W123 hydraulic motor mount trial & result

[BillGrissom]

I got the silly idea that a hydraulic mount, as used in a few high-end cars would eliminate the cabin noise from my diesel, especially that bothersome 3000 rpm highway whine at 65 mph (great gear selection M-B geniuses!). The only one that jumped out w/ in-line studs was the 1997-2012 Corvette (& 2002-2004 GTO), which was surprisingly cheap ($12 ea), so I figured why not try. I haven't read that anyone else ever tried similar.

The biggest mod needed was to rethread one stud from M10 to M8 coarse to thread up into the M-B mount. I ASSumed the ~1" extra height wouldn't be a problem, and shouldn't a gushy hydraulic mount sink a bit. Wrong, I could barely close the hood and had to wire in the radiator shroud high to not hit the fan. I also had to jack the engine up to the max to get the mounts in, even after cutting both studs as short as I dared. It was also very hard to get the stud started since the mount rubbed the inner fender at that height, especially with one side slightly higher and tilted. I had to crow-bar the engine over to get it started, and also to drop the lower stud into the hole. I was so geared up that I found a stainless bowl that fit perfect, after cutting one side to clear the K-frame. I could fix the height issue by machining off ~3/4" on a spare set of mounts, but read below first.

How did it work? Terrible. The whole car body shook so bad I could hardly stand to even idle, and didn't get much better at highway speed. Initially, I left off the engine shocks, thinking who needs those w/ hydraulic mounts. Installing them didn't help. I also wondered if I had somehow changed the engine, like when I knocked off the fuel filter and might have got air in the hose, but cracking each injector until it bled didn't help. The problem appeared to be that the mounts are actually stiffer than the factory ones. I tested them on my hydraulic press and they need some force to compress, almost like they are solid rubber.

I finally gave up and re-installed factory mounts, w/ polyurethane fill, as before. But, this time I installed 1/4" backer plates underneath to support the poly since otherwise it will eventually put too much force on the thin sheet-metal underneath and crack it, as it did in my 1984 (earlier post). The engine now idles and runs much smoother, but no better than before the hydraulic mount fiasco.

Probably for the best since hydraulic mounts are a continual problem in Corvettes, as well as Lexus (different design). Owners regularly find them leaking brown gook and collapsed. Many change to after-market solid polyurethane mounts for hundreds of dollars and live with some buzz.