Revolutions fluctuating about 50 rpm rythmically about every second at highway speed (1 Viewer)

[96LX]

I noticed that while I'm using cruise control above 70 mph that the engine rpms go up 50 rpm and holds for no more than a second and then down 50 rpm and holds no more than a second, this repeates seemingly endlessly, or at least while the engine is under load. It is so slight that I could barely notice it at first. I can't even see the tachometer move up and down and had to use the bluetooth OBDII device to confirm. If I sit up straight without my back touching the seat the acceleration / deceleration will cause my upper body to barely sway back and forth.

I can't help but think this may have something to do with torque converter locking and unlocking but would it be this gentle?
I changed the transmission fluid last October using this method: Transmission Fluid Exchange Writeup - https://forum.ih8mud.com/threads/transmission-fluid-exchange-writeup.19299/

I somehow managed to screw it up and didn't replenish a pint of fluid during one of the cycles, then drove about 15 miles with it slightly low and had some weird shifting, I posted about it here: RTH: Transmission Dipstick Reading - https://forum.ih8mud.com/threads/rth-transmission-dipstick-reading.1229439/#post-13490766

I've been checking the transmission fluid level often and it has been correct and looking as fresh as last October every time.

I'm not sure what else this could be, otherwise the engine sounds and feels good, maybe a slight lack of power but that could just be in my head, it has been much warmer lately so higher intake temps might account for that. I did just check the spark plugs and they looked great.

Anybody else have this problem?

Thanks in advance

 

[ppc]

I would check for the proper amount of slack in the cable connecting the cruise control actuator to the throttle body and make sure the cable operates freely.

 

[Kernal]

Any engine codes?

 

[96LX]

There are no engine codes, I will look into the cruise control actuator and report back.

 

[BILT4ME]

Do what @ppc said. The Cruise Control monitors the speed, RPM, TPS, vacuum, and all that stuff while engaged and if the cable is loose, it allows "movement" in the actuators and will cause a variance like this.


BTW, this is how my wife drives all the time, except it's more like 200 RPM. I can't stand to ride in the car when she's driving on long trips unless I'm asleep.

 

[jonheld]

Use your OBD2 device to monitor throttle. It will tell you if the actual throttle is pulsing or something more sinister is afoot.

 

[Rusty Marlin]

Do what @ppc said. The Cruise Control monitors the speed, RPM, TPS, vacuum, and all that stuff while engaged and if the cable is loose, it allows "movement" in the actuators and will cause a variance like this.


BTW, this is how my wife drives all the time, except it's more like 200 RPM. I can't stand to ride in the car when she's driving on long trips unless I'm asleep.

HORK city! I worked for a guy that drove like that. He wouldn't let me drive to lunch and every day I'd be motion sick all afternoon. It was a 1/2 mile to lunch.

 

[96LX]

I finally had some time this weekend to take a look at it. The cruise control cable seemed taught, but slightly less taught than the accelerator cable, so I adjusted it to be a bit more tight. On my test run I didn't seem to have the issue with cruise control on at 80mph, but it quite hard to tell. I tried to monitor the throttle on the scanner but the road I'm testing on doesn't seem to hold the same pitch long enough for the results to be conclusive. However I did notice the rythmic fluctuation is happening at idle now. Furthermore, when I started the truck today it was throwing a P0120 "Throttle/Pedal Position Sensor/Switch "A" Circuit"

Could this code be from my adjusting the cruise control cable, or could my throttle position sensor have been the problem all along and the computer coincidentally just now threw the code?

To name all possible variables, I also changed the oil, flushed and refilled the coolant, and adjusted the transmission kick-down cable this weekend.

P.S. The transmission kick-down cable was quite loose, after tightening that it is like driving a completely different truck! Is it possible to over-tighten it? The shifts are now quite snappy, maybe too snappy, could the firm shifts be bad for the transmission? The first to second shift happens much sooner, and it seems to hold second gear much longer while aggressively accelerating. I could only imagine firmer shifts would be a positive but I figured I'd ask.