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Bucking between 30 and 40 MPH

I have a 1997 V8 Thunderbird with 195,000 miles. I have a bucking problem between 30 to 40 MPH. This condition started about 10,000 miles ago. I tried to narrow it down to certain conditions and came up with times that it DOESN’T seem to happen. It seem when the engine is cold it runs smooth. If I need to speed up it runs smooth threw the 30 to 40 MPH window. The car runs better with premium gas and for awhile that seemed to work but not with all premium gases.

I keep the car in excellent condition. I had a complete tune-up about 25,000 miles ago, with new power packs, cables etc. I had the transmission replaced about 5000 miles ago. I change oil every 3000 miles, fuel filer, screen fuel pump changed.

Any help would be appreciated!



I would go after the egr passages on this engine. They are highly prone to plugging and can cause your complaint. r&r throttle body, clean passages.

Anytime I hear of bucking in that specific speed range I can’t help but wonder about torque converter clutch (TCC) shudder. I don’t know the specifics of that transmission but if you have a way to turn off overdrive you might try that to see if the bucking disappears (this often turns off the TCC lockup). On some transmissions driving in something like 3 instead of D will keep the TCC out of the picture. If you can figure that out and drive without TCC that will tell you if its that or not.

If at a loss for that you can try this - on a nice uncrowded and safe portion of road get it to where its bucking, keep your foot steady on the gas, and just nudge the brake pedal with your other foot - not enough to actually put the brakes on but just enough to engage/disengage the contacts in the brake switch. If it stops bucking there’s a pretty good change it is TCC shudder.

You said the transmission was replaced 5K ago. I would revisit whomever did it for a check up.