severe bucking on throttle change

[Keisari]

I've had an issue that's slowly been getting worse, now to the point where my neck actually hurts after an hour or two of driving...

Say you're heading towards a red light and you're slowing down in 1st or 2nd gear at around 2000rpm without using the brake. You realise you're going to come up a bit short, so you give the accelerator a dip. My issue is that no matter how softly you flutter the accelerator (in fact, it's actually worse the softer you tap it... if you floor it, it's still there but not so bad) the whole car bucks and oscillates violently for 3-5sec. Similarly, if you're accelerating (particularly in lower gears) and you take your foot off the accelerator, the car bucks again. This only happens during driving, ie: sitting in the driveway in neutral it revs up and down perfectly happily. The only solution I've found is to ride the hell out of the clutch so as to smooth out the changes as much as possible. There's also a slight but noticable hesitation between about 1800-3400rpm under low to 2/3 throttle, which smooths out completely above that.

TPS checks out fine using a multimeter and feeler gauge, also disconnecting it results in the usual symptoms of a bad TPS: seeking idle, poor low-end, no fuel cut etc, with the same initial problem. There's no obvious vacuum leaks or intake cracks, and I recently cleaned out the throttle body and IACV. Timing is spot-on, no codes. I don't have a fuel pressure gauge but that's the next thing I'm going to check. Fuel filter was recently changed, air element is pretty clean. Distributor, rotor, cap and spark plugs are all good (though there is some oil in one of the tubes and blow-by on another), and I think the coil tested fine. Any other suggestions or similar experiences?

Thanks!

[SuperDave]

Does the ECU in that model Celica use O2 sensor feedback? Sounds similar to O2 sensor oscillation that can occur with the PowerFC on the 2zz.

[Guest]

Does the ECU in that model Celica use O2 sensor feedback? Sounds similar to O2 sensor oscillation that can occur with the PowerFC on the 2zz.

it does, this could be something to check

[Keisari]

Thanks for the suggestion guys. O2 sensor appears to function normally - voltmeter connected to E1/VF1 oscillates more than 8 times at 2500rpm with E1/TE1 bridged and stops as soon as the rpm drops.

[Keisari]

I wonder if it might be a bad transmission mount? The cabin does vibrate noticably at idle (and worse with the lights on with idle-up).