Hello,
I have a 1978 Ford F-150 2WD, 400CI with a C6 Auto Trans, 133,000 Miles. The engine starts right up and is seeming to run great. Trans shifts properly. The issue is sometimes at around 45 MPH the truck starts to shutter and vibrate. The engine does not seem to lose power and does not spudder. It does not happen all the time. It seems to come and go. It happens more after the truck has reached normal operating temps. I just all four tires replaced and wheels balanced, thinking this maybe the problem. Other then the shudder and vibration coming and going the PU drives really smooth down the road with good tight steering.
Quite a few years ago I had a similar problem with a 1965 Marlin. I took it to a chassis shop where they were equipped to diagnose that kind of issue, they ran the car with the drive wheels on rollers and checked for vibration sources underneath in a pit. They found the vibration was from a bad universal joint inside the torque tube. So, it might be worth checking your universal joints especially if they are original or very old. (At least they're easily accessible since you don't have a torque tube to deal with.)
Sounds like it might be torque converter shutter. Do you see any check engine lights on? Does letting off the gas pedal stop the shudder? Does pressing on the gas padel more so the transmission downshifts also stop the shudder?
@oskool Bear in mind there is no "check engine" light in a 1978 vehicle. Something of that vintage would probably only have oil pressure and alternator warning lights.
Thank you for the responce. Letting off the gas does not stop the shudder. And yes more gas pedel does seem to take some of the shudder away.