Stock Tune: Why does my idle dip so low?
04-16-2008, 03:43 PM
#1
Stock Tune: Why does my idle dip so low? My car is a bone stock 98 Trans Am WS6 M6. 40K miles on it.
Here is the problem scenario:
Steady cruising at 25mph in 2nd gear
Take foot off gas, leaving clutch engaged
engine-brake down to about 5-10mph
disengage clutch and coast
idle drops to 500rpm, stumbles, and recovers to 1000rpm
Interesting points:
It only happens when I engine-brake. For example, if I were to disengage the clutch immediately after letting off the gas at 25mph, the idle drops smoothly and stays at about 1100-1200rpm until I'm stopped, and then it settles down to 1000rpm once the car is completely stopped.
The problem occurs in any gear in which I'm engine-braking
Basically if the car is moving with the clutch engaged and my foot off the gas pedal, as soon as I disengage the clutch the RPMs drop like a rock to about 500rpm before the computer "catches" it and brings it back up to normal.
The car has always done this. What the hell is going on, and how do I fix it?
I HAVE HP TUNERS but I have not messed with anything (yet).
PLEASE HELP ME!
Here is the problem scenario:
Steady cruising at 25mph in 2nd gear
Take foot off gas, leaving clutch engaged
engine-brake down to about 5-10mph
disengage clutch and coast
idle drops to 500rpm, stumbles, and recovers to 1000rpm
Interesting points:
It only happens when I engine-brake. For example, if I were to disengage the clutch immediately after letting off the gas at 25mph, the idle drops smoothly and stays at about 1100-1200rpm until I'm stopped, and then it settles down to 1000rpm once the car is completely stopped.
The problem occurs in any gear in which I'm engine-braking
Basically if the car is moving with the clutch engaged and my foot off the gas pedal, as soon as I disengage the clutch the RPMs drop like a rock to about 500rpm before the computer "catches" it and brings it back up to normal.
The car has always done this. What the hell is going on, and how do I fix it?
I HAVE HP TUNERS but I have not messed with anything (yet).
PLEASE HELP ME!
04-17-2008, 05:13 PM
#5
Many M6 cars do this from the factory at certain speeds.
04-18-2008, 07:21 AM
#7
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Just make a log, log things like RPM, MPH, etc and then as you go back, look for the dips. Look at the RPMs and MPHs they occur at. Match em up with the TC table's cells and possibly adjacent cells and increase, rinse, and repeat. There is no other way to "tune" that table other than trial & error. Start with adding larger units where you have problems like .5 to .75g
Many M6 cars do this from the factory at certain speeds.
Many M6 cars do this from the factory at certain speeds.
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04-29-2008, 07:26 PM
#10
There are two ways to go on this. You can add to the throttle cracker like you did, or you can get rid of the throttle cracker completely so the pcm goes into its idle routine as soon as you close the throttle even if you're still moving. I opted for the second way.
04-30-2008, 07:57 AM
#12
Big picture, this is getting you into a closed loop idle system sooner (not closed loop fueling, closed loop IAC based on RPM) I set the enable speed to 10 mph and zeroed out almost every cell in the airflow table, except for a few "parking lot speed" cells. So now say you're getting of the highway and you shift to neutral. If your throttle cracker is too low or high your rpms could dip to 400 rpm or hang at 1500 rpm all and the pcm won't make a correction until you get to 1 mph, where the throttle cracker disable speed is. The pcm doesn't think it needs to make a correction because it's not in idle mode. This is really mostly a problem when you turn your A/C on or off while cruising so your idle trims don't have a chance to react till you come to a complete stop.
The only real reason to do this instead of massage your throttle cracker table is if you're going to zero out your throttle cracker cells anyway so your rpms drop faster between shifts. I hated how gm built in the throttle follower and cracker for slow rpm drops for easier shifts. So now the engine revs down faster and I'm in gear faster and easier.
The only real reason to do this instead of massage your throttle cracker table is if you're going to zero out your throttle cracker cells anyway so your rpms drop faster between shifts. I hated how gm built in the throttle follower and cracker for slow rpm drops for easier shifts. So now the engine revs down faster and I'm in gear faster and easier.
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kyoytey1693 (10-15-2021)
11-05-2008, 03:29 PM
#13
I know this thread is old, but I wanted to post an update. It turns out the problem I was experiencing was not from the throttle cracker. Fooling with that table did mask the problem, but that is not what was causing it. It turns out that it was the DFCO exit table that needed to be adjusted. Basically, when I was engine-braking up to a stop sign, the fuel would stay cut off all the way down to 1200rpm. I was hitting the clutch pedal at about 1400rpm, which would leave the motor still in DFCO and let the rpms drop very quickly before the PCM could correct. I raised the DFCO exit value to 1800rpm, and put my idle and throttle cracker back to stock. The problem is now completely gone. I hope this helps somebody!
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kyoytey1693 (10-15-2021)
11-05-2008, 07:56 PM
#14
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I know this thread is old, but I wanted to post an update. It turns out the problem I was experiencing was not from the throttle cracker. Fooling with that table did mask the problem, but that is not what was causing it. It turns out that it was the DFCO exit table that needed to be adjusted. Basically, when I was engine-braking up to a stop sign, the fuel would stay cut off all the way down to 1200rpm. I was hitting the clutch pedal at about 1400rpm, which would leave the motor still in DFCO and let the rpms drop very quickly before the PCM could correct. I raised the DFCO exit value to 1800rpm, and put my idle and throttle cracker back to stock. The problem is now completely gone. I hope this helps somebody!
