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Old 09-23-2004, 12:53 PM
SL Owner
Join Date: Sep 2004
Location: East Coast U.S.
Posts: 131
Possible HUGE issue: MB dealers issuing potential problem spark plugs, rough idle

Folks, after endlessly chasing-down an intermittment miss on my car at idle only, you won't believe what I discovered. For my 1984 Mercedes 380 SL, Bosch and the original Mercedes manual recommend a Bosch spark plug part number W9DC (standard copper plug). But for the dealerships I've visited in the Washington DC area, who twice sold me replacement plugs, they provided me Bosch W9D0 (that's a zero on the end) copper plugs.

After trying to resolve this intermittent miss for hundreds of my own hours and some substantial mechanic work by myself and a shop, today I asked myself if perhaps the dealer could have made a mistake. I removed the plugs from my car, noted the last digit as a potential issue, tracked-down some W9DC plugs, put them in the car, and guess what. The idle is smooth as silk. You can put a wine glass on the engine. Intermittent miss at idle is GONE.

If you are having a rough idle problem, I highly recommend you carefully inspect your plugs. Match them up with the ** exact ** part number from your owners manual and only use that plug. Do not accept a "super-ceded" plug unless you absolutely have to. After reading 1000's of posts on the Internet about people trying to resolve rough idle problems, I cannot fathom how many may be caused by the dealership providing plugs "slightly different" from the original but causing issues in your car. Plugs issued by manufacturers that claim to be "compatible" may not be. Another example of that is in the case of platinum plugs, which are recommended by Bosch but many MB owners claim it too causes a rough idle.

I spoke with a Bosch technician on the phone. According to him, the Bosch W9DC0 has a bigger center electrode for "longer life." He stated that Bosch does not specifically recommend that plug for the 380 SL. He states there could be performance differences in the plug.

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Old 09-24-2004, 07:33 PM
Registered User
Join Date: Jul 2000
Posts: 638
spark plug and idle problem

the wrong spark plug will (too cold) will develop carbon deposits very rapidly and will leak electrical spark energy instead of jumping the gap, resulting in misfire.

If fact, if low speed driving is the diet, go at least one heat range hotter than standard. The 'standard' plug is guaranteed to not cause preignition from being too hot at extended wide open throttle. As if we can do that over here.

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