Why would a car jerk during deceleration?

Well if its an automatic, that's easy... I have this too. first answer this...

how fast are you going?

does it have overdrive (auto only)?

if manual what gear you get up to when this happens?

well if its auto, and your going fairly fast and suddenly let go of the gas... its going to change down quite quick due to gearbox design. This would give the sensation of the car braking...

if its a manual car, then you are most likely in too low a gear to start with, this is why we change down when we loose our brakes, speed reduction is effective immedeatly without brakes...it slows you down to save emergency speed...1 min or so.

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I am having a similar problem with my grand am! I have been told that it could be a motor mount

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My 1993 Buick Skylark had the same problem last year-- if I took it above about 40mph after the engine was warmed up, every deceleration thereafter caused the car to shudder. After researching it a bit, I decided it was probably the Torque Converter Clutch Solenoid in the automatic transmission, so I had it replaced and the problem disappeared. I've read that this is a somewhat common problem with Overdrive, which my car does not have, so apparently the problem doesn't play favorites.