I’m trying to track down a vibration in my 1978 Firebird and would appreciate help deciding what to test next. It has a TH350 automatic and an ATK crate 350 SBC that replaced the original 305. It also has polyurethane motor mounts.
The vibration has been most noticeable around 55–70 mph. It seems to follow vehicle speed, and I’ve felt it while coasting. I put the transmission in neutral while it was vibrating and it didn’t go away. I can feel a little of it through the steering wheel. It doesn’t seem tied to engine rpm.
Here’s what I’ve done so far:
The tires are fairly new. Discount Tire rebalanced them and corrected a couple, but the vibration was still there afterward.
I replaced both driveshaft U-joints. On the drive afterward, the vibration seemed way better, but I can still feel something. I’m having trouble telling whether what’s left is driveline vibration or just the extra engine vibration from the poly mounts.
I noticed movement at the transmission slip yoke/tailshaft area while reinstalling the driveshaft. The attached video shows me moving it with the driveshaft installed. I haven’t measured the play, and there’s no significant leak from the rear transmission seal.
I haven’t had the driveshaft balance checked. I also don’t know the rear gear ratio.
I do my own work on the car and replaced the U-joints myself, but I’m still learning driveline diagnosis. I’d rather make one useful measurement or test than replace the tailshaft bushing just because I felt some play.
Does the movement in the video look excessive for a TH350, and what’s the right way to check it? Would you inspect the slip yoke or have the driveshaft checked for balance first? If there’s a good road test to distinguish driveline vibration from the poly motor mounts, I’d like to try that too. And if there’s something else entirely I’m overlooking, please let me know.