Bucking when letting off the gas...

[Olered]

My 97 Disco has developed an odd habit. When I'm driving in all types or traffic be it little or none, when I lift of the gas I get a bit of a jerk and the RPMs fluctuate up and down as if I'm revving the engine. It only fluctuates about 300-500 rpms. It will rev up and down usually about 2-3 times. Anyone experience this?

Thanks

 

[BaldEagle]

mine "bucks" alot during the same situation. this is due to a little slack in the transfer case that builds up sometimes. a little annoying, but nothing i worry about. i think it's pretty common.

 

[Olered]

Yet, it kinda feels like I'm running in 3rd gear or something. It's like it's trying to "engine brake" or something.

 

[Olered]

Okay still looking for anyone that has experienced this condition. Maybe a little more explanation is in order. I'll be driving around at speeds 45 and under. When I move my right foot from the gas to the break, the rpms drop quickly to idle, there's a clunk in the drive-shaft, and the engine will rev to 1500 rpm and back to 800 rpm no more than 3 times-2 usually, then level off at 1100-1000 rpm or whatever rpm coincides with the speed. Sometimes the truck surges with the rpm change.
Does the IAC control the times when you are moving your foot from the gas or removing your foot from the gas? I can't tell if its the engine or the tmission. One is not keeping up with the other.
Closely related-There's this big hill on the way to my street. I start going down the hill and can get to around 60mph. In any car, I would normally accelerate down the hill, release at the bottom, coast up the hill and apply the brakes at the stop sign. In this truck, it accelerates down the hill, I release at the bottom, start coasting up, and then get surge-jerk-surge-jerk (no gas applied), brake for the stop sign.
I know, it's funny to read but I had to send it. Just looking for feedback.
thanks. 1997 D1

 

[DiscoJen]

I have a 97 and mine did the same thing. Mine turned out to be the PROM chip in the ECU. There was a TSB relating to this as part of the PRIDE campaign - I think it only affected 96-97's. I bought my 97 used in 2004 and it did it from day one, I don't think it's a problem that "develops" - you either have it or you don't.

Instead of buying the new PROM chip, I found a used ECU for $70 bucks from a 98 (98's didn't have this problem) and it's been golden ever since.

Might be worth looking at. You may be able to find more information on this by using the words PROM and PRIDE in a search.

Good luck,
Jen

 

[Chris-St Louis]

Hey Jen! You got it on the head! TSB your refer.....

No: 19/04/97/NAS

Ref: WSM section 19TIB 19/08/96/US

Issue: 2

Date: 06/20/97

PRIDE IV 6-11

Engine Speed Fluctuation at Moderate Speed Closed Throttle

AFFECTED VEHICLE RANGE:
DISCOVERY (LJ) 1997 MY Advanced EVAPS Equipped
VIN: up to VA727076 Automatic
up to VA726789 Manual
up to VA558898 Manual and Automatic

SITUATION:

IDLE FLUCTUATES DURING MODERATE SPEED CLOSED THROTTLE OPERATION There have been reports of engine flare or surge when the vehicle is decelerating. The engine speed may fluctuate between 1200 and 1600 rpm (up to 400 rpm total fluctuation) when the throttle is closed and the vehicle speed is between 35 and 50 mph (56-80 kph.). This may occur in GEMS equipped, Advanced EVAPS vehicles in the above VIN ranges.

RESOLUTION:

REPLACE PROM

Upon customer complaint only, install a new PROM set which has been developed to eliminate the problem. Replace the PROM set using established Operation Pride procedures. (TIB 19/08/96/US)

PARTS INFORMATION:

PRM9640 PROM (Automatic)
PRM9641 PROM (Manual)
LRNASTATK Anti-Static Mat

WARRANTY CLAIMS:

18.30.89/26 ............... Time 0.50 hrs.
Fill out DPR and FAX to LRNA, Replace GEMS PROM

COMP CODE: .............. 7G1A

Normal warranty policy and procedures apply

REPAIR PROCEDURE:

REPLACE PROM

NOTE: Make sure to complete the DPR and FAX to LRNA.

1. Call the GEMS HelpLine.

2. The HelpLine will issue a new specification PROM set that will eliminate the concern.

3. Use procedures outlined in earlier phases of Operation Pride to replace PROM set.
(TIB 19/08/96/US)

4. Following installation of the PROM set, start and warm up the vehicle from full cold to hot at idle, in Drive with the Park Brake applied. This will adapt the engine management IACV system and minimize any noticeable changes to the customer upon return of the vehicle.

 

[Olered]

Thanks for the info! That's helps so much! I thought that it was a deadly fix but hopefully will be easy!

 

[agro1]

Nice post Chris...What's the number for the GEMS HelpLine ??

 

[Olered]

Is changing the ECU fairly simple? Like an unplugg/remove/plug-in new...
I'll begin the search.

 

[Olered]

Update---
Took it to the dealership (LR Buckhead) twice.
First time they told me they couldn't find the problem. I told them the problem. They still couldn't find it.
Second time, I told them the problem and provided them the information on paper with the price and all. The service agent looked at me, looked at the paper, looked at me, and said "hmmmmm". I then asked him to humor me and replace the prom. He said okay.
Day later...he called me to let me know the truck was ready and the prom had fixed the problem.
It's bad that the dealership cannot help anymore.

No more bucking truck!!!!! Thanks everyone for the info!!!!

 

[alex3324]

No: 19/04/97/NAS


4. Following installation of the PROM set, start and warm up the vehicle from full cold to hot at idle, in Drive with the Park Brake applied. This will adapt the engine management IACV system and minimize any noticeable changes to the customer upon return of the vehicle.

Is this how the dealer resets the adaptations after replacing a fuel injection system componet? I thought one needed testbook or rovacom to accomplish this.