I have an issue which occurs about 80% of the time.

When I take my foot off the gas to decellerate/brake, the engine RPM's doesn't drop as fast as it should making braking harder and also wasting gas and doing who knows what to the transmission. Allot of the time even when I do stop at a light, the RPM's will be over a grand and after a few moments then drop to the normal level. This happens no matter if I was going 25mph, or 65mph. I can also notice this when revving it in park or neutral. I can hit the gas real quick but the engine take FOREVER to idle back down. *Sometimes* if I tap the gas pedal super fast, the idle will come down like normal too while decellerating.

It seems to me it may be my IAC? Maybe, big maybe, the TPS sensor? If I plug my notebook into the ALDL, what kind of step numbers should my IAC be reading? It also seems I remember I can adjust the IAC decay rate in the PROM? Would that affect this or just the IAC following steps while driving and not under decelleration?

Any ideas?

 

Its normal on my car, but I have a stick shift. At idle, your IAC steps should be as low as possible ( less than 10 ) to allow the IAC a full range of motion to open up and get more air in. Adjusting the IAC decay rate in the prom will affect this. How long has it been going on? Have you used a different prom or modified your stock one?

 

I had my IAC working perfectly and I haven't changed anything in almost 2 years. About a year ago is when this started happening. Since I didn't change anything in the PROM I was simply guessing that something got out of whack. I don't want to try adjusting the PROM to compensate until I've resolved what it is.

I sold my notebook and need to get another one just so I can see better what's going on and make changes if needed.

 

I have the same problem on my rs. What is the solution? I also have a 5-speed, does not bother the trans., but it is annoying.

 

the first thing i would look at is if the throttle cable was sticking. just rev it from the throttle body and make sure the spring is pulling the throttle back down right away. after that i would look into cleaning the IAC maybe or checking to make sure it still adjusts properly. just some ideas, hope it might help a little

 

I have a very similar situation, but I'm attributing it to my modified, self-programmed GM engine computer (1227730, $8D). This didn't occur with my TPI when it was stock or even after I initially started modifying code, so I know it's something I introduced myself recently. So now I'm just trying to isolate this behavior to which program change I introduced.

once the engine goes to closed loop, and I've come to a complete stop so that the VSS is at zero, then the engine idle will step down to my preset programmed idle setting (750rpm), takes a couple seconds while I'm stopped. If I don't wait for that and start rolling again (VSS not at 0) before the idle settles down, then it never gets down to idle at successive stops, and very annoyingly stays above 1,000rpm, sometimes 1,200 or higher. I have to do this drill only once per engine run cycle, thereafter, it idles appropriately low at stops.

incidentally, my throttle body and IAC are relatively new and my lap-top and data logger show that those are fine (responding to ECM commands), so I believe it's in the program code.

I had to get into programming my GM ECM when I put a new cam in it and the mail-order chip designed for my engine parameters ran very poorly.

 

I'm having a similar problem, during engine tear down I noticed a intake leak.
Hope my problem is due to a vacuum problem. Do all above posts and if problem still exists check for vacuum leak.

Good luck.

 

I have been chasing the same problem, for four years plus. I seem to think it has to do w/ my park/neutral setting. I never had a problem until i did my 5 speed swap. Since then it has been maddening. I am going to try a different T/B here soon, and then maybe try burning a new chip, with any auto tranny info deleted. I am just now getting the chip stuff....
I will subscribe to this though,everyone thinks i want to race at the stop light. Gimme a break.
shawn