Throttle response

[Trae]

I am having a problem with my throttle. When parked and in neutral if I press gradually on the throttle the engine will surge and is very hard to maintain a fixed RPM. If I gradually let off the throttle the engine will drop strait to idle. When the engine is under load I am able to accelerate smoothly but deceleration is tricky and usually I just disengage the clutch instead of trying to gradually let off the pedal. There are no codes. I have verified lift pump operating and gone through the electrical connections. Has anybody seen this? Any suggestions?
1994 GMC 3500 6.5td 5speed stock.
Thanks,
Trae

[More Power]

My experience has been that what you're reporting is somewhat normal.

For example, trying to maintain, say 1000 rpm in neutral with an electronic 6.5 is nearly impossible. The engine wants to return to idle.

MP

[Trae]

I think 1000rpm is perhaps to low to try but it would be nice to smoothly roll back from 2000 down to 1000.
What really bug me is if I am in a parking lot and in first gear hovering around 2k rpm and I want to slow it down a bit I cant because it just drops it down to idle. I can also hit a point where it will rpm up then down causing to truck to buck unless I ether add power or take my foot off it. This also happens with cruse control going down hills. I have to add fuel or take it out of cruse to keep it from bucking. I

[arrowheadracing]

I experience the same symptom. Though mine is an auto. If I am on the highway and have the cruise on, it goes up a small hill and down the other side, it bucks the rpm up and down like its running out of fuel to maintain the mph. I notice this at slower speeds in town once in awhile. As well as the neutral thing. I believe its just the electronic part of the throttle that sucks. Hopefully this is something they fixed with the Dmax, as I ll be owning on soon hopefully.

[JohnC]

If it happens in cruise control, it is not related to the APP. The PCM synthesizes the pedal position under cruise. I'd go through all the troubleshooting for stalling as many of the causes could also cause the behavior you're describing. Also, mechanical issues with the injection pump can cause the same behavior, although many of them will set diagnostic codes. These codes may not light the SES lamp, so be sure you read the codes and don't assune there are none in the absense of the light.

That said, the electronic throttle is very touchy at low fuel delivery rates, and you may be stuck with it as is. My standard had a mechanical pump, so I can't compare it with anything.

One more thing. What resistor are you running on the FSD? There have been a few reports of surging at light throttle with #9 or higher resistors. Maybe just a fluke, but worth considering.

[Trae]

Good news.
The guys at DIS suggested checking for a fuel restriction and among the items to check was the fuel tank cap. Well when I arrived at the air port tonight I got in my truck and headed home. Well about half way I was thinking about my issues and thought I would give the cap a try. Well it was like night and day the performance improved I was blowing a lot less smoke and it appears the deceleration issue may be improved if not gone. Ill have to drive it around some more to be sure.
JohnC Im not sure which resister I have. I have not had the FSD out to check. I bought this truck in December and am still working through all the little problems the seller forgot to mention.
Thanks for the input guys ill post any further results.

[Trae]

Took my truck to Diesel Injector Service here in Dallas they ended up replacing the Injector pump and wow I had no idea the truck should run this well.
It fixed both the throttle issue and smoking power issue I was having.
Now the truck runs so good I might have to keep it.

[rameye]

trae,
how much for the pump R&R??

Labor alone is about $800 up here on the gold coa$t.

Anybody have a good diesel shop in CT?

[99gmccrew]

Trae, I haven't had to replace a pump yet. How much did it run you?

Oops, You beat me to the punch Rameye!

99gmccrew

[rjschoolcraft]

Trae,

That sounds a lot like the symptoms that I was having with my Suburban when I first bought it. A warranty replacement pump cured my problems then, as well.

[Trae]

$1400 for pump crank sensor and labor.

[charliepeterson]

trea: you caught this in time. The problem you explained can get worse. Eventually the truck can seem to run away. The rpm's will/can go to 4500 from 1500 with no additional throttle pressure. It's most noticeable with no load on the drive train, like in neutral or very slippery conditions. On dry pavement it's like having cruise control when you don't have this option or turned off. I've seen two trucks do this with no ECM codes logged, very strange indeed. The IP's were swapped out and they're making money now.

[rameye]

Hold everything...Trae do youmean to say pump, crank sensor and labor 1400??

Or did they replace the crank sensor for the pump and charge you 1400??

I think I'm gonna ship my truck to Dallas and save 500 bucks! after the shipping!!

[Trae]

Just went to the truck and pulled the receipt out of the glove box.
DS4-S5288 pump $888.80
Sensor crank $64.39
Seals $14.50
Labor $322.50
Scan charge $50.00
Tax $106.44
Total $1446.63
Oh and 1 year parts

And to make things even better I Just finished replacing the AC hose assembly now I have AC to boot.
Oh DIS is one of the places that rebuilds the DS4 right in the shop so I guess I cut the middle man.

[Barry Nave]

Had the RPM problem also in the winter of 2003-04. Eng would sound like a DI and wanted to take of on is own. Replaced crank sencer only and truck was back to normal. Also when cold truck would not want to go with out sounding like a DI. Once up to temps. it would run normal. Did have the code for the crank sencer as well as others though after clearing the crank was the common code.