Own a car with the M103 Engine-Must Read!!!
 

If you own a car with the M103 engine, this information is very important. I'm not sure if this information has been shared here before, but it bears repeating.
I own a '92 300E which has recently been experiencing a longer-than-normal crank time both in the morning and after initial warmup. Engine idle has also been rough at times with an occasional miss. The problem was getting worse with time. I had installed the recommended Bosch spark plugs about 10,000 miles ago. I was browsing another MBZ forum and read a thread stating that Bosch no longer makes the correct, non-resistor plug for this engine. Using the Bosch plug or any other resistor plug can cause problems with the ignition system. The spark plugs wires themselves contain a resistance already built into the spark plug boot. The two types of non-resistor, pure copper spark plugs recommended were the Denso T20EP-U and NGK BP5EPS. I purchased the Denso plugs and installed them. WOW-What a difference!!!! Instant engine startup, smoother engine idle, and no occasional miss. My engine was truly transformed. If you own a car with this engine, you should definitely check to see what kind of spark plug you are running. No platinum or anything alse. Just a straight, non-resistor, pure copper plug. It worked wonders for my car!!!

Yep, Kay; this has been an ongoing topic for a long time.
There is actually a "sticky" message at the top of this forum I think.
I first heard of it in 1991, when the dealer told me under no circumstances to run Bosch Platinum plugs in the 103. The wires/boots have resistance, and the dist cap has resistance, which are the required values for the system to funtion at peak performance.

DG

Dude, are you kidding me?

Well, its good to know... I will keep this in mind when changing the plugs in my 88' 300SEL (which I think it needs - it has a slight and occasional sputter at idle). They are so hard to get to you know. :D J/K. I have never seen plugs to easy to get to. All that concerns me is that I hate changing plugs on a car with aluminum heads. I am always afraid they will get stuck or something and I will ruin the head. :o

The plugs on the M103 engine are very easy to replace. The key is using the proper spark plug wire puller (I own the Hazet tool which grabs the plug wire down on the metal part). Rotate the plug wire a little bit while it is still on the plug before gently and firmly pulling off the individual plug wire. Clean out the inside of the plug connector (I used cotton swabs with a little alcohol). Then, use a good quality spark plug socket, make sure the socket is completely on the plug, and slowly but firmly loosen the plug turning counterclockwise (I used a 1/2 inch ratchet with a 1/2 to 3/8 adapter to get a little bit better torque). Remove the plug and check its condition. Check the gap on the new spark plug (.32-.35 inches), then put a small amount of anti-sieze compound on the spark plug threads. Put the plug on the socket and carefully, by hand, thread the spark plug back into the hole. It should thread easily. Next, using a torque wrench, torque the plug to 15 ft/lbs. DO NOT OVERTORQUE!! (I did all this with a cold engine). Before you put on the plug wire, take a cotton swab and put a small amount of dielectric grease on the tip of the newly installed plug. Take the plug wire and firmly push it onto the spark plug. You should hear or feel a click to let you know that the plug wire is completely seated on the spark plug. Hope all this info helps.

Good plug changing advice. I'll add one comment; do it with a warm (not hot) engine. Everything seems to unsnap and unscrew easier if the motor has been warmed up a bit. Cold wire boots and cold thread plugs just seem to really resist more.

DG

This info is the same for 420's and 560's because they use the same ignition system as the 300SE/L's.