this is just driving me nuts, while driving mostly above 40 mph and at 65--70 if I let up on the pedel ever so slightly it go'es to an idle and slightly touching it goes back to power again this is wearing out my neck & u joints 1997/3500 / 6.5T

mine was doing that until I installed Heath's reflash. I used my laptop and checked the 3 accelerator pedel position sensors and all are good. Might have something to do with factory programming?

home mech...go to user cp and update your signature to reflect what you're driving...

my '94 does this too...I have always assumed it was normal, and left it be until I get a chip.

Mine does the same. It's as if it goes into "coast" mode. Once I step on the throttle, it then "re-engages" the transmission. I've noticed that if I set the cruise control (even at 40mph) it does not do this.

I thought it was just normal.

Somewhere (another forum?) somebody said this was to reduce vibrations on the transmission input shaft during deceleration (via unlocking the torque converter). I have no idea if that is true or false. My old 5.7L with the 4L80E never did that.

How long have you owned the truck?

somewhat similar experience with my 93 3500hd ... i dont have cc tho...
so im forced to deal with it, i thought it was turbo at first but i think its the TC lock thing i read about on these pages and if im not mistaken there is a DIY solution and a retail fix too.. ill search for the links for you.

I was having throttle issues as well myself, would go to wide open throttle and back to idle without touching the pedal and was VERY touchy at the beggining of it's movement. I started a thread here about it and the overwelming majority said I needed a new pump, replaced ECM about 8 months ago and has been fine since.

I bought the truck new for the company I worked for in 97 and then I bought it from the co. when I left
currantly has 119000 and im getting it ready to take to the dealer for an injection pump problem and I hoped that would fix the surging too. the last couple of days it has got real jerky 97 serria 3500 6.5 turbo
thanks for all the help I can get

Any time you release the accel, the PCM unlocks the 4L80e TCC, and rpm will drop to below the torque convertor stall speed.

Press the accel to resume speed, rpm comes up and the TCC locks again.

TCC unlock and lock should be a smooth transition, as line pressure varies according to demand.

Pressure regulator could be stuck, or the fluid temp sensor could be defective, calling for max pressure, or the TCC is sticking.

Trip to the dealer's Tech-2 is probably a good idea, at this point.

He may check TDCO, while he's at it - should be -0.5deg, see if he will set it to -1.5deg, to broaden the power range a bit.

Thanks GM for that info now heres what happened today' I removed the filter canister to clean it out for my visit to the dealer for the inj pump problem [hopefully they will replace it without to much hassle] changed filter and put all back togeather. very little sedament, no water and the filter was only a month old. and it now appears to run ok on a short test drive,right from the start 30 to 50 mph, Ill find out tomorrow how it is on the highway around here it's 90 to keep up with the flow WILL SEE!

Stock PCM, your tranny will definitely unlock the Torque Converter and return to idle... and no, you can't do the manual fix thing on yours 'cause it's OBD-II (so the tranny guys tell me), although they say it does work on OBD-I (so Tracy tells me).

You can try the BD Autoloq, but they have had less-than-ideal results with it on OBD-II trucks. There's a gadget called a Pressure-Riser that I've been told works well. They go for about $130 + installation.

Course, a new relash on your PCM sort of solves the issue, too, and you get some new ponies under your hood at the same time.

my 94 just started doint this last week.

I cannont maintain 60 MPH on the highway. Either 50 or 65+.

I'm pretty sure its not an APP problem because it occurs with the cruise engaged as well.

It is DEFINATLY isn't just the TCC locking and unlocking.

I havent checked it out yet, but I'm thinking it may be the lift pump is INOP and the IP wont suck enough fuel untill you reach a certian 'throttle' setting.

I'll keep you informed

tim

I don[t think it's the lift pump mine carries 4#at all times mine cuts back to idle as soon as i reach the speed that the pedal is at -like when I crest a hill and not move the pedal and the truck picks up speed going down the hill it will cut back then lunge forward again. as long as I'm pulling it's ok but tring to coast it jerks

What happens in cruise?

What happens in cruise?


Going along uphill, or flat level at 70-75 things are peachy.

But down a slope, the engine will 'hunt' between idle and cruise speed.

Its exactly the same sensation as if you were going down a slope, and lifted your foot off the pedal, then applied enough throttle to accelerate ever so slightly, then lifted off again at about 2 cycles a second.

I imagine its pretty tough on the u-joints.

Tim

WELL just got it back from the dealer friday and they replaced the injector pump with a reason of internal failer, I don't know what else the mech. did but the old truck runs great now but I do have to replace the u joints! and the best part it was free!!!!! 119500 miles on it and done under warrenty

same thing WELL just got it back from the dealer friday and they replaced the injector pump with a reason of internal failer, I don't know what else the mech. did but the old truck runs great now but I do have to replace the u joints! and the best part it was free!!!!! 119500 miles on it and done under warrenty


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Going along uphill, or flat level at 70-75 things are peachy.

But down a slope, the engine will 'hunt' between idle and cruise speed.

Its exactly the same sensation as if you were going down a slope, and lifted your foot off the pedal, then applied enough throttle to accelerate ever so slightly, then lifted off again at about 2 cycles a second.

I imagine its pretty tough on the u-joints.

Tim

IIRC i had these same issues but with a bad surge at idle also. Taking out the #9 resistor and putting back in the #4 or 5 cured it all. Mind you it was on the 5068 pump in OBDII but its worth a shot if you've changed your resistor.

(other) CR...

I never messed with the resistor. Still bone stock. The issue has gotten worse. Truck is almost undriveable on the interstate. Now there is a surging. About a 2-3 second cycle where the rpms rise and fall slightly with steady load and throttle pressure.

I have had an intermittant noise in my serp belt. I thought it was the tensioner (which was defective) but the noise came back. Last night I noticed that my crank pulley had seperated and I could rotate the serp belt by hand almost half a rev.

Any chance this is connected to my throttle issue? I don't know how, but I never assume it cant be with these electronics...

Tim

Cowracer:

The "hunting" you are referring to can be manipulated with the optic bump. Have you messed with the optic?

If you did the optic bump and moved it too far, the truck will "hunt" excessively on downhill slopes exactly as you described. Moving the optic back closer to its original position should eliminate the problem.

Doesn't cost you a penny to try.

Cowracer:

The "hunting" you are referring to can be manipulated with the optic bump. Have you messed with the optic?

If you did the optic bump and moved it too far, the truck will "hunt" excessively on downhill slopes exactly as you described. Moving the optic back closer to its original position should eliminate the problem.

Doesn't cost you a penny to try.

I had heard that the optic bump causes hunting, and an overly sensative throttle, which is what mine is doing, but I never messed with the sensor.

Strange

tim

Try an optic "unbump" and see what happens, if nothing else cures what ails ya.

(other) CR...

I never messed with the resistor. Still bone stock. The issue has gotten worse. Truck is almost undriveable on the interstate. Now there is a surging. About a 2-3 second cycle where the rpms rise and fall slightly with steady load and throttle pressure.

I have had an intermittant noise in my serp belt. I thought it was the tensioner (which was defective) but the noise came back. Last night I noticed that my crank pulley had seperated and I could rotate the serp belt by hand almost half a rev.

Any chance this is connected to my throttle issue? I don't know how, but I never assume it cant be with these electronics...

Tim

Crank pulley drives all the other pulleys inc the alternator which picks up rpm, its suppose to only drive the tach and the ecm gets rpm from the sensor at the crank but it could be related, especially if your belt is slipping and causing low voltage. Replace the crank pulley, the harmonic balancer may need replacing also, check it out real good. Make sure you have good power to the batts as everything relies on having good voltage.

Try an optic "unbump" and see what happens, if nothing else cures what ails ya.


Worth a try, and if it fixes it then fine. But I'd like to know what changed in the first place...

Tim

Mine always had a touchy throttle and hunted since I got it. Connected a scanner and found my fuel rate was about 7-8 stock at idle. I think that some of the STANADYNE rebuilds didn't have the optic set right to start with.