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.

__________________
300CE 1989 Euro Model.
ML350 2011.

The 7 and 9 refer to the heat ranges of the plugs if my memory is correct. I was recently looking at the Bosch web site and one of the pages there gave the meanings of all the letters and numbers in the plug designations. If you Google on "Bosch spark plugs" you'll find this information.

I don't know if the HR7 plugs would cause stalling but the wrong heat range on a plug can certainly result in the plugs getting fouled. What was the condition of the plugs you removed? A plug which runs too cold will not burn off deposits and will eventually foul and misfire. A plug which is too hot can eventually have damage to the ceramic insulator at the center point and can also cause pre-ignition (pinging). Sometimes an engine which has a problem with oil fouling of the plugs can be improved, for a while, by switching to the next hotter range of plugs.

I ran into a similar problem getting the H9DC plugs as you did. My on-line supplier sent me HR9DC and told me that Bosch was recommending them to replace the H9DC plugs. However, when I went to the Bosch site I found a whole list of plugs including the H9DC, H9DC0, and several resistor plugs. The supplier told me that Bosch had told him that they were not making the H9DC plugs anymore! The Champion S12YC are also recommended by Mercedes but I've not read any comments pro or con on these plugs. Most on this forum seem to recommend Bosch. I've used NGK plugs in my Volvos in place of Bosch and have liked them.

If you do some searches on spark plugs in this forum, you'll find that folks are having problems running resistor plugs in engines with ignition systems designed for non-resistor plugs. The plug connectors and also the rotor ( I think) have resistance built in so resistor plugs may add too much resistance to the ignotion system. The purpose of the resistance is to reduce radio interference.

I have the same engine as you do. I finally went to a parts house locally and they carried H9DC0 plugs which I installed and my engine idles so smoothly I can't feel it running.

I doubt that the plugs are causing stalling unless they are fouled very badly. Incorrect or fouled plugs would be more likey to give a rough idle, poor acceleration, and poor economy. Your experience with the cleaner may be an important clue and others may have some ideas on that. In any case, I'd go ahead and install the proper H9DC or H9DC0 plugs.

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1986 300E
1993 Volvo 940 Wagon
1984 Volvo 240 (daughter's)
1976 Dodge 1/2T pickup, gas hog

H9DC plugs are the recommendation for USA engines.

Rest of world engines are likely different.

You should obtain local information.

__________________
Kent Christensen
Albuquerque
'07 GL320CDI, '10 CL550. '01 Porsche Boxster
Two BMW motorcycles

This family of plugs is available in heat ranges 6, 7, 8, and 9 ,and the higher the number, the "hotter" the plug. For a given set of operating conditions a "hot" plug should be used if average engine output demand is low and a "cold" plug if average output demand is high - such as racing or high speed autobahn driving.

As stated, Mercedes recommends heat range "9" for most USA M103s. If you do lots of high speed driving - say over 160 KPH - a colder plug than "9" would likely be more suitable. Also, as stated, check Mercedes heart range recommendation for your typical local driving conditions.

Dukec

Thank you all for your replies.
The plug was a bit fouled.
I used to drive around 160 kms/hr but now they have radars all over the highway limited to 120 km/hr.
City driving would be 60 - 80 km/hr and freeway driving at 120km/hr.
I travel daily total 90 kms to work. i:e 45 each way.
So I believe I should find some H9DC or H9DCO plugs.

The summers are quite hot here averaging 40 to 45C. Would that affect the sparkplugs choice by any way.

__________________
300CE 1989 Euro Model.
ML350 2011.

Spark plug tip temperature is primarily a function of engine output, not coolant temperature. The proper tip temperature operating range is 400C to 900C. Below 400 the plug will foul, and over 900 can cause preignition, which can lead to detonation.

Low speed driving in hot weather can cause high coolant temperatures, but plug tip temperature will be at the low end or the range since average engine output is low.

It sounds like your driving conditions are similar to the US, so a heat range "9" would be suitable.

Duke

NO.

Only USA cars use H9DC and H9DC0 plugs.

Get the correct plugs for your car.

__________________
Kent Christensen
Albuquerque
'07 GL320CDI, '10 CL550. '01 Porsche Boxster
Two BMW motorcycles