Clean97Z

Could too much spark advance cause the car to have idling issues when coming to a stop? I am not seeing any knock retard, but the idle jumps to about 1700 when I come to a stop and then gradually settles back out to 950 while sitting and idling. If I am rolling and put in the clutch it will settle out at ~1700. I followed the steps to adjust closed throttle spark advance looking for the lowest MAP value, and that is working as expected. I also blended in the main spark advance table based off of that. The car has a XFI280 cam, p&p heads, LT's, no cats, Gmmg, 52mm TB. The closed throttle spark advance is set at 31 deg from 400 - 1200.

I am wondering if the TPS is bad, or just not very good. Last year I had a shop look at the some of the cold start idling issues while the oil cooler was being installed, and they adjusted the throttle body to reduce the IAC counts. At that same time my original TPS went bad. I purchased a new one from NAPA to ensure that was the issue, and it seemed to fix the issue. But after that it never seemed to respond as well when coming to a stop, where it would tend to hang a little when cold but nothing like I am seeing now.

I may put the car back to the stock tune, or back off the closed throttle spark advance to see if it changes anything as well. I may also pick up a TPS sensor from GM because I don't have much faith in aftermarket sensors.

bobdec

Not sure how it will run on a 'stock' tune going back to 20* timing and 650 idle RPM with that cam (230/236 113 LSA). Pretty sure your current timing at closed TPS setting should not be causing high idle after deceleration. Do you have logging capability ? Things like TPS voltage and IAC counts when it's running at 1700 would help isolate a TPS, IAC, vac leak problem. Are you sure the 52 mm TB IAC port is feeding the intake idle air runners w/o leaking/channeling any air through the blade area. There are some idle problems due to this w/aftermarket TB's.

Clean97Z

I do have data logging capabilities, so I will do that. I tried this morning but my ol' XP laptop was not cooperating. It is possible that the TB is the issue. I will log those items on my way home today to see if it happens again. After I loaded the program last night I drove the car and had no issues, but it was a little cooler this morning.

I am not 100% sure what the stock idle was (it has been too many years), but the idle was set to 1000rpm with about 19 or 20%u00B0 of timing. But I had the car tuned by a local shop after the cam install 6 years ago, so I am not sure if they messed with anything. It always seemed to idle fine when warm, but smelled really rich. The increased timing has cleaned up a good portion of the smell, and the car seems to run a lot better part throttle, so I if I can get this issue addressed I can continue with the main spark advance table.

I did try setting the idle to 900, and the car would stumble a little when coming to a stop at times.

Thanks for the help.

bobdec

Problem is you are OBD2 so I'm not going to be able to look at logs unless you have them in CSV format. OBD1 has a Throttle follower IAC Offset that jacks up the closed TPS RPM a couple hundred when speed is over 12 MPH. and slowly lowers it back to idle speed as the speedometer drops to 0 MPH. This is to prevent stalling when coming to a stop. OBD2 should have something similar. Easy to test just coast at 12 MPH w/clutch in or in N and RPM s/b 2-300 higher than idle, as you slow RPM should drop. If you IAC or idle is messed up it may be causing the stalls.

Clean97Z

I can post the recent log in CSV format, but after logging it on Friday I noticed the TPS was reading .7 volts with at idle. From what I can tell that is normal?? If not, then I found the issue. Otherwise I might adjust the throttle screw which is what the shop did last year.

I also believe you are correct about the 12mph offset because I have noticed that while driving in the past. I haven't looked for it in the tune software yet though.


Thanks.

bobdec

I believe GM specs on closed TPS voltage is something like 0.45-0.75 so you are OK. I ran mine at 7.2 for a while. Prior to playing with closed TPS timing I closed up the TB stop to 0.60 volts and my IAC counts now run in the low 40's with a MAP of 39-42 Kpa at idle with a Hot Cam (214/228 112 LSA) and 1.6 RR's (.525/.525 lift).

Clean97Z

I attached a link to the CSV file. You can see an example of the problem starting at row 300. No throttle input but the car is idling at ~1700 and then very gradually comes down. I did notice the IAC counts seem quite high.

CSV File

I should also mention that the high idle doesn't start until I actually start moving in the car. If I just start it and let it idle, it is just fine.