2000 rpm flare up on start up - bad IAC?

["Turbo-T"]

Every time I start my TR when it has been sitting for some time, the idle will flare up to 2000 rpm or so, for a about 5 seconds, and then it will gradually come back down. On the Scanmaster, the IAC, when flared up, will show in the 100's.

My ECM is not losing power or anything like that. If the engine is warm, it flares up maybe 1/4 to 1/2 way and quickly comes back down.

Of interest, anytime I have changed a chip, the idle has not flared up near as bad as it has once the ECM has had the chance to relearn the IAC.

I checked some of the youtube vids of others starting their TR's and none of them seem to have that issue.

Here's a youtube vid I made of mine, you can hear it when I start it up and you can also hear it straining against the brake.

Turbo Buick Regal 001.avi - YouTube

The car has always done this the last 4 years of ownership.

Any ideas?

Thanks in advance.

 

[mikestertwo]

What is your IAC count when the motor is idling normal? What chip are you using?

 

["Turbo-T"]

Mike, I'm using a turbo tweak 5.7 chip. As for IAC count, I just went out and started the car, the engine water temp was 150 degrees and it was 72 degrees, however the idle was not as high and the IAC count started off at 52 and dropped down to 0. Yet it idles fine at 0.

I will have to wait until the car is bone cold and then try to start it and see what the IAC is when I start it.

 

[3.8L V-8 eater]

I have the same issue and my IAC's are fine at hot idle....Curious to see what others say about this...Thought all cars revved up higher on start up when cold all my other cars do??

 

[TurboDave]

High cold start up idle is normal.

 

[PARKBOY]

My car does the exact same thing at start up. And I had an 86 T Type that also use to do that. I always thought it was normal.

 

[mgmshar]

A couple of points...

1. When starting cold, the ECU initially sets the IAC to a certain amount of steps. On a stock chip, I believe it's 150 steps (i.e. really far open). On the aftermarket chips, it's usually lower to prevent the huge rev-up on cold start.
2. There are two functional reasons for this rev-up when cold - a) to get the catalytic converter hot as quickly as possible, and b) to spin the camshaft at a higher speed during the time when it may be starved for oil (low speed + no oil = wiped cam lobe nose).
3. Since most aftermarket chip makers don't worry about catalytic converter warm-up, they target a lower IAC step target at cold start than a stock chip.
4. Once the car starts, then the ECU starts to lower the IAC steps to bring the engine speed down to a target RPM. It takes several seconds to make the transition from the initial IAC position to the target idle speed after a cold start.

Now, we know that your throttle plate idle screw is adjusted too far open, because the car idles fine with the IAC completely closed (i.e. zero steps). So, at a cold start, you have a throttle plate that's open too far plus an IAC that is set to a high value (over 100 steps). The result - you are getting a lot of air at cold start, so you are getting a lot of RPM. It takes a while for the ECU to bring the IAC down to hit the first target RPM, so your car is revving for some time.

The fix?

Do an IAC reset procedure, which actually means that you are adjusting your throttle idle screw and TPS so that at hot idle, your IAC counts and TPS readings are both correct. The instructions are here...

IAC Reset Procedure

Try this and report back. I'm guessing this will help your problem a lot.

Good Luck,

 

["Turbo-T"]

Update:

This morning it was 45-50 degrees. The scanmaster was showing 140 on the IAC before I started the engine. As soon as it fired the IAC dropped to 123, then it eventually dropped to 50 when idling cold. Does this sound about right?

mgmshar, the car idles at 0 steps IAC when fully warm, is this not normal?

 

[TurboDave]

Ya, sounds good. 50 is a smidge high but I wouldn't worry too much about it. It's working.

 

[mgmshar]

Update:

This morning it was 45-50 degrees. The scanmaster was showing 140 on the IAC before I started the engine. As soon as it fired the IAC dropped to 123, then it eventually dropped to 50 when idling cold. Does this sound about right?

mgmshar, the car idles at 0 steps IAC when fully warm, is this not normal?

The progression of your IAC after cold start sounds about right, except settling at 50 while still cold seems a little low. So, it's trying to do the right thing.

Hot idling at zero steps is NOT normal, although many people's cars do it. Once the IAC reaches zero, the ECU can no longer control idle speed effectively, because it can't lower the idle speed any more. Therefore, it is likely that your car's warm idle speed is actually running faster than what the ECU is trying to target.

As stated in the IAC procedure, the IAC should be at around 15 to 40 (20 to 25 target) steps with the engine fully warmed up. To do this, I am sure you will have to CLOSE your throttle plate a little bit using the idle adjusting screw. Once completed, your IAC will still go to about 140 on your next cold start. However, since the throttle plate will be a little more closed, the initial rev-up won't be as high.

Try adjusting the idle screw per the procedure (it's easy when you have a scan tool), double-check that your TPS is below 0.44 volts after you make the adjustment, and I bet you'll be fine. Here's a tip that will help - make sure the car is fully warmed up and all unnecessary electrical accessories are off (i.e. A/C, headlights, radiator fan, etc.) when you make the final adjustment. If you get the IAC to be around 20 steps with the car fully warm and all electrical accesories off, you should be fine.

Good Luck,

 

["Turbo-T"]

Thanks I will try the IAC reset/TPS reset as you mentioned. FWIW I went ahead and checked the TPS with the engine off key on and found it's currently between .40 and .42.

Is it possible someone before me deliberately set the throttle up this way? If so why? Or do they get out of whack from age/use?

 

[mgmshar]

Thanks I will try the IAC reset/TPS reset as you mentioned. FWIW I went ahead and checked the TPS with the engine off key on and found it's currently between .40 and .42.

Is it possible someone before me deliberately set the throttle up this way? If so why? Or do they get out of whack from age/use?

It's likely that somebody before you set it this way. I've seen other cars set to run this way. I'm not entirely sure why. My best guess is somebody had a problem with it (maybe it was stalling or something), and they adjusted the throttle plate as a band-aid to fix it. Or maybe they thought the warm idle speed was too low, so they increased it artificially by opening the throttle plate so far that the ECU could no longer lower it. That's what we used to do on carburetors, right?

If, for some reason, you're not happy with the way the car idles after you adjust the throttle plate, you can always turn it back. Just keep note of how far you end up turning the screw to make the adjustment. Also, any time you mess with the idle screw, you need to double-check the TPS reading at idle. With the car running at idle, it should not be higher than 0.44 volts.

Let us know how it goes...

 

["Turbo-T"]

Hey....before I attempt to do this adjustment....I wanted to clarify....the IAC should be 10-40 in park right?

I just noticed even though my IAC reads 0 while idling hot, that number was with the car in park; drop it into gear and the IAC goes up anywhere between 10-20.

Thanks in advance.

 

[mgmshar]

Hey....before I attempt to do this adjustment....I wanted to clarify....the IAC should be 10-40 in park right?

I just noticed even though my IAC reads 0 while idling hot, that number was with the car in park; drop it into gear and the IAC goes up anywhere between 10-20.

Thanks in advance.

Not sure if you read the IAC procedure that is in my earlier post, but you should target 10 to 40 while the car is in park.

Take a look at the instructions in the link in my earlier post. I think they are pretty clear.

Good luck,

 

["Turbo-T"]

Hey thanks, yes I must have over looked that.

Ok well I had to turn the throttle screw ccw 1 full turn. With the car running hot in park that put the IAC at 24.

Next I shut off the car and turned on just the ignition and set the TPS to .46. Once I tightened down both screws I went WOT via the gas pedal inside the car but the TPS won't go past 4.47. So I tried under the hood grabbing the throttle and going WOT, still 4.47 is as far as it will go.

Then I tried bumping down the TPS to .42 but still 4.47 is as high as it goes.

I even tried .40 but again I cannot achieve anything over 4.47.

As a last resort I pulled back the carpet and still 4.47 is as high as it goes.

The directions say 4.55 to 4.85.

Is 4.47 going to pose a problem?

 

["Turbo-T"]

Update....I fiddled around with the TPS some more and got it at .40 with no pedal and 4.76 when I floor it inside the car. Now we will see how much better she is or isn't.

 

[mgmshar]

I'm surprised that you had to turn the screw a full turn CCW to get the IAC readings right. That's a pretty big adjustment - must have been way off. Keep an eye on your IAC readings as you play around with the car for the next couple of days - you may find that you want to adjust it back somewhat (or maybe not), depending on where things settle out. With that much of an adjustment, I will be very surprised if you don't see a significantly lower flare-up tomorrow morning when you start it.

If your TPS is at 0.40 volts closed and 4.76 volts WOT, then you are doing great. That's a good range of values. When you start the car up tomorrow morning, double-check that your TPS volts stay below 0.45 with the car running and the throttle closed. The software in the ECU uses 0.45 volts as the threshold between "idle" and "throttle open". I usually make sure mine is below 0.44 volts with the car running just to have a little safety factor. The ECU can handle closed throttle readings below 0.45 volts, but not above 0.45 volts.

Let us know how it goes...

 

["Turbo-T"]

Thanks again, yep this morning it didn't idle as high as it did in the past. I forgot to check the rpms. I did though at lunch time when it was 73 degrees out. It now starts off at 1500 rpms. I didn't think though to check the IAC (no sleep last night) though i did check TPS, it goes between .42 and .44 with no pedal.

I also noticed when you rev the engine in park, the idle immediately comes back down, unlike in the past it would stay a tad high for a split second once you let off the pedal until it came down.

 

[mgmshar]

Thanks again, yep this morning it didn't idle as high as it did in the past. I forgot to check the rpms. I did though at lunch time when it was 73 degrees out. It now starts off at 1500 rpms. I didn't think though to check the IAC (no sleep last night) though i did check TPS, it goes between .42 and .44 with no pedal.

I also noticed when you rev the engine in park, the idle immediately comes back down, unlike in the past it would stay a tad high for a split second once you let off the pedal until it came down.

Sometimes GM did know what they were doing... Most chips use the GM code for controlling idle with just a few tweaks. So, if the TPS and IAC are set to the way GM wanted, the car should idle well. Glad things got better for your car! Enjoy it, and hopefully you learned a little bit along the way...