My car jerks forward when I completely let off the brake. Its been doing every since I switched to a built auto trans. None of the trans sponsors no what it could be and its really getting annoying. Does anyone know if it could have something to do with the tune?

 

anyone

 

I don't understand the complaint, most autos "creep"
and if your idle speed is too high for the converter,
it could push pretty hard. Is it anything more than
that?

 

This happens while I moving. I could be doing 70mph and put on the brakes. As soon as I'm 100% off the brakes it revs up a few hundred rpms and jerks forward. I'm not talking about when I'm at a stop.

 

Wonder if it's the vac booster "recharging", air surge.
I've noticed this on other cars though not enough of
an effect to bother me. Maybe try plugging the line
one time to see.

If a small air surge makes a big difference you may be
idling / cruising at an over-rich condition, where a bit
more air makes a lot more torque (if you were over-lean
already, more air would tend to make a bog or ping
instead).

 

If it was air surge wouldnt it rev the car up as I'm letting back on the brake. If I press the brake in all the way and let off 99% nothing happens. It just happens when the brake is completely released.

 

Sounds like it might be a very harsh converter lock up. Just a guess. Did any of the trans builders you spoke with think that might be a possibility? At speed the converter should unlock when you hit the brakes and lock back up when the brake is released. Don't know what kind of internal trans problem would cause that though. I'm not a trans builder.

 

One of Trans sponsor said the converter clutch is releasing and then applying. The other sponsor said the converter unlocking would happen when you applied the brake not when you let off the brake. Is the converter supposed to automatically lock up when you let off brake? If not can it be fixed by the tune? or does that means something is wrong with converter.

Just to add some fuel to the fire about three day ago the car seems to have another problem. If I get the car up to about 45 mph it goes into overdrive. If I keep the mph at 45 it seems like its going in and out of overdrive. The only way to make it stop is to give it a little more gas. Does anyone know what could cause that?

 

What converter is in the car? Who built the trans? A converter locking and unlocking can sometimes feel like going in and out of OD. I think it's time to have your trans builder check it out. Also you give very little info about what else has been done to the car.

 

What converter is in the car?
Vig 3200
Who built the trans?
Performabuilt Stage 2

Also you give very little info about what else has been done to the car.

Its just a few bolt-ons. LT, 10 Bolt w/3.73, and few little free mods.