Guys, what are some possible causes for this? It's a bucking/jerking during low speed crawl- no throttle applied, brakes partially applied, car is basically idling forward.

I've been assuming it's due entirely to my misfire/rough-idle, but now I'm starting to wonder.

Trans is a TH700R4 with a factory GM LT4 Vette torque converter. Camshaft is a 212°/218° duration with a 112° LSA. Nothing that a stock converter should have trouble with. Everywhere else in the rpm range and under heavy or light load, the trans operates perfectly. Could this jerking be at all due to something in the tranny?

 

Poor tune. Bucking and jerking could be expected from a stick, but not like this from an automatic. Something is definitely out of wack!!

 

I know the LT1's are known for air distribution problems at low speeds. Cop'ers always use to cry about a "Misfire" in the caprices, but that pulsing was just due to how the LT1 intake was deisgned. The minram I can't imagine would be much different. Has this been on going - or did it just pop up?

If that is not the case - you never stated if you were running maf or SD? Secondly - Investigate vac leaks..... Do you have any one spark plug looking fouled? Have you tested the resistance of all of the spark plug wires? Lots of options here.

 

Sh*t, I didn't notice it was FI. What computer, and who tuned it? It's got to be way out of whack if it bucks with an auto...that or a vac. leak like GOY posted.

 

Yeah, I've posted a lot about it in the DIY PROM section. Guys over there know I've done every practically every diagnostic known to man. I've been over the entire engine and EFI system with a fine tooth comb. It's why I'm poking at the trans as a possible cause for the bucking/jerking.

It's a speed density system.

 

Could be the brakes themselves.

could be the ocnverter if u have something that is a quality unit like a yank or a Vigi. they both tend to have some tight and loose spots depending on throttle, load and braking at speeds under stall. Price you pay for having a "tight" stall converter(meaning high stall yet still maintaining as low as slip as possible)

Also IAC issues or excessive steps could do it.

Also go thru your tune very carefully and datalog and graph the poop out of it. MAybe some closed throttle timing settings and main table arent quite jiving. Also the same with fueling tables.

Little blips sometimes make a large difference depending on the combo.

Also as I know you are aware of from the prom board, the lt1 without individual cyl tuning does have some known issues that primarily show up at low speeds and idle.

later
Jeremy