Mounts vs Idle w126
 

1987 560sel with bad idle either warm or cold on startup only then "cleans up after about 3 minutes"

The mounts are bad and will replace asap.

I understand bad mounts will cause a bad idle condition "How so"?

Thanks
John1

When I replaced them on my 560SEL I noticed a big difference on both idle and (D). So the answer is yes.

Mounts don't change anything about idle conditions of the engine. They only affect the coupling of engine vibrations to the body. Changing the mounts may make it seem that you have a smoother idle but you have done nothing about the idle. Its better to evaluate idle conditions by observing the engine directly and not thru the body. So the answer is no.