P Mack

iTrader: (6)

I feel stupid that I never tried this before. I had my idle airflow tables pretty much dead on and my ve table was damn close. But still when doing parking lot maneuvering and coming off the gas and putting the clutch in, my rpms would drop. It severely pissed me off that I could watch my rpms sit at 300 rpm and never recover while rolling and my IAC/idle trims just sat there like they didn't care. As soon as I came to a stop the IAC would jump into action, but with any speed at all it wouldn't respond. Seems to me that the idle feedback loop should kick in depending on throttle position, not speed. But enough ranting, here is the solution. You have to trick the car into using the timing to recover from rpm dips instead since the IAC won't do it for you. You can do this by making the timing at rpms below your idle (400, 600, maybe 800) higher than the timing at your idle rpm. This applies to the main and base spark tables.

For example, the stock main spark table has 19, 22, and 27 degrees of advance for 400, 600, and 800 rpm. So when you're rolling at 5 mph and the rpms dip, the lower the rpms get, the lower your timing gets. Change it to 35, 30, 25 and like magic no more dipping. Bonus: you shouldn't even need to change your idle airflow tables if you only change the timing at rpms below your idle speed.

Cliff notes: more timing below idle rpm in main and base spark tables

Ls1Bait

Would this also work with LT1-EDIT. Also: When I accellerate the rpm's dip a little, would it cure this?

Patrick G

iTrader: (14)

Best tip EVER!

RedHardSupra

iTrader: (1)

HA! good one! got any before/after logs?
does it also work for when you go P->R? i've had some cars that would dip rpms when you go quickly P->R. either doing it slowly, or stopping for a moment in N fixed it, but I'd rather know a real solution.
great find!

MECHAM

iTrader: (4)

nice..im gonna try this...my rpms would sometimes hang a little high...sometimes dead on perfect...and sometimes dip down looowwww..and stay there...gonna try your theory.

ringram

Nice idea, might beat messing with spark under and overspeed tables and extra airflow tables etc.

Billiumss

iTrader: (11)

Makes sense, thanks for posting...

P Mack

iTrader: (6)

Edited to include the base spark table too. I changed the main and base at the same time. I'm not sure how well this applies to automatics but I think it would have the same effect. I'll try and look for an old log file for a before and after.

Michael55123

iTrader: (3)

I couldn't get it to help me. I changed the main, and base as well, atleast I think I did. Still surges down and sometimes dies. Am I missing something?

Mike

WAHUSKER

iTrader: (4)

Good tip.

I also just discovered the Idle Timing table. (never saw this with HPT 1.7) Mine was WAY off compared to the main spark tables. So I adjusted it to be similar & now the PCM doesn't hunt anymore. I presume it was using that, the idle would go up, and then it would go to the main table when the rpm hit 1200, drop timing (& rpm) & then revert back. It was bugging the **** outta me!!!

And I had a hot idle issue that I finally resolved. The car would idle fine when cool or warm, but when hot (like at the track) it would hunt & try to die. I went into the airflow table & multiplied the cells above 140* by 1.5 & now it's fine.

Now if I can just get the initial start-up worked out....runs like a carbed car with a stuck choke, so I have to rev it to 1500 for 15 sec & then it's fine. We think we found this too, just need to download the file & see.

SSpdDmon

iTrader: (10)

Unless your foot is on the gas or you're going faster than 4mph, the car uses the base spark tables. Once you give it gas or exceed 4mph, it switches to the high/low octane tables +/- an spark modifiers. The trick is to monitor what spark corrections are being made as well as what the IAC is doing to compenstate. Once you see where the over-corrections are being made, pull back on the numbers some. Just remember, small changes may take time. But, they'll get you there faster than big guesses.

WS6FirebirdTA00

iTrader: (33)

good post

I also found that the throttle cracker can screw with this. On my OS it wasnt bad but the 2002 it showed up really weird.

P Mack

iTrader: (6)

I should add that this may not help your issues if your idle airflow table is off. It's not really for surging, its for rpm drops that don't correct themselves.

Bink

Thanks P Mack!! Great tip . Mine does it at lights-at idle. I'll try it tomorrow. 2001 LS6 Fly by wire.

chrisj6

Also adjusting the stall saver may help you some. Just raise those values a little higher in and around your idle region.

Frost

iTrader: (45)

I am still sorta fighting this on one car myself... I know EXACTLY what you are saying...

Bink

Idle 1200 RPM. In gear and Park Idle spark is at 30*. At 800 RPM I bumped it to 34* and at 400 RPM I bumped it to 36*. Idle seems more stable at long lights and with AC on. 0 RPM 36*, also

SgtB

iTrader: (3)

I tried this and it almost completely fixed my issues.
Would anyone want to look at my tune and help me out some?

SSpdDmon

iTrader: (10)

Sure...post it up

CamTom12

iTrader: (1)

Good tip!