Hello have a 1987 Oldsmobile cutlass supreme bruougham.Having.problem with cab vibration in cab,Parts replaced carb professional built new coil,module,coil,egr,wires,ac delco plugs vaccum lines,runs great in park idles great timing set at 20 degrees only vibrates in drive check compression drives down road great only happens in drive engine lift up some with torque 74,000 miles carb was calibrated wiring I’m stumped don’t hear clunks but thinking maybe motor mounts ,Anyone ever have this problem?Or a problem with lock up on converter replaced brake switch also
Hello! And welcome. You've already done a great job throwing the parts cannon at the ignition and fuel systems! Because your Cutlass runs perfectly in park and drives down the road smoothly, we can safely rule out things like vacuum leaks, misfires, or carb tuning. You're on the right track with your suspicions. Here is what is likely going on:
The most likely culprit is the motor mounts. Your instinct is spot on, and worn engine mounts are the most frequent cause of vibrations at idle. Motor mounts use rubber insulators to secure the engine to the frame while absorbing and dampening vibrations. Over time, this rubber deteriorates from age, heat, and oil exposure.
When you shift into drive, the transmission engages and puts a drag or load on the engine. Because you mentioned the engine visibly lifts up when torque is applied, that's a textbook symptom of a broken driver's side motor mount. The broken mount is failing to cushion the load, allowing the engine's vibration to transfer directly into the cabin. If the shaking decreases or disappears when you shift back into neutral or park, it's a very strong indicator that the mounts need to be replaced.
That being said, don't forget to inspect the transmission mount at the rear of the transmission, as a worn transmission mount can also cause vibrations that feel very similar to engine mount failure.
You also asked if this could be the lock-up torque converter. While it's true that problems with a torque converter or pump assembly can cause rotating vibrations, a failing TCC (torque converter clutch) solenoid usually presents a much more violent symptom. If the converter was staying locked up when you came to a stop, the engine would shudder heavily and completely stall out (exactly like forgetting to press the clutch pedal in a manual transmission car).
Since your engine idles great in drive and doesn't stall out at a stoplight, your lock-up system is likely functioning exactly as it should be. If you ever need to isolate the transmission to be absolutely certain, diagnosing it usually involves unbolting the torque converter from the flex plate and pushing it back toward the transmission to see if the vibration persists when the engine is started.
Start with those motor mounts and the transmission mount. At 74,000 miles and nearly 40 years old, that original rubber has almost certainly given up.
