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Old 08-21-2001, 09:21 PM
Registered User
Join Date: Nov 2000
Location: Houston, TX, USA
Posts: 185
Exclamation 400E Rough Idle when cold? WHY???

Ok techies....

92 400E :

Within a couple of months ago, I replaced the plug wires, plugs, fuel filter, oil and oil filter, engine wiring harness and air filters. When the car is cold, and I first start it, the idle bounces up and down and the car runs sorta rough. This problem is even WORSE if the car has not been driven in a week or so. When it's doing this, if you give it pedal, the car dies when the RPM's come back down. This lasts about 5-10 minutes until the car gets warm. What could be causing this???

Peter Balias
1992 400E
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Old 08-21-2001, 09:48 PM
Ashman's Avatar
Service Advisor
Join Date: Sep 2000
Location: Sherman Oaks, CA
Posts: 4,748

Have you checked that?

Cold start enrichment vavle or whatever it is that helps cold start?

Fuel injector nozzles?

maybe a bad plug that smooths out when it gets warm?

My bro's car sounded like it was choking the other day, changed plugs and its perfect now.

Its a range rover though.

'92 300CE - Sold
2004 C240 - C7 Wheels
2015 ML350 - P1, Pano, Ash poplar wood, Sport Dinamica interior, Running Boards Keyless go.
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Old 08-22-2001, 04:04 AM
akry's Avatar
W140 Maniac
Join Date: May 1999
Location: Burnaby, BC, Canada
Posts: 960
Distributor Caps?? Check them, they might be wore or dirty... Also, check your battery as well as all ground wires....

Andy Kuo
1992 Mercedes-Benz 400SE

2002 Acura TL Type S
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Old 08-22-2001, 10:44 AM
Posts: n/a
I have the same problem on my 190e. Has been there for 10 months. Have spent nearly 2K. Still no change. Two mercedes dealers and two independent mercedes techs have not been able to figure it out. have changed plugs, wires, cap/rotor, injectors, OVP relay twice (everyone seems to blame everything on the OVP relay, but it doesn't always help!!), air/fuel filter, thermostat, installed a 470 ohm resistor on cis-e circuit, reset timing, NOTHING WORKED. These are all the standard things everyone replaces, without even checking if they need replacement!

This is a fuel system problem, but the dealers would rather throw parts at you than diagnose the real problem. If you solve it, do let me know!

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Old 08-22-2001, 03:12 PM
Michael's Avatar
Join Date: Jul 1999
Location: CT, USA
Posts: 2,699
Could be carbon buildup...went thru this with my 500E (same M119 motor). When you first start your car, your electronics set you up with a rich fuel:air mixture until the car's warm. However, carbon in an engine that hasn't run in a few hours is like a sponge (as represented to me by a BG Products engineer) and absorbs much of this fuel, resulting in a lean mixture. And when the mixture's too lean in a cold engine, the engine stumbles.

I used BG Products' "44K", which is specifically designed to clean combustion chambers. After 2 applications, my cold-engine stumble went away.

Just my experience...and by the way, Techron had no impact
"If everything seems under control, you're simply not going fast enough" --Mario Andretti

1995 E500 street car
1986 Porsche 944 Turbo S track car
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