This is for the 91 300E W124:

In there right now (changed about 5 years ago): Bosch 7578 HR9DC

Pelican parts says to order: Bosch HK-9-DC 7978

O'Reilly's just sold me: Bosch 9659 HR9 B1133V

Owner's manual says: Bosch H9 DCO
Beru 14 K-9 DUO
Champion S 12 YCC

Gap = .8mm (0.032 in)

I'm assuming I have a 3.0L because of the 030 after the 124.

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1991 300E 124.030 103.983KE 722358 03 412178
207K

1979 240D 123.123 250K (Project car)

2000 Ford Ranger, 187K

2015 Dodge Ram 1500 EcoDiesel 37K

Obviously the owner's manual is correct. Bosch HR9DC0 are the replacement to osch H9DC0. The replacement being a resistor type. You have a resistor type plug in your car now as the "R" in the part number of the plug commonly means it is a resistor type. You have the option to either go to the dealer and get the Bosch H9DC0 or you can go to any local auto parts store and get any of the other brands that are recommended in the owner's manual which are now mostly all resistor type. I currently have the Champion RS12YC in mine. A lot of controversy about running resistor type plugs in these older cars that call for non-resistor type plugs, and your car apparently has had resistor type plug in it for the past 5 years and if you have not had any problems with the car then I would assume its safe.

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1990 190E 3.0L

Sorry last week I had to pull the resistor plugs out of my W202 as they were causing a slight misfire when cold. Plugs were only 12,000 kilometers old.

Installed correct NGK copper non- resistor plugs - BCP6ESF all is now well.

These same plugs are the correct variant for a M103, early KE M104.

NGK BCP6ESF. If you do a lot of stop start traffic you can install the BCP5ESF

Before you posted Ismalley, I went back to O'Reilly's to double check before installing the spark plugs. They switched them to the Champions (you have); they admitted they sold me the wrong plugs initially.



Ivanerrol,
O'Reilly's didn't have the NGKs in stock, so I went with what they had in the store.
(Lesson learned, I need to order the correct plugs in advance, maybe I'll buy 12 next time just to have extras on hand.)

As a side note on keeping spares, I have 2 brand new Bosch 7578 HR9DC I won't be needing - maybe I should give them to somebody who can use them?

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1991 300E 124.030 103.983KE 722358 03 412178
207K

1979 240D 123.123 250K (Project car)

2000 Ford Ranger, 187K

2015 Dodge Ram 1500 EcoDiesel 37K

You do know you can get faulty parts even in a new box. I have lost count over the years of how many faulty ignition components I have returned. Seems you had same problem.

However, I reckon you will find your ignition/fuel system below par and spark plugs took that crap in the face and failed.
Of course it could be the motor is not tight any more....same results. Oil/carbon/fuel and a weak ignition...wires etc, will kill any plug.

.

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