High idle will not back off 1200 RPM while stopping - TBSSOWNERS.com Forums - The #1 TrailblazerSS Dedicated Owners Forum

Warwagon · May-14-08, 09:00 PM

Anyone experience this or find a solution? (2007 TBSS 2WD bone stock.)

Randomly coming to a stop the usual add power till 1200 - 1500 RPM that the computer holds with no throttle will not back off. (I know it is normal to be cruising along and have the computer holding 1200 RPMs with power added, until the brakes are tapped. Then the computer backs off.)

I have verified this high idle by shifting to neutral and the RPM Jumps to 2000. It will hang there for 30 seconds and then slowly drop or drop instantly if I come to a complete stop.

Result = Excessive braking force needed to stop.

If I am compression braking in 2nd gear it will slow down till the 2K RPM mark then it will accelerate. Shift to Neutral again hangs it at 2K RPM.

The AC being on makes this worse. The AC clutch kicking in results in 2 misfires being picked up on a random cylinder.

100% stock and stock tune. I did try BBA's tune and get the same result.
Water spray on the intake manifold did not indicate a vacuum leak.

Dealer has a log from their data logger showing this and think they can't duplicate the problem. IE they don't have a clue.

Log shows the computer has a great idea. While slowing down in 2nd gear lets open up the throttle (ETC)! We go from 29MPH to 32 MPH with 16% to 21% ETC!

My guess is it is not seeing the brake applied or thinks the engine is going to stall.

Logs of this are here:

http://www.hptuners.com/forum/showth...977#post128977

**Ohnoitsa60** · May-14-08, 09:20 PM

Are you 100% sure there isnt a vac leak somewhere??

I had the same issue with my Camaro, it drove me NUTS for a few months. It ended up being a Vac leak.

Did you try spraying the intake down with carb cleaner while it is running?

Do you have any mods? Cai ? Everything is tight? No air getting past maf that is not being metered?

Warwagon · May-14-08, 10:12 PM

Makes sense. ESP with 11% Positive LTFT.

I used water to check for vac leaks. I'll try the carb cleaner. It is possible there is a crankcase air leak. But I don't see any oil leaks from the 2nd dipstick plug etc. I'll plug hoses next in case the booster etc is leaking.

**Ohnoitsa60** · May-15-08, 06:09 PM

Any mods?

When you are using the carb cleaner idle will fluctuate and theres your leak.

BigRobSA · May-17-08, 09:11 AM

This may sound stupid, but do you have big feet?

I ask because I have similar things happen...but it's due to my superhuge, clodhopper clownfeet.

Size 16.

Sometimes when I'm braking, I notice the rpms are still up. I look at the pedals, and my foot is still touching/pushing on the accelerator.

These pedals are WAY close together.

larryv · May-17-08, 02:52 PM

Mine does the same thing! I took it into the dealership and they test drove it. Did a reprogram and the same thing happens. The mechanic told me thats the way the SS is supposed to be. I told him that it wasn't like that before they put in a new battery. Im going to another dealership and test drive an SS and see for myself. I know for sure it was not like this. If this is they way it should be then I will need front brakes real quick. I will post as soon as I find a SS to test drive.

BlkSSon22s · May-17-08, 03:38 PM

Quote:

Originally Posted by BigRobSA

but it's due to my superhuge, clodhopper clownfeet.

Size 16.

lmfao

Warwagon · May-17-08, 08:57 PM

Unless there is a common failure affecting a few SS's: Something is screwed up in the computer calibration that is commanding too much airflow via opening the ETC, throttle. I watch the ETC % open up while compression braking in 2nd gear. It is like the idle air motor is sticking. Yet we don't have an idle air motor. Someone on another forum said this went away with a tune.

The dealer, service manager, says this is normal as in the SS's tend to do this. This doesn't make it right. I do not know if it is possible or worth the trouble to beat a new ECM cal out of GM. Yet this seams to be a safety issue.

I used a can of carb cleaner on the intake, brake booster, evap, pvc piping. I did not notice any Idle change at all, but I FEEL BETTER for some reason...

I hear you on the big feet. I am logging the pedal position -- it goes to 0 and stays there. So not the issue. Neither is the not seeing the brake as I logged those PID's today. Cruise shuts off with brake tap...

The simple act of turning on the AC makes the problem appear. It does not do this with the AC off. I have the auto temperature controls.

This problem is not the result of any repairs - It just appeared. Not only did it appear - it is getting worse and easy to duplicate. My 2006 did not do this - it would back off the second you tapped the brakes. My 2006 also surged when first started and is the reason I have a 2007 now.

I did notice the #6 and #8 plug wires are hitting the heater hose heat shielding. The wires are black in this area, but I don't see any holes blown in them from arcing.

FWIW: 91 octane gas is being used.

No mods. The Yank 3200 converter and electric fans (I am still debating the electric fans) are sitting on a shelf waiting for all the bugs to be worked out before I void the warranty and drop a built trans to take it. With this not being solved and the rear end noise not being solved with new parts thrown at it the warranty is of little value on the SS.

Warwagon · May-17-08, 09:16 PM

Photo of spark wire and area it is rubbing. #6 wire off and #8 is on in photos.

Warwagon · May-18-08, 06:06 PM

Got lucky and found the solution...

Cut the values in the Final airflow vs. RPM vs. Gear by 40%. HPTuners has a bug that will not multiply by -.4. So I had to put this value in an excel table full of the number -40 and cut and paste special %.

We will see how it turns out but so far testing shows it is having an effect.

Looks like I will return the lower range below 1000 RPM to stock. While the engine was colder, 162 degrees, I logged 546 RPM while in gear and the engine shuddered. I am guessing it was going to stall, but the computer opened the throttle and recovered it. Otherwise it completely backs off when it should! Talk about a Sunday Drive all day today...

Warwagon · May-24-08, 11:25 PM

"When I say Whoa, I mean WHOA!" Solution to the idle/throttle will not back off issue.

Change two tables. This applies to 2007 TBSS, other years are slightly different.
Main difference is by cutting down the Final Idle Airflow Minimum vs. RPM vs. Gear.

Table #1 to change: Final Idle Airflow Minimum vs. RPM vs. Gear (This could use further reduction around 2000 RPM in 2nd gear - but works well enough now.)

Yes this looks nasty on a curve, but it works, the problem is at 1200RPM on up. I cut the entire table and the engine would start to stall at times hitting 535 RPM and having the throttle opened by the stall saver program. So I put in stock values in the idle speed area and smoothed out up to 1000 RPM. Then I broke the kneecaps out of any throttle idle air over 1000 RPM.

Whack the entire table above 600 RPM by 40%. (Select all values and multiply by .6 )
Use 6.30 for the 800 and 1000 RPM rows.

Table #2 Idle Proportional Airflow vs. RPM Error:

Get rid of the zero values in the Idle Proportional Airflow vs. RPM Error: coastdown table from -256 through the -64 RPM

Copy the normal table from -256 to -64 and paste to the Coastdown table.
Select -256 and make it 2. then multiply that value by -1 to make it negative. (I tested -1.5996 and it wasn't enough.) Do not put a "-2" in the other -256 tables like I tried. It will cause your LTFT's to go -17 the second the throttle snaps closed.

This will allow the throttle to snap closed with your foot input. No hang at 1200 RPM and slow down to idle when the PCM feels like it. Rev it in neutral and watch the RPM drop back to 650 RPM immediately.

Test this out by slowing down from 40 MPH in 2nd gear on a slight hill. DFCO will end about 22 MPH and give you a slight bump before finishing the slowdown. Without this 'fix' you will hit 2K RPM and the engine will start to take off. Neutral drop and you get 2K RPM till you stop or 30 seconds pass. With the fix the RPM will drop immediately when shifted to Neutral.

Side effects: You get a driveline 'clunk' when going from no throttle to applying throttle. From engine dragging you down to hitting the other side as you accelerate. Perhaps this clunk elimination is why these tables are so tilted to keeping some engine power on?

2nd Side effect: Better cackle - backfire with trailer.
Down shifting into 2nd with a trailer can be noisy. The cackle has improved before DFCO and a couple down shifts have resulted in short duration loud backfire cackles 1 sec or so. Might back off the Coastdown table if this bugs you.

If anyone has a better idea I am all ears!

related thread:
http://www.tbssowners.com/forums/showthread.php?t=12173

**speedmaster747** · May-25-08, 01:28 AM

Thanks for the info!

BBA · May-28-08, 05:39 PM

Quote:

Originally Posted by Warwagon

"When I say Whoa, I mean WHOA!" Solution to the idle/throttle will not back off issue.

Change two tables. This applies to 2007 TBSS, other years are slightly different.
Main difference is by cutting down the Final Idle Airflow Minimum vs. RPM vs. Gear.

Table #1 to change: Final Idle Airflow Minimum vs. RPM vs. Gear (This could use further reduction around 2000 RPM in 2nd gear - but works well enough now.)

Yes this looks nasty on a curve, but it works, the problem is at 1200RPM on up. I cut the entire table and the engine would start to stall at times hitting 535 RPM and having the throttle opened by the stall saver program. So I put in stock values in the idle speed area and smoothed out up to 1000 RPM. Then I broke the kneecaps out of any throttle idle air over 1000 RPM.

Whack the entire table above 600 RPM by 40%. (Select all values and multiply by .6 )
Use 6.30 for the 800 and 1000 RPM rows.

Table #2 Idle Proportional Airflow vs. RPM Error:

Get rid of the zero values in the Idle Proportional Airflow vs. RPM Error: coastdown table from -256 through the -64 RPM

Copy the normal table from -256 to -64 and paste to the Coastdown table.
Select -256 and make it 2. then multiply that value by -1 to make it negative. (I tested -1.5996 and it wasn't enough.) Do not put a "-2" in the other -256 tables like I tried. It will cause your LTFT's to go -17 the second the throttle snaps closed.

This will allow the throttle to snap closed with your foot input. No hang at 1200 RPM and slow down to idle when the PCM feels like it. Rev it in neutral and watch the RPM drop back to 650 RPM immediately.

Test this out by slowing down from 40 MPH in 2nd gear on a slight hill. DFCO will end about 22 MPH and give you a slight bump before finishing the slowdown. Without this 'fix' you will hit 2K RPM and the engine will start to take off. Neutral drop and you get 2K RPM till you stop or 30 seconds pass. With the fix the RPM will drop immediately when shifted to Neutral.

Side effects: You get a driveline 'clunk' when going from no throttle to applying throttle. From engine dragging you down to hitting the other side as you accelerate. Perhaps this clunk elimination is why these tables are so tilted to keeping some engine power on?

2nd Side effect: Better cackle - backfire with trailer.
Down shifting into 2nd with a trailer can be noisy. The cackle has improved before DFCO and a couple down shifts have resulted in short duration loud backfire cackles 1 sec or so. Might back off the Coastdown table if this bugs you.

If anyone has a better idea I am all ears!

related thread:
http://www.tbssowners.com/forums/showthread.php?t=12173

I'm doing this to my tune but something is weird...when I first looked at your pictures, your Final Idle Airflow Minimum Vs. RPM Vs. Gear table was missing teh 2500 RPM column, the second time I opened it it had that column...maybe it's an AMD jpg bug? LOL.

It's all good, now I just have to flash it.

Silver06SS · Jun-03-08, 06:37 AM

Quote:

Originally Posted by Warwagon

"When I say Whoa, I mean WHOA!" Solution to the idle/throttle will not back off issue.

Change two tables. This applies to 2007 TBSS, other years are slightly different.
Main difference is by cutting down the Final Idle Airflow Minimum vs. RPM vs. Gear.

Table #1 to change: Final Idle Airflow Minimum vs. RPM vs. Gear (This could use further reduction around 2000 RPM in 2nd gear - but works well enough now.)

Yes this looks nasty on a curve, but it works, the problem is at 1200RPM on up. I cut the entire table and the engine would start to stall at times hitting 535 RPM and having the throttle opened by the stall saver program. So I put in stock values in the idle speed area and smoothed out up to 1000 RPM. Then I broke the kneecaps out of any throttle idle air over 1000 RPM.

Whack the entire table above 600 RPM by 40%. (Select all values and multiply by .6 )
Use 6.30 for the 800 and 1000 RPM rows.

Table #2 Idle Proportional Airflow vs. RPM Error:

Get rid of the zero values in the Idle Proportional Airflow vs. RPM Error: coastdown table from -256 through the -64 RPM

Copy the normal table from -256 to -64 and paste to the Coastdown table.
Select -256 and make it 2. then multiply that value by -1 to make it negative. (I tested -1.5996 and it wasn't enough.) Do not put a "-2" in the other -256 tables like I tried. It will cause your LTFT's to go -17 the second the throttle snaps closed.

This will allow the throttle to snap closed with your foot input. No hang at 1200 RPM and slow down to idle when the PCM feels like it. Rev it in neutral and watch the RPM drop back to 650 RPM immediately.

Test this out by slowing down from 40 MPH in 2nd gear on a slight hill. DFCO will end about 22 MPH and give you a slight bump before finishing the slowdown. Without this 'fix' you will hit 2K RPM and the engine will start to take off. Neutral drop and you get 2K RPM till you stop or 30 seconds pass. With the fix the RPM will drop immediately when shifted to Neutral.

Side effects: You get a driveline 'clunk' when going from no throttle to applying throttle. From engine dragging you down to hitting the other side as you accelerate. Perhaps this clunk elimination is why these tables are so tilted to keeping some engine power on?

2nd Side effect: Better cackle - backfire with trailer.
Down shifting into 2nd with a trailer can be noisy. The cackle has improved before DFCO and a couple down shifts have resulted in short duration loud backfire cackles 1 sec or so. Might back off the Coastdown table if this bugs you.

If anyone has a better idea I am all ears!

related thread:
http://www.tbssowners.com/forums/showthread.php?t=12173

Doesn't make sense on your idle proportion table....in that table, when your rpm error is high (the negative numbers), the the computer subtracts the said amount of air from the final minimum airflow...but with your table, when you subtract a negative number, you are actually adding to the final minimum airflow. But who knows GM's math....

Warwagon · Jun-07-08, 01:27 AM

Quote:

Originally Posted by Silver06SS

Doesn't make sense on your idle proportion table....in that table, when your rpm error is high (the negative numbers), the the computer subtracts the said amount of air from the final minimum airflow...but with your table, when you subtract a negative number, you are actually adding to the final minimum airflow. But who knows GM's math....

Wild ass guess:

You have a 2006. I have a 2007. They changed the ECM for 2007.

Desired Idle RPM - Actual RPM = RPM error

650 - 700 = -50 so computer goes to -50 table area and get value.
650 - 600 = 50 so computer goes to 50 table area and get value.

The 2006 keeps the sign for the speed error. Speed error variable = 1 or -1
Airflow + (speed error variable * table value)

The 2007 ECM just adds no matter if you are under or overspeed. So you have to give it a negative value to cut airflow.

airflow + table value = airflow.

Computers can only add. They have to get fancy to subtract, multiply, and divide. Just think fewest clock cycles to add.

Phildo · Aug-02-08, 12:29 PM

little old, but you mentioned the "driveline clunk" i have experienced this since day one of having the truck, tuned or not. is this clunk anything to be worried about? it just really does not sound good at all...

Phildo · Aug-26-08, 03:13 PM

just FYI, I finally got my SS in about the clunk and a groan in the rear end. so far they found the front diff to be bad. they said they heard a noise in the front, checked it, and found metal shavings inside. once they get that fixed they are going to check out the rear diff again.

Warwagon · Oct-18-08, 11:07 AM

Although the above fix helps out it still can give you unwanted power while stopping from time to time. Just not as long as stock.

Clarke295 · Nov-12-08, 11:23 AM

Quote:

Originally Posted by Warwagon

Anyone experience this or find a solution? (2007 TBSS 2WD bone stock.)

Randomly coming to a stop the usual add power till 1200 - 1500 RPM that the computer holds with no throttle will not back off. (I know it is normal to be cruising along and have the computer holding 1200 RPMs with power added, until the brakes are tapped. Then the computer backs off.)

I have verified this high idle by shifting to neutral and the RPM Jumps to 2000. It will hang there for 30 seconds and then slowly drop or drop instantly if I come to a complete stop.

Result = Excessive braking force needed to stop.

If I am compression braking in 2nd gear it will slow down till the 2K RPM mark then it will accelerate. Shift to Neutral again hangs it at 2K RPM.

The AC being on makes this worse. The AC clutch kicking in results in 2 misfires being picked up on a random cylinder.

100% stock and stock tune. I did try BBA's tune and get the same result.
Water spray on the intake manifold did not indicate a vacuum leak.

Dealer has a log from their data logger showing this and think they can't duplicate the problem. IE they don't have a clue.

Log shows the computer has a great idea. While slowing down in 2nd gear lets open up the throttle (ETC)! We go from 29MPH to 32 MPH with 16% to 21% ETC!

My guess is it is not seeing the brake applied or thinks the engine is going to stall.

Logs of this are here:

http://www.hptuners.com/forum/showth...977#post128977

I have the exact same model as you and mine does the same thing at random times; its preety annoying

**SSilveronyxx** · Nov-28-08, 06:51 PM

Ironically, a day after I came across this post mine just started doing it on Monday. Oddly it happened just after I had the dual exhaust work done and a road trip. I thought to myself, "DAMN this think picked up a whole load of ballz...it's fighting the brakes!!!" lol

But, then I realized that it was intermittent. Oddly, it only seems to be happening up here in NJ and right when the cold is hitting. As I'm hitting 30k soon I'm thinking maybe it needs a good TB cleaning.