Spark Plug Riddle 1991 300SL
 

I am looking for an explanation for why just replacing the spark plugs in my 1991 300SL 65k miles would correct an engine stumble at throttle tip in from a stop. This imtermittent stumble would only happen until the engine reached normal operatin temp.

The plugs that were removed after 10k miles look clean and to be the proper heat range. Every time this problem has occurred it was fixed by either replacing a cracked dist cap or changing plugs.

sldan

Did you remove Platinum plugs?

Scott P.

Scott P

This engine did not use Platinum plugs. I have always used the OEM plug.

sldan

A new dist cap can make a big difference. I have experienced this on other cars.

Mike

I am wondering what is it about the engine ignition control, fuel system control, etc that makes this engine so sensative to the condition of the spark plugs?

Just wondered if any of the senior techs out there have put any brain power to this issue!!

sldan

hows yer eng wire harness?
you should not need sp plugs that often.

george

Possibly the coil output .
With new plugs, the profile of the center electrode is sharp and square, which allows a quicker (Lower.) voltage/amp discharge across the gap.

As gas errosion rounds off the electrode profile, voltage requirements increase and a weak coil will need more time to build up a charge to spark the plug....which sometimes will be a non spark event...missfire...or even the turbulence in the chamber blowing out the weaker spark.

Another reason why multi-ground electrode plugs have come into use on late model engines so manufacturers can prove/provide no tune up during the DOT mandated emmision complience distance.

You'll find loads of interesting reading if you Google 'spark plug'.



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dkveuro

I think you are on to smething. I had not thought of the coil but spark voltage and duration are a fnction of the coil's ability to provide the energy for a long hot spark.

The weak spark could cause a lack of engine response with a cold engine and a rich mixture. Initally the problem appeared to be a a classic lean mixture issue.

Thanks for the suggestion.

sldan

Does your vehicle have the 'G' coil design or the conventional type ?

I have found the 'G' type to fail more often than the conventional type.



.

The coil does not look like what I would call a conventional coil.

The part number is F3000-35769.

I think I have attached a photo but not sure I did it correctly!

sldan

Conventional coils look like this..

http://www.sec-shop.com/parts_126/coil_1.jpg

I would recommend renewing the coil you have with another...used, from wrecker yard.
Local company here sells them ( Mercedes type.) for $50.00.



.

The coil must be what you are calling a G coil.

I will replace it.

Thanks,

sldan