The idle of my 91 300SE has been less than glass smooth for some time now. If a Chevy idled like this, I probably wouldn't care, but I expect this car to idle like the engine was off.

As it idles now, the vacuum gauge is 1/3 - 1/4 from the max vacuum reading. Neither the vacuum gauge nor tach fluctuate. It's the same whether idling in drive or neutral.

If I raise the revs to ~850rpm in drive or ~1050rpm in neutral, the vacuum gauge goes to full vacuum and a passenger can't tell if the engine's running. I can tell because of slight vibrations through the pedal.

I noticed something new today. I had the opportunity to bring the engine to redline on a short merge lane. I can't remember the last time I revved the engine so high. Further along, I came to a stop and the idle was more rough than before this romp.

It's difficult to start after it's been sitting for a while. Then the engine knocks about almost violently for a couple of seconds then settles to a smooth idle. It doesn't get to the mildly rough idle until the engine is at operating temperature of ~82C.

The OVP relay fuses are fine. Engine mounts were fine 6 months ago.

Any ideas?

Thanks,
Sixto
91 300SE
81 300SD