HotZ06

I live in a neighbourhood with lots of speed bumps. So I have to cruise around at 10mph and de-clutching to navigate the bumps. I notice that if I'm running at around 1500 rpm in 1st gear and de-clutch, the engine seems to take control of itself and slowly slow down. At the same time, there is a certain amount of mechanical activity and jerkiness going on as though the transmission is having difficulties transitioning from drive to non-drive. Anyone else noticed this?

Nanook

Did you do Jim's tune ?

HotZ06

Sorry, don't know who Jim is.

Supersubes

Jim as in Jim Hall of Halltech. As you may know cars that are throttle by wire give the ECU some freedom when controlling idle and such. You are just feeling the car trying to stabilize the idle. There is lash in the drivetrain which you can work around by staying in the friction zone of the clutch.

Nanook

Jim's tune tends to tweak throttle response and driveability

AtlBlkZ06

I know exactly what you're talking about.

It happened to me as I was entering the garage slowly. As I clutched and braked, the engine accelerated to 1200RPMs even though my foot was on the brake. I was wondering about this too, thanks for posting.

I think it does that because the first gear is pretty tall and its just a part of the idle control.

1962 corvette

Me too - I have a November 07' production car - noticed the 'over-run' on a road trip last month
RPM hangs up' around 1100 - 1300 it seems after light throttle

Anyone asked a dealer about this ?

Bill

Poor-sha

I know with some (most?) of e-gas cars they come programmed to slowly bring down the revs when you push in the clutch. It has something to do with burning all the fuel that would normally get expelled out of the exhaust.

I'm probably not explaining that real well but basically it's for emissions. On some cars it can make smooth shifts difficult to do quickly.

ayousef

this will be a real problem when you're trying to brake hard and the RPM hangs at that position, happened to me be4 the supercharger install, my tune is so messed up now, the rpms dont hang up high, they drop below idle and cause more trouble lol.

HotZ06

I understand your explanation and have had cars that do as you say for emission reasons. However, that used to be a pretty consistent effect. This is inconsistent, as is the transmission system 'backlash'. The only 'fix' I've managed to get to work is to de-clutch early. I've not had chance to see if it varies with air/engine temperature or other variables (I live in a hot part of the world).

Maybe if Jim is reading these threads he can shed some light.

Ayousef, have you found a fix for the 'Overspeed Warning' on the driver display yet?

tjwong

I beleive what you are describing is what is commonly called throttle follower action. In an old school carbureted engine, it was called the dash-pot. The throttle follower delays the closure of the throttle plate as you let off the throttle, another action is called the throttle cracker, most of these strategies are designed to keep an engine from stalling when the throttle is snapped closed. Another reason is emissions. In the LS2/LS3/LS7 calibrations, there is a table the can address this very issue. In HPT its call the "final air flow vs gear vs RPM"

HotZ06

Interesting - but can anything be done to change it? Or rather change the annoying resultant effect?

outnumbered

My 06Z does not do it.

But my Trailblazer SS does it and it is an automatic. Can be quite scary in the process of stopping and the throttle is working against the brakes and you are off the gas.

At least with the clutch you aren't being pushed.

tjwong

Well, I will know this morning. I am in the process of finalizing a tune on an 07 Z today. One of his complaints is this "cruise control" feeling on decel. In the older calibrations reducing the amount of air flow in a given RPM cell or in some calibration vehicle speed or TPS position will cure the issue. My personal car does this as well but not to the extent that it bothers me.

tjwong

I tuned a Trailblazer SS earlier this summer, I installed a ported intake, a LS2 90mm TB and tuning. The owners major complaint about the truck was just this issue. It was cured by tweaking the air flow tables.

outnumbered

Thanks, I thought a tune would fix it. It also stinks like after WOT.

It is like there is a huge amount of unburned fuel or something. The Corvette does this a little but the SS is awful. I assumed the tune is very rich.
I bet the SS runs better now.

tjwong

Yes,a the PE (WOT) enrichment is very rich on the SS, try a LS2 90mm TB it greatly enhances throttle response!

itsaz

Were you able to make any improvements? I'm asking because I have the same problem in my new 08. This was not an issue in my 06.

Tom

tjwong

It was definitely an improvement using the table I posted above.

HotZ06

Congratulations on the improvement. I'm pretty dumb at these things so what di I do? Thanks.