I have an intermittent stalling problem.
My engine is 103.983
Today I decided to check my sparkplugs after reading on this forum that I needed to use H9DC and not HR9DC. I wanted to make sure that my plugs were not the resistor types. Imagine my surprise when it was neither .My plugs were H7DC.

Now please tell me what is the difference and should I be changing to H9DC.

I checked with BOSCH here and they carry only the resistor types.
Also the first and the last plugs were loose.It came easily in my hand.
Could this be the cause of the stalling. It has always started right up.
Also I have noticed that the stalling goes away for few days when I add injector cleaner to the fuel.

Please help.

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300CE 1989 Euro Model.
ML350 2011.