QuestionI am slowly restoring a stright six automatic coupe. Engine sounds good, but when I decelerate I hear a noticeable clunk. I am concerned it is the transmission or rear end, but it seems to shift well when crusing or accelerating. Any easy way to determine what it might be without spending an arm and a leg or being told the transmission needs to be replaced? Could it be a u-joint or simply in need of a tranmission flush or rear end maintenance? I am not a mechanic but want to be ready when I have to approach a mechanic.... Odometer indicates 19K miles, but I assume it has already been around once. Engine and transmission appears to be original. No leaks that I can determine.
AnswerIf you hear just a single "clunk" when you first back off the gas, this is very likely a bad motor mount, either on the engine or on the transmission.
With the engine off, grab the top of the engine valve cover, and try to rock the engine side to side, to see where there is looseness. Typically on these engines, the driver's side front motor mount will be weak or broken, as these are the ones that take the most stress, and also get a bath in oil every time there is a leak.
If you find any problem with the motor mounts (and check the other two also), fix that first, before looking any further.
If, on the other hand, decellerating causes a continuous racket, it probably is a bad universal joint, or possibly a bad pinion bearing at the front of the differential. Climb under the car and shake the driveshaft, at both the front and rear - if you feel any looseness, take the car to a "driveline service" place - there are nationwide chains that specialize in tracking down this sort of problem.
Don't ignore the problem, in any case. If it is the motor mounts, driving it that way may cost you a radiator, if the engine slides forward on a quick stop. If it is in the driveline, it might be much more damaging than that, if it comes apart while you are driving!
Dick
