytime. And as many speculate, the "low end hesitation" has disappeared too, and "partial throttle" throttle response has been so much better in hot conditions.

So is this theory possible? Or was I just lucky today?

[z28_xxx]

Let us know in a week

Or just move it back and see if it happens again

I think I found and cured the problem. I don't know if this makes sense but here goes...

Ever since I installed the Vortech elbow, the IAT sensor has been heavily pushing on one of the coolant lines.


Obviously that line gets really hot, so it soaks the IAT sensor, which is probably reading an insanely high temp at start up. Why it would kick the idle that high, I don't know But as the air comes rushing in, slowly cooling the sensor, the idle slowly comes back down to normal.

So I relocated mine a few inches above the filter, even though I hated drilling the CAI. Well, after a day of driving, under the same conditions as before, it started up perfectly everytime. And as many speculate, the "low end hesitation" has disappeared too, and "partial throttle" throttle response has been so much better in hot conditions.

So is this theory possible? Or was I just lucky today?

[Rob B (shoebox)]

Maybe that will fix your problem. Some time ago, I had the thought that the IAT had something to do with it. I honestly can't remember now, but I think I tested the scenario by doing the "resistor mod" (really worthless as a mod), just to make the IAT reading a constant. Again, I think it made no difference. I may try it again, just to be sure I even did it the first time, lol.

[edit] Now that I have thought more about it, I think I may have swapped another IAT that I had laying around in there. I know whatever I did, didn't help. I may still try the resistor thing.

[Craig 94 TA GT]

I was actually thinking about putting a 4.7k ohm resistor on it, but thought I'd better get the actual temp, rather than tricking it...which is why I resorted to drilling the hole in the CAI elbow.
However, the car has been acting fine for only a day now...knowing my luck, it will soon start acting as before lol. But if things get better or worse, I'll post the results on this thread.

[Joe 1320]

I happen to use the resistor in warm weather. It does sharpen the throttle response for the first 1/3 of the throttle travel.

[Craig 94 TA GT]

After a few days of evaluation, the problem still does slightly exist. It does it only about 1/4 the time now, and only shoots to 1300 RPM instead of 1800+. But i still don't see how this is related to the IAT.
Here's an example of a warm start (before the IAT relocation):
After a ~4 hour cool down, the MAT reads 110 degrees at start up...the RPMs shot up to 1375. As it cooled to about 98 degrees in 5 seconds, the idle kicked backed down to 875. I don't see how 12 degrees can cause a change of 500 RPM in idle. So I still don't understand why relocating the IAT made the problem better. Maybe I'll just try the 4.7k Ohm resistor trick to see what happens...