# Unstable throttle (rapid up/down movement) while in overdrive, 1989 Isuzu trooper

**URL:** <https://community.cartalk.com/t/unstable-throttle-rapid-up-down-movement-while-in-overdrive-1989-isuzu-trooper/12417>\
**Category:** Maintenance/Repairs\
**Created:** [March 31, 2008, 6:55pm UTC](https://community.cartalk.com/t/unstable-throttle-rapid-up-down-movement-while-in-overdrive-1989-isuzu-trooper/12417 "2008-03-31T18:55:55Z")\
**Posts on this page:** 1\
**Page:** 1

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**Author:** ![kapexp10](https://avatars.discourse-cdn.com/v4/letter/k/5f9b8f/32.png) [@kapexp10](https://community.cartalk.com/u/kapexp10)\
**Post date:** [March 31, 2008, 6:55pm UTC](https://community.cartalk.com/t/unstable-throttle-rapid-up-down-movement-while-in-overdrive-1989-isuzu-trooper/12417/1 "2008-03-31T18:55:55Z")

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The trooper has a 4 cyl. fuel inj. w/auto tranny. As of late, the tranny will shift intermittantly from overdrive down to 3rd then back up/ back down until I step on brake lightly then it stops shifting. This all happens when above 30 MPH and ONLY AFTER the first 15 minutes when started cold. The up/down shifting occurrs at low as well as high engine vacuum. When placing the tranny select in 3rd the violent up/down shifting stops but it runs at 3000 + RPM. The shifting while in overdrive is rather constant. I have checked and rechecked for vacuum line/hose leaks but found none. Can’t find anything on subject in repair manuals either. HELP!!!