Is it normal for the engine and car to shake very slightly and irregularly when at idle? I wonder if the motor mounts are broken. (1985 244 sedan).

A steady, slight rocking back and forth is normal since 240 motor mounts are composed of a softer rubber than most cars. This is intentional so they will break away in a severe frontal accident and the motor will drop to the road and not end up in your lap!

Irregular is the key word here. If the rocking is irregular it could indicate a few possibilities as mentioned by the other posters. Namely a bad motor mount or vacuum leak. It could also mean that your fuel pressure regulator is starting to go. Saw that on a friend's 740 once.

If the RPM's are fluctuating at all, you will get this. Often it is a minor vac leak that the IAC is constantly trying to compensate for (assuming the '85 is a LH...can't remember).

A bad motor mount is pretty easy to spot on a 240...check the clearance to the oil filter.

No matter what, an L4 motor with no balance shaft mounted longitudinally will have a less than silky smooth idle.

I'd say that some vibration is to be expected, and if the operative words are "very slightly" I wouldn’t worry too much. That does not mean that you shouldn't bother to give them a closer look, though. The one that typically tends to go out first is the one on the passenger side, as it is too easy to drench it with oil during oil filter changes (it is also easy to avoid it, if you take measures). Take a look at this mount below the oil filter. Does it bulge much? Is it cracked? Are you able to wrap your fingers around the oil filter, or is the space too tight? The thing you absolutely want to avoid, is having the oil filter resting on the motor mount, which will eventually happen if you don't replace in time.

My two cents.
Erling.

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