Collapsed motor mounts can affect idle speed in that they can allow the engine to tilt while accelerating, decelerating, or idling. This can result in a pulled or released accelerator cable, that in-turn can lead to unexpected acceleration, and high, or unsteady idle RPM. Resulting in what you describe as poor idle.

Start by replacing both your motor mounts (If deemed necessary, than see if you still need to deal with the belt tensioner, and engine shock.

Also check your accelerator, cable and make sure its not too tight or too loose.


Phil

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'95 E300 Diesel, 264,000 Miles. [Sold it]