My 420 (149,000 miles)was idleing rough and the timing was making noise so I replaced the timing chain, chain guides, water pump, belts and hoses and installed new platinum spark plugs. The distributor cap and rotor looked fine so I did not replace them. After the last bolt was tightened I started the engine and noticed it was still idleing rough at ~600 rpm. If you kick it up to ~1000 rpm or higher it smooths right out and I must say purrs with that new chain. Any how, even after driving it for a while the Idle is still rough, cold or hot, does'nt seem to make a difference. Acceleration is great, cruising is great, just rough idle. Any suggestions? are the platinum plugs ok for that model year? What is the recomended gap for the plugs? Any help would be appriciated.

Do not use Plats. Mercedes says Do Not. Use the Bosch copper core. Set the gap at about .035 or so. A little higher is said to help with idle. I don't know if that is true!

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Benzmac:
Donnie Drummonds
1992 500E (very soon I hope :)
1981 280GE SWB
ASE CERTIFIED MASTER AUTO TECHNICIAN
SERVICE MANAGER FOR 14 BAY FACILITY
MERCEDES SPECIALIST 8 YRS
PARTNER IN MERCEDESSHOP.COM

bob u since them was no email i am telling you trorgh this if you click on my profile and go to my hompage you will find a link to a s -class club on yahoo if you wnat to join.


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1985 euro 500sel 220k
1981 300sd 278k sold
1989 chrysler
leabaron 130k
1990 320sl (hopefully)

Bob,

I too was experiencing rough idle on my 4.5, while running plats, although I didn't notice the roughness until after the car was down about 3 weeks for repair. After reading multiple responses here regarding platinums, I picked up a set of Bosch copper cores. The idle has smoothed considerably and power is increased. I also noticed increased mileage on the last tank, which fed my normal drive as well as two full throttle runs past the century mark to blow out the crud (exit to exit on the interstate).

Prior to changing the plugs, an interstate run would clean the system for about a week. After a week, I would notice black smoke under hard acceleration. Always thought it was just build up from stop and go runs to and from the office. Besides causing a rough idle, the plats apparently caused the engine to run a bit rich, as evidenced by black deposits around the electrodes. It's been a little over a week since the last run and I had no black smoke today under acceleration.

The book I have states my gap should be between .028 and .032, I set them all to .032.

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Mike Tangas
73 280 SEL 4.5