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Old 11-02-2004, 01:42 PM
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Join Date: Jan 2004
Location: Raleigh, NC
Posts: 14
Angry spark plug question

I replaced 2 W8DC Bosch spark plugs today with 2 W7DC Bosch spark plugs today. The guy at the auto part store said that the W7DC was just a "colder" plug and it should'nt be a problem to mix these with the others. Well, my car will barely start now and will not stay running. What did I do?

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Old 11-02-2004, 02:07 PM
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Razorback Soccer Dad
Join Date: Apr 2001
Location: Dallas/Fort-Worth
Posts: 5,711
MB selects particular plugs for a reason. Stick with what was already installed.

I can't imagine how you could run an engine with different plugs with varying heat ranges...combined with the octane you are using, could make for some pretty interesting timing issues. At best, use a complete set of plugs of the same heat range.

But. I'd go back to the 2 W8DC. If you can't find them, check with the MB parts you get a different answer.

I would imagine that you may have a problem with old plug wires too. Pulling them off and reinstalling after replacing the plugs could have disturbed their integrity...
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Last edited by G-Benz; 11-02-2004 at 02:11 PM. Reason: Adding text
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Old 11-02-2004, 10:50 PM
Join Date: Oct 2004
Location: CT
Posts: 3,513
Spark Plug Question (Dont do this)

It is unlikely that the guy at the auto parts store has a degree in in Automotive Engineering or Thermodynamics. The guy who selected the correct plug for your engine is likely to not only have such a degrees, but also has a labratory to do engine testing and access to feedback from the field, essentially millions of miles of experiance with just your engine. Now I'm not saying the guy at the parts store is wrong, just not the guy I would take my advice from.

Unless you do a lot of city driving and those two cylinders were the only ones working to begin with, I doubt putting the next colder range plugs in two of the cylinders will have a noticable effect on your cars start perfomance. Colder range plugs are designed to dissapate heat quicker, they have a shorter insulator. Typically a colder range plug would be used for high speed highway driving. This plug would have a tendancy to fowl in low speed short trip city driving.

By the way why were only two plugs replced.

John Roncallo
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Old 11-02-2004, 11:02 PM
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Join Date: Aug 1999
Location: Florida / N.H.
Posts: 8,804
My bet is you have those two plug wires crossed or the boots not all the way on..
One heat range should not make such a big difference ..
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Old 11-03-2004, 10:32 AM
SL Owner
Join Date: Sep 2004
Location: East Coast U.S.
Posts: 131

Arthur is probably right-on. First, you should only use the spark plugs stated in the MB manual-- not platinum ones (in the opinion of many and not all) and not ones with a 0 on the end of it (as-in W9DC0 instead of W9DC) based on my experience.

Also, you are mixing heat ranges since you said you only replaced 2 of them. So not only did you switch heat ranges, but you also mixed heat ranges. All bad. Even if you fixed the crossed wires or plug wire connections (per Arthur's post) you will still probably have problems.

My 84 380SL is very sensitive to spark plugs. While I have a set of NGK's with no resistance and haven't tried them, I can say that the car runs perfectly with a W9DC, the same plug the car was originally built with. If you put for example a W9DC0 in there, it develops an extreme irregular miss at idle-- the whole car shakes.

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