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'96 GMC Sierra 5.7L Vortec annoyance


quick69gto
09-18-2009, 06:28 PM
As the title says, I have a '96 GMC Sierra XC Z71 4X4 pickup with a 5.7L Vortec/4L60E automatic that has a rather annoying engine braking behavior.
Basically, when you lift off the gas pedal the truck will maintain it's speed, especially below 55mph. On a down hill grade the truck will actually speed up.
Above 55mph, it will occasionally decelerate (engine brake) but only about 10 to 20% of the time, and about 10 seconds after you lift off the gas.
It gets very annoying to have to use the brakes to scrub off speed when I need to. When I use the truck to trailer something, it gets very annoying.

I am a 30 year veteran mechanic so I've searched for any problems with the truck (vacuum leaks, etc.) but I have found nothing. It idles perfect, it shifts perfect, and at low speeds it acts fine. It usually acts like I described when it's been at a steady speed for a few hundred feet or more.
The truck is in perfect shape (except for that one annoyance) with just over 146,000 miles on it.
I've done some research and it could possibly be tied to the ECU programming for emissions sake. There might be a recall to fix the problem, However, I ran into a dead end concerning that.

Anyway, I hope I've described the problem sufficiently, and hopefully someone could shed some light on the subject.

Thanks,
Quick

maxwedge
09-18-2009, 06:52 PM
Are you saying the idle will not drop down, the trans is coming out of lock up, not clear what the true symptoms are here. In o/d all cars/trucks experience very little engine braking as a result of the gearing and especially when the tcc disengages. My TB does basically the same thing, I downshift to 3rd to get some engine braking, now of course if this is a new symptom, a full scan with a trans scanner will look at what is happening in the trans during this event.

Godofwarmike
09-18-2009, 09:07 PM
My car does this if i let off the gas too slow.

all i do is tap the gas real quick and it usually engine brakes.

quick69gto
09-19-2009, 01:16 PM
It's not a new problem. It's done this since I purchased the truck two years ago with 130,000 miles on the clock.
It's almost like the throttle body blades get stuck just above idle. I've cleaned the throttle body and I have tried tapping the gas/ brake pedal. The throttle body does close fully.
You can really feel it at 75mph or above on the highway. If you lift the throttle, it will maintain it's speed until about 10 seconds after, then it's almost like you hit a jake brake.
I've owned other 4 speed auto cars and SUVs (imports) but they decelerated immediately after lifting off the gas. My GMC seems to have a delay before the engine will fully decelerate.
I don't know how else to explain it. It seems the truck will wait a while before it closes the throttle or slow the fuel delivery to the injectors.
By the way, dropping down to third usually helps on the highway, but it doesn't at city speeds (below 50mph).
I have adapted my driving habits to the trucks behavior so it's not a big problem.
It's just a small annoyance.
I just thought some of you might be aware of this behavior.
I may start playing around with some performance programmers to see if they will cure the annoyance.

Anyway, thanks for the input.

Quick

Godofwarmike
09-19-2009, 03:39 PM
Maybe your idle air control valve is going bad?

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