Wrong spark plugs

[MD5]

I've just changed the spark plugs on my 96 E280T (along with some other work), which was "serviced" just before I bought it late last year.

The problem I had was after running on a journey for an hour or so, the engine would slightly misfire at higher revs when overtaking, with the odd backfire when kicking down to overtake, but not all the time. Otherwise, the car drove beautifully, with a quiet, smooth tickover. No fault codes showed up after a Star check at my local indie.

I removed NGK plugs (which looked okay), and replaced them with Bosch (plus an air filter etc), and the difference is amazing. Pulling away is much more repsonsive, but I will have to wait until Thursday before I do a long journey (to Le Mans - anyone else?) to see if the misfire has been cured.

One question arises from this, in that when stationary, in drive and on an uphill slope, the car would roll backwards (prior to changing the plugs, and unlike my other MBs). Since the change, it now remains stationary (as do the others). I'm wondering how the technicalities work to explain this? I used to have a DS23 Citroen Safari, with a semi automatic box, and the smooth gear change was totally dependant on the carburettor and tickover being perfectly set up. Is it the same here (not the carburettor bit!)?

 

[grober]

What grade/type were the NGK plugs? You also changed the airfilter so it might have been that.
The "rolling back" is probably idle related in that the engine is producing a little more torque to drive the torque converter which will "hold" the car on a slope or cause a little transmission creep on the flat. One of the "handy" characteristics of torque converter based auto boxes

 

[Dave Lewis]

I would also be very interested to know the number/heat rating of the old spark plugs. I had similar running problems on my 280, cured by changing from the wrong plugs( fitted at a 'service') to the right ones. There was disbelief here that this could make much of a difference.

 

[w124coupe]

I would also be very interested to know the number/heat rating of the old spark plugs. I had similar running problems on my 280, cured by changing from the wrong plugs( fitted at a 'service') to the right ones. There was disbelief here that this could make much of a difference.

Not from me (as I said at the time!)...

Amazing how often the wrong plugs are installed - especially when the cars all left the factory with a sticker under the bonnet giving the right part numbers by brand!

 

[MD5]

What grade/type were the NGK plugs? You also changed the airfilter so it might have been that.
The "rolling back" is probably idle related in that the engine is producing a little more torque to drive the torque converter which will "hold" the car on a slope or cause a little transmission creep on the flat. One of the "handy" characteristics of torque converter based auto boxes

Aha, the torque thing makes sense to me, thanks. I changed the plugs whilst my mate changed the brakes all round, and I think he put the old ones in a bag in the back of the car, so I'll check and report back. Sadly, plugs and filters are about my limit, but I like to have an understanding of these things.

Actually, it's a pity the car is doggy, because it drives beautifully, and the geometry was sorted after having a new ball joint for the mot. I'm waiting for a new oil pressure sensor to arrive from Switzerland (ebay, £16.64), and it has a bearing noise from the alternator, otherwise it's sorted mechanically.

 

[lynall]

Wifes old m104 mengined w210 was also fussy on plugs anything other than the bog std cheapies caused it to run terrible.



Lynall

 

[MD5]

Just to let you know, the spark plugs I took out are NGK BCP 6E S. Would these have been the "correct" equivalent?

 

[Dave Lewis]

Just to let you know, the spark plugs I took out are NGK BCP 6E S. Would these have been the "correct" equivalent?

No, the correct equivalent is BCP 5E S.

I took out the BCP6ES ones fitted to mine, and changed them to BCP5ES, and it made a huge difference.

 

[MD5]

Were you not tempted to use Bosch, Dave? I think I read that MB and NGK weren't a great match.

 

[w124coupe]

Were you not tempted to use Bosch, Dave? I think I read that MB and NGK weren't a great match.

Merc list the Bosch, NGK and Champion equivs on the bonnet label and in the handbook as suitable.

 

[Dave Lewis]

Were you not tempted to use Bosch, Dave? I think I read that MB and NGK weren't a great match.

I've heard reports of Bosch quality control on plugs being a bit iffy nowadays, as they are made at different locations around the world. NGK have always been good for me, and a lot of owners in the USA swear by them for the M104 engine. The correct Bosch one is F8DCO.

 

[MD5]

I must admit, I bought mine from the dealership, as I like to keep my face in, as it were!

The part no.on the packet is A 003 159 67 03 26!