I've got a perplexing situation with the 82 SD. The engine has roughly 160,000 miles on it, as far as I know the original injectors have never been serviced.

The car has a LONG history (I have partial service records going back to the 80s) of having parts changed to cure this problem. I'll list just the "interesting" items.

1983 24,000 miles: "rough idle" listed on service chart, no parts changed
1992 87,000 miles: air filter mounts had to be changed; implies vibration
1992 88,000 miles: one valve had to be replaced. Isn't this a bit unusual for a car that was, at the time, so young?
1998 96,000 miles: Replaced motor mounts AND shocks
A few months ago, 155,000 miles: replaced air cleaner mounts
Last saturday: replaced motor mounts again.

And it still persists. The thing is, it does not do it consistently. Some days, it idles 100% smoothly and you sit there and don't feel a thing. Other days, at idle and at idle only, the engine SOUNDS fine, but it gives you a back massage from the seat bouncing up and down. Passengers are literally visibly going up and down in their seats a fraction of an inch.

The one "trend", if there is such, is that TYPICALLY, it is more likely to exhibit this behavior after a hot day, or after a long, hot interstate drive. If you putter to the grocery store on a cool day at 35, it typically idles smoothly. If you go 100 miles at 70 and then roll to a stop light, it's usually bouncing you around. But these are not "absolute" behaviors.



I am starting to wonder if this is just a typical 5-cylinder behavior. I have never owned or even driven one before we got this one. All I know is that you can't feel my worn out 4 cylinder or my nicer 6 cylinder in the seats, but you do feel this one.

The air filter vibrates so hard under the hood when you look at it at idle that you can't see the nut on top, just a blur as it moves back and forth.

Once you put a LITTLE rpm into the thing, it smooths right out. But I don't think it's a slow idle problem; it appears to idle at 750 or 800 rpm which I think is where it should. I don't want to "temporary fix" it by elevating the idle speed, because I like hearing them idle normally. Just not used to feeling it.

John, if I'm ever near Charlotte in the thing I may ask you to look just to compare it and see if your 5 cylinders are similar in behavior, but IDK when that will be so I'd rather just poll the forum as usual, first.

I'm wondering if this is just something all 5 cylinder engines do; I would think it would be perpetually out of balance compared to even numbers. Since this car has had a history since 25K miles of having a rough idle, I'm wondering if I just got a bad build or something.

Any thoughts? What else could it be since mounts didn't fix it like I thought they would?

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~Michael S.~
Past cars:

1986 300SDL
1987 300SDL
1982 240D
1982 300SD


Current:

1987 300SDL