Low to Mid Speed Bucking - CDL Applied and Open


Well-known member
Dec 11, 2017
Fort Worth, TX

I've been chasing down some rather frustrating issues with my 5spd D1. I regularly encounter bucking in 1st-3rd gear in high range when transitioning from on throttle to off throttle. This is always accompanied with requisite engine fluctuations (100-300 RPM). I.e. off throttle down RPM, buck, up RPM, buck, down RPM, buck... repeat until brakes applied or declutch. A solid clunk from the front end diff or transfer case (really tough to tell which) is heard as well.

This issue is exacerbated in low range with the diff locked. In fact it makes driving over difficult terrain very frustrating as I am constantly having to declutch to avoid this back and forth rocking motion. I know the consensus is usually driveline play, however the diffs and transfer box were recently replaced with known good units. Same with the axle flanges. All the splines looked good. The output shaft on the R380 looked perfect.

The truck is a pre-AEL '96 5spd. I'm wondering if some or most of this issue is due to the engine RPM changes. For example, occasionally I can replicate an engine surge by idling forward in first gear, high range no CDL, (no gas applied) and then smoothly declutch. The engine will rev up to ~1200 RPM for several seconds and then slowly settle down to regular idle speeds.

I've seen a TSB related to closed throttle engine management (see here) posted on DWEB but it looks to be for '97 model years which are AEL only.

In summary, thought I had driveline issues but wondering if I have an engine management problem. Looking for advice troubleshooting.
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Well-known member
Apr 19, 2004
Bloomfield twp, MI
could this be a failing Speed sensor? i had intermittent lurching and idle problems when the speed sensor went bad. I unlpugged the sensor and the issue went away. sucks to not have a speedometer though.


Well-known member
May 24, 2007
Boise Idaho
I would look at your TPS. Then maybe your IAC and make sure that is clean and functional. Certainly unplugging the VSS is easy and worth doing for troubleshooting. Might check out your base idle. Could also be a vacuum leak.


Well-known member
Dec 18, 2010
Is ‘96 a gems or 14cux? I experienced a similar issue that ended up being a failing alternator but it was a 14cux.