Hi, have a problem with a 1999 Trans Am that seems to increase idle intermittently when the air condition is turned on. Had the vehicle serviced by a speed shop who were utterly incompetent and couldn't resolve the issue. Did a comparison on a base idle and from what i understand most of the values are zeroed out concerning the AC. This is an area I need some help with the car runs great untiL you turn on the AC. I have VCM Editor file from the car I'm sure most of you are more experienced then I am. Thanks
Here is the file, not sure what you mean by a data log of the issue. Its a 1999 Trans Am that has an intermittent idle issue when the AC is turned on the idle increases from 700rpm to over 1000rpms I have attached Editor and scanner files .
Appreciate any help
Start logging IAC counts, and also ALL air channels.. You need to know if the PCM is commanding more air through any of the channels at A/C turn on... Then you can figure out how to address the problem. You can see that when the A/C comes on and the idle goes UP, that the PCM is pulling timing trying to get the idle speed back down.. WHY is it going up? That's the question.. Start monitoring more channels and hopefully you can drill down on the problem. You can also compare your settings in Hp Tuners/Engine/Toque model/Loss/A/C Compressor to see if they are the same as other cars that are the same year as yours.. Is this an all factory car? Changes to the intake, or throttle body, or?
It originally didn't have ram air, it does now. It has the factory intake and throttle body. Also I have a factory tune from the repository for the same year vehicle. Attachment of the factory tune on left mine on the right they are different.
In my mind, the ONLY way the engine RPM is increasing on A/C turn on is if the PCM is commanding it.. Otherwise, just turning the A/C on will usually result in a LOWER engine idle-as evidenced on carb equipped vehicles that DO NOT have an A/C solenoid that bumps idle. It HAS to be that the PCM is adding air-either through the IAC, or some other parameter.. If you are monitoring all the air channels hopefully you can see where the added air is coming from, and then work backwards from there. If the IAC counts increase right at A/C Turn on-you KNOW for sure that the increased idle is being commanded by the PCM. You know there is a channel for "A/C Request" right? You can actually monitor when you turn the A/C on so that it can be easily/readily witnessed in the scan log. That Torque Model that you searched, I don't fully understand how that affects engine RPM/IDLE, but I know for sure that it does. I have an LS swapped 94 Silverado that would essentially act like it was on cruise control when you turned the A/C on while driving down the road.. Seriously, it would hold 80+MPH with my foot off the gas.. As soon as I turned the A/C off, the truck would then start slowing down normally. Whichever of the two tables that you posted above has the LOWER settings, maybe try changing to those settings just for a test, burn it, test it, and see if that fixes it. Worse case, you can go back to what was there previously if it does not fix it, or makes it worse. I still think a good scan log of ALL the air channels to include IAC requested, IAC actual, A/C request, and all the usual stuff, and you will be able to see where the extra air is coming from. I don't think the PCM is adding spark at A/C turn on because it looks like the PCM is actually pulling spark trying to control the engine rpm. Anyway, please post back your results as I'd like to know what and how you end up fixing it. Makes it easier for people with this same problem in the future when they find posts like this that have the "fix".
Last edited by HATEHPTUNERS; 07-02-2023 at 03:58 AM.
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