SL 500 Spark Plugs - PeachParts Mercedes-Benz Forum

js1sj · #1 02-28-2011, 08:50 PM

So here is an interesting one,
I have had an interesting time finding the correct spark plugs for my sl500, and while this might not seem like to big of an issue, please read on. So my 1994 sl500 is in good shape having all the updated preformed which this model needed (wiring harnesses, ect) however after replacing the spark plugs I had a rough idle and miss, but only when the rpm was at idle either in neutral or drive. This was very annoying and sent me looking for the problem because of course my local shop would not supply me with the wrong plugs. But after talking to a family member who is an electrical engineer and also has this model they said the auto parts supply companies have incorrect data in their system. Supposedly according to their system Bosch FR8DC+ replaced the F8DC4 required for the car. This is just not right. As the car requires non-resistance (F8DC4) plugs and putting resistance plugs creates weak spark at idle. This is because on this model (1994-1995 sl500) the resistors are in the spark plug boot and putting resistor plugs in raises the resistance of the ignition system to the point of creating a weak spark( the wires have a built in resistance of 2000 ohms).

I tested in the plugs with an ohm meter:

FR8DC+: 6000 ohms of resistance.

F8DC4: 0 ohms of resistance

So with the stock system, F8DC4 and the resistor spark plug wires the total number of ohms for the ignition system should be around 2000 ohms. With the incorrect plugs which all auto parts suppliers in my area say are the correct plugs for the car the total resistance for the ignition system is 8000 ohms. With the correct plugs installed the car runs fine. I found all the correct data in the EPC.

Earlybird · #2 03-02-2011, 07:47 PM

That's the most timely post I have ever seen. I just got through installing a set of FR8DC+ plugs and the old ones were FR8DC4. I now have a low speed chug and a check engine light on my 1996 500sl. Thanks, you saved me from diagnosing everything under the sun except the new plugs I just installed.

cth350 · #3 03-03-2011, 08:03 PM

There are plenty of threads here about resistor vs non-resistor plugs. Bosch is pushing the resistor ones heavily. Most of us push back.

Benz spec'd out a total ignition impedance measured from plug to coil. Adding resistance to the plug means removing it from the wires. Gotta keep them matched.

While you're at it, make sure you have copper core plugs and not the platinum ones.

-CTH

Gusfazekas · #4 03-04-2011, 06:39 AM

I have the '95 SL500 with 107K miles on the OD. I replaced the plugs with Bosch Iridium, don't remember the model number. I have had no problems with idle, missing or check engine lights. Runs great. I don't know if these are resistor or not, doesn't seem to matter in my car if it is resistor.

rs899 · #5 03-04-2011, 08:59 AM

I understand Bosch discontinued the non-resistor plugs. NGK hasn't (yet).

I picked up a set of NGK BP5ES which is supposed to be one of the correct plugs for our '85 380SL (others being BP6ES and BP7ES) , but haven't tried them yet.

Benzyne · #6 03-04-2011, 10:23 PM

I replaced my Bosch plugs with NGK Irridiums. Great ! No probs at all. Not certain as to which type they are.