Today I replaced the plugs on my recently purchased 85 380se (121k mi.) because after fixing the fast idle problem (thanks guys), it idles normal but now misses. I replaced the distributor cap and rotor. I noticed that the gap in the old plugs were quite wide,so I went ahead and replaced them w/ bosch supers. Slight change to worse resulted. Were these plugs set at the factory or did I need to regap them? I noticed on some threads, some of you went wider than spec, why? I will check your answers before buying new wires but they seem fine. (no arcing) Slowly but surely building a beauty!

First let me say: Conrats on your purchase. Sweet car.
As for the plugs you bought, I had to gap the Bosch Supers I plugged into my block 3 weeks ago. One thing that I've learnt (among many other things) is this: check each plug with a gap gauge. I've even had to re-gap a plug insalled by the MB dealershop.

Your owners manual will tell you the proper electrode gap, look in the back. If not, set each for 0.8mm. (thats what my 85-95 workshop manual tells me)



Cheers for now
EADG

On almost all MB after 1980, gapping the plugs at 1.0mm(.040") will increase flame travel. This allows more air/fuel molecules to be in the GAP area.

Does anyone have a link to the Bosch website where you can obtain code #'s for MB's???

I'd really appreciate it because I could use some new plugs on my 190E.......

Mike McKinney

http://www.boschusa.com

A. Rosich
S320, 1998
E320T, 1995

Many thanks to EADG & MB DOC for the info. After removing the new plugs, they were gapped at .8mm, however the boxes they came in said .9mm. So I took MB DOCS advise and widened them to 1.0mm. Said beauty purrs like a kitten now.

Thank you Mr. Rosich!!!

bosch plugs gap
 

Are there any harmful effects of using a wider gap, such as lower fuel economy, less acceleration, damage to oxygen sensor, catalytic convertor? Why would the manufacturer give the wrong spec?

If following M. B. DOCs advice, then a properly gapped spark plug would in essence run better when it was worn out to .040". Of course the plugs do work better when new due to a sharp center electrode edge rather than a worn out rounded edge. I would still recommend a proper gap, the surrounding molecules will be involved in the burn "in due time". This may be a better way to ignite the fuel as it can create a longer sustained burn and actually help smoother running and more power.The electrical system was designed with a predetermined air gap, and I feel it is best to stick with it. Modular vee engines and I believe the updated 111 "EVO" motor use 1 mm air gap. Some of the "hybrid" FSS cars used Bosch F8KTCR plugs, which are dual ground lug plugs and were not supposed to be adjusted, unsure if there is a listed gap spec or not. I've found the normal Bosch plugs to be a little on the wide side when new, all plugs with a service spec on the gap should be inspected and regapped if neccesary as a matter of course. The plugs may be used for applications with varying gaps, so i'm sure Bosch (or any other manufacturer) doesn't really sweat what the gap ends up being as long as it's close. No disrespect intended towards M.B. DOC, thought I would give my view on this.....
Gilly

Wide Gaps Work.
 

Out of the hundreds of cars I've serviced, I have heard of only one(1)complaint from one(1)owner. A 560SL owned by a guy in Atlanta called back after a service and said he lost 3-5 Mph off his top speed....really. The car idled great! But the speedometer was a needle's width or so lower than it used to be on his favorite stretch of road.
The rationale behind the wider plug gap is that it uses more of the system's "potential" output. This gives a hotter/larger spark even at low speeds(like at idle, when you need it most). The ignition systems on Mercedes since around 1980 (with a few M104 exceptions, but those had a recall on the coils anyway)have more than enough reserve to make use of a larger plug gap without any adverse effects on the components. As far as the "out of the box" spec goes, both Bosch and Mercedes go for a "middle of the road" setting. Back in the 60s, most cars ran well with a .5mm plug gap. That was "spec". Then in 1990 MB decided that all cars should have a 0.8mm plug gap(the same "spec" that was on the Bosch box). The older cars with conventional ignition systems still run better with a 0.5mm plug gap. Think about it, if you "require" Techs to regap plugs(the ones that care do it anyway)by giving different specs for each application(like they should),that's $1 of labor on a $3 part. Life is much easier from the manufacturer's point of view to just screw 'em in.
The bottom line is, for whatever reason....wider plug gaps work. But hey...that's just my opinion(and what I've gathered from empirical data over the last 12 or 13 years).
Regards,
Randy D.

W8AC vs W8D
 

The label on my engine says to use W8D or W7D plugs. The closest ones available at my local checkers parts store was a W8AC. What is the difference? Would it be acceptable to use the W8AC?

Get the right plug. The Bosch catalog (which the parts store should have if they are selling them) will explain what the difference is between a "D" and an "AC" designation.
Gilly