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Old 10-06-2002, 03:48 PM
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Join Date: Nov 2001
Location: west orange, NJ
Posts: 47
Rough start, rough idle

Another rough idle problem for the troubleshooters. Here's the scoop:
1991 420 SEL (197,700 miles)
When cold- hard to start on first try, catches instantly on second try, then settles into idle with a noticeable, consistant miss. Very smooth above idle speed and drives otherwise smoothly. Mileage 16/20.
When warm-starts instantly, but idles and drives as above.
No sign of fouling on plugs, no obvious (visible) sign of a vacuum leak, tried water spray on wires and injectors but no change in condition. New fuel pumps and filterabout 5k miles ago. Trans doesn't always downshift as it should.

Any thoughts on where and how to look next would be appreciated.

1991 420 SEL (daily driver, 197k)
1991 560 SEC (Sunday Driver, 70k)
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Old 10-06-2002, 09:12 PM
haasman's Avatar
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Join Date: Jun 2001
Location: San Francisco, CA
Posts: 3,097
I'd say try cleaning your injectors. I had a car with even worse than yours in symptoms. Following another forum member's suggestion I put TWO bottles of Techron into a less than full fuel tank and within minutes the car ran much better. Warm it up and go for a drive. Preferrably running a little harder than usual.

Just remember to change your oil shortly after you have run through the tank of fuel+Techron.
'03 E320 Wagon-Sold
'95 E320 Wagon-Went to Ex
'93 190E 2.6-Wrecked
'91 300E-Went to Ex
'65 911 Coupe (#302580)
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