I have a what I thouhgt was a miss at idle, but I am now wondering if it could be a bad motor/engine mount.

I have some steady, almost loping vibration, and I will randomly get a slightly more violent shake. This happens when in gear idling (sitting at a light). Also, the severity and frequency of the miss and loping change intermittently (randomly).

This happens warm or cold.

Things done to date:
-timing chain
-tune up (plugs, cap, rotor)
-ignition coil
-all lifters (had one that would not pump oil)
-Plenum gasket
-up stream O2 sensor
-TSB plug wire routing
-Cleaned throttle body and IAC (removed, on bench)
-Replaced Fuel Injector O-rings (had vacuum leak)

What does a bad motor mount feel like?

I did some searching and found the best way to test a bad a motor mount is to have someone put it in gear, step on the brake, and give it a bit of gas (to load the driveline), then to watch the motor and motor mount for movement. Is there a better way to test a motor mount?

Is it common on these trucks for motor mounts to go bad?

My truck is a 2001, 2500, 5.9L gas, 46RE (auto), 170000 km. I believe the previous owner used to tow a horse trailer on the highway.

Thanks,
Chris

 

If an engine is running smooth, you shouldn't get any motor mount movement at idle.

Doing the rev test with the brake on is good. Also you can jam a prybar where you can move the engine and watch the mount. A real wasted motor mount will be visibly broken. Also, if you've had an oil leak onto a mount (like the trans mount) for a while, it's safe to say the rubber is wasted. Oil breaks rubber down.