Once in a while, maybe once or twice a week, the RPMs oscillate about 50-60 RPM. Accelerator pedal is held steady. Flat, straight road. A/C on or off. Going about 65 mph in this video. I can feel it but just barely. What could it be?

 

Tach is always related to the alternator but I've also fixed a loose connection behind the face card. Very easy to check once the binnacle's off. Tach could be loose/frayed alternator/tach wire. It could be something causing the motor to surge, drag or something like a belt tensioner loosening up. What condition is the belt in?

 

Check the white wire that plugs into the alternator. My 96 was doing weird stuff and it was loose. I squeezed the female connector some to tighten it up, plugged it back in and it was fine for years.

 

Belt is a Gatorback with about 6000 miles on it.
I can feel the motor surging when this is going on.
I'll check out the alt wire.

 

Fuel filter canister been replaced? Fuel pump and sock filter? Crank sensor wiring condition? It's a small drop so it's not serious yet. I wonder about the fuel delivery or possibly an injector or it's connection. You could run some Techron through the fuel system to see if that does anything.

 

Fuel filter has about 6,000 miles on it also.
The fuel pump, according to the PO, was replaced about six years ago.
I pour a can of sea-foam in the tank every oil change. So far I have changed the oil twice but have run about four cans through. Two right off the bat.

No idea of the crank sensor wiring. I'll look at it while I'm r/r the rear universals.

I am going to get rid of the cats here shortly and use the resistor capacitor scheme for the post O2's.

 

Ok, so it's been a while since I last posted on this issue. Kinda have it narrowed down a little more. Recent work on truck has pretty much ruled out anything engine related.
It only happens in 4th, between 50-ish and 60-ish, not at highway speeds, 65+. Cruises right along at 75.
Thinking torque converter or valve body. Transmission doesn't have any electrics in it, right?

 

Shifts by pressure. Direct drive clutch or it’s control valve, torque converter control valve, Hysteresis valve sticking.....

 

ihscouts, thanks for the reply.
That's what I thought. Didn't think I read that section wrong and I didn't see any wires going to trans.
Changed fluid and filter while engine was out. That didn't affect surging though.
Going to fiddle with the kick-down a bit. My downshifts are a bit crisp.