The idle on my '86 300E smoothed out a lot when I replaced the injector seals. I identified the seals as leaking by spraying carb cleaner on each seal and listening for any change in the idle. If the idle changes, you know the carb cleanr is being sucked in around the seal.

My old seals were so hard they shattered like glass when I removed them. The new seals were nice and soft and inexpensive too. This isn't a hard job to do at all.

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1986 300E
1993 Volvo 940 Wagon
1984 Volvo 240 (daughter's)
1976 Dodge 1/2T pickup, gas hog