[Tman2331]

My 1990 gmc k1500 350 was stumbling pretty bad today, and the truck downshifted going up a hill and I heard a klunk no rpm gauge but was definitely over 2500. It sounded like someone smacked the fire wall with a wrench. I didn’t hit nothing on the road lol, Still runs but stumbles bad, changed the fuel filter, set at base timing, and nothing has changed, got her to down shift and heard the same klunk, no engine lights, all fluids look good. Any suggestions? Thanks

 

[95 Truck]

I think a little more info is needed,

1) can you get it to clunk any other ways? Such as going from park to drive or reverse to drive
2) any engine codes
3)will it clunk if you brake torque it a little bit
4) stumbles when? Just normal acceleration? Only when it down shifts under heavy load?
5 you look at fire wall or under the truck to see if you can see anything broken and hitting the firewall or transmission housing

 

[BogiesAnnex1]

I’d start out checking engine and tranny mounts.

Bogie

 

[alwill923]

U joints

 

[LATECH]

Check inside the distributor

 

[Tman2331]

My 1990 gmc k1500 350 was stumbling pretty bad today, and the truck downshifted going up a hill and I heard a klunk no rpm gauge but was definitely over 2500. It sounded like someone smacked the fire wall with a wrench. I didn’t hit nothing on the road lol, Still runs but stumbles bad, changed the fuel filter, set at base timing, and nothing has changed, got her to down shift and heard the same klunk, no engine lights, all fluids look good. Any suggestions? Thanks

It’s doesn’t sound like a mechanical klunk like a bad diff or something. Only had it happen twice, both while downshifting, I won’t know if it will klunk tell it downshifts, it doesn’t happen every time. no nothing is hitting the firewall, ujoints replaces last year, all mounts look good, stumbles through out the rpm range, stumbles worse anytime u push the petal. no engine lights. Havent done a lots of testing after the klunks other then listening to it idle.

 

[95 Truck]

I am beginning to think you have 2 separate problems,

I would first start with stumble, I am thinking pull plugs and see if one is fouled out with oil or something, also inspected all plug wires, distributor cap and rotor. In fact Latech could be spot on with cap and rotor, if cap has cracked or rotor has broken it could be misfiring inside cap.

As for clunk during down shift, I am thinking something trans related, and seeing it could be internal, the sound could be muffled.

I would focus on the misfire or stumble first and go from there.