[Starman97]

My recently rebuilt 95 ExtCab V6 Auto has developed a strange problem.
When I start it up in the morning and after work, it idles smooth, but from about 20% throttle to 60% it seems to miss rhythmically , sounds like a StarWars Pod racer is the closest description. No CEL since the rebuilt motor was installed.

Once it warms up, then it's smooth, there's a little fluctuation in the 30-50% range, but it's slight, if the radio's on I don;t even notice it.
If it's cold and I floor it, it pulls hard once the RPMs come up. But the vibration does not seem to be RPM or speed related. I tried re-seating a bunch of the connectors to the coils/igniter/MAF/throttle sensor.
I've run several cans of injector cleaner and premium gas seems to help a little, part of the rhythm seems to be pre-ignition.

Fluids all look good, and mileage seems normal, other than I leadfoot it a bit to avoid the rough spots.

 

[buck720]

I would change out the temperature sensor

 

[Starman97]

Fixed it.
The crossover pipe to downtube joint was leaking.
Two of the nuts had completely worked themselves off, the last one
was only finger tight. I had reused the nuts after my rebuild, they looked OK,
but the tops are crimped slightly to prevent them from spinning off.
The old ones had worn the threads enough that the crimp was not effective
and the nut could spin on the stud without any resistance. The new ones went on fairly stiff.

I guess the gap in the flange was sucking in air between exhaust cycles and confusing the O2 sensor which then tried to tweak the mixture. That would explain the 'cycling' effect.
Also under acceleration, the engine torques against the motor mounts and would open up the gap, at full throttle, there's no time for any suction and it would run correctly.