jjgurley

My wife's F150 has been acting strangely for over a year now. The mechanic that rebuilt the engine has "given up" finding the problem. One other mechaninc also declined the job.

300ci V6, fuel injected, 1990 1/2 (apparently this matters for some parts).

The truck runs in either "fine" or "badly" mode. "Badly" means it bucks when at a constant speed on level ground. Accelerating (or going up a hill) is smooth, and idle is rough.

Upon a cold start, it runs in "fine" mode. After a minute or two, it switches to "badly" mode and continues to run badly until it is thoroughly warm AND the ignition has been turned off. Typically, this means the second trip on a errand runs "fine", but just momentarily turning the ignition off while moving switches it to "fine" mode. It stays in "fine" until cold.

It feels similar to running one of the tanks dry, but since acceleration instantly smooths it out, I'm inclined to doubt a fuel problem. I also considered water in the gas, but it's gone through dozens of tankfulls with no improvement. I read something here about EGR valves, but the mechanic tested the valve. Also, the "reset" phenomenon points towards some component that "reboots" or something similar - maybe the ECU?

There's no engine check light or other idiot light.

It's had new fuel/air filters, plugs, distributor rotor (I think).

I've considered a bad ignition switch, but I would think the idiot lights would flicker if the switch was intermittent?

All suggestions considered!