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My bucking truck

I have a 1989 Toyota Pickup that drives pretty well generally. It originally came from the west coast, but we’ve had it in Maine for 6 years. For the last two it has had the following problem:

While driving in 3rd, 4th, or 5th gear(maybe in 1st and 2nd, but I can’t detect it) at a certain RPM level the truck bucks like crazy(like the engine violently shaking under the hood causing the whole truck to jerk back and forth). To be more descriptive I will give an example: I am driving along in 4th at 40 MPH and it will start to buck, now if I push the pedal to the floor I can accelerate out of it, but when I let off the gas and go back to just maintaining my speed it will start to do it again… Also, if I’m driving at 40 MPH and it starts to do it, I can let off the gas and it will stop, but the second I get back on the gas to pick my speed back up it will come right back. It is like this almost every time I drive it, but I would say it is more prone to this behavior during/ after it rains… and it will continue for days. I would generally say it never really goes away.

I’ve thought about plugs and distributor, but that doesn’t seem to be it… wondering about fuel injection, but really I don’t know. any ideas?

Does this truck have the 2-bbl carb 4 cyl or the EFI 4 or 6 cyl engine?

If it is the carb’ed 4 cylinder, the carb probably needs to be rebuilt. If it is EFI, you may have a fuel pump going bad. Or in either case, a clogged fuel filter.

It could also be bad ignition wires. They may be arc-ing all over the place.

Have the motor mounts and u-joints checked.