Brown spark plugs, what could cause this?

[aliharry]

Hi all,

I’ve recently replaced the spark plugs in my 2006 CLK 500, I’ll attach some pictures below but they are all quite dry and brown. Is this normal? Or could this be lean fuelling. I should add two of the plugs are only 2000 miles old (cylinder 3) and are being replaced due to wanting to replace the others which were not done. However even the new ones looked just as old.

if so what could cause this. I should add the car does have a rough idle and misfires under load / harsh acceleration. It always says cylinder 3 misfire however cylinder 3 had brand new plugs, coil pack and leads to no avail which makes me think the car is throwing cylinder 3 when it is in fact a broader issue.

Any help would be appreciated and here are the pics below. Thank you

 

[alecmascot]

A Brave Search :

https://www.autolite.com/docs/default-source/tech-specs/understanding-spark-plugs/autolite_plug-tips.pdf

 

[johnsco]

I would expect plugs from an engine with correct mixture to be light-brown.
White is too lean.
Black / sooty is too rich.
Black and oily suggests an engine fault.
From your post, it sounds like you have not changed all plugs at the same time.
This is a good place to start from ... Replace all plugs together ... Be sure that they are the correct plugs ... If there is any doubt, buy them from MB.
New plugs should come with the correct gap.
Fit them and torque them correctly.
If the engine still missfires, it is not the plugs.
Next place to look is the coil-packs and plug leads.
Then the engine management system which sets the mixture and the timing.
You may need a diagnostic tool ... Possibly STAR.
But - Get the basics correct first.

 

[johnsco]

The Autolite link in Post #2 is a good guide.

 

[grober]

Sure you got the correct heat range plugs? I favour NGK IRIDIUM TIPPED PLUGS

Mercedes M113 V8 Engine Problems, Reliability, Specs

Learn about the Mercedes M113 4.3, 5.0, and 5.4L V8 engine. M113 reliability, engine problems, specs, and performance.

tuningpro.co

 

[Muckypaps]

Google should really be you first port of call.
Move with the times.

 

[Dickster]

I’d say that’s normal, they should be brown.

ngk spark plug reading - Google Search

 

[Phil1968]

That's what I generally aim for when jetting the carbs on my motorcycles, a Rich Tea biscuit / brown colour. Usually indicates a spot on mixture. So no, I wouldn't say you have anything wrong there.

 

[Jobsworth]

Look perfect to me. Nothing wrong with that.

They have a glazed appearance if running lean.

 

[Jobsworth]

What’s the part number of the plug you have there?

 

[optimusprime]

Wish mine looked like those do .

 

[BoristheBounce]

Looks good to me

 

[grober]

Cars & Light Commercial | NGK Partfinder

www.ngkpartfinder.co.uk

 

[aliharry]

What’s the part number of the plug you have there?

Mine are the Bosch FR8DPP33+, I’ve heard ngk run a little better but these were the oem ones at the time so I’m running original.

 

[aliharry]

That's what I generally aim for when jetting the carbs on my motorcycles, a Rich Tea biscuit / brown colour. Usually indicates a spot on mixture. So no, I wouldn't say you have anything wrong there.

Ok sure, I replaced them all anyway as maintenance but was wondering if that brown colour was meant to be so soon. But that clears it up thanks

 

[aliharry]

Google should really be you first port of call.
Move with the times.

Have already done the research, my main issue being they looked just as worn as the rest even though two were brand new, that was my point of questioning. They were all replaced and the misfire still consists and many have had big struggles diagnosing misfires on the m113 block due to the ecu often having issues as well as the fuel lines and cat being clogged triggering the same issue. So I’m trying to diagnose it in a smart and effective way before dumping money into it.

 

[aliharry]

I would expect plugs from an engine with correct mixture to be light-brown.
White is too lean.
Black / sooty is too rich.
Black and oily suggests an engine fault.
From your post, it sounds like you have not changed all plugs at the same time.
This is a good place to start from ... Replace all plugs together ... Be sure that they are the correct plugs ... If there is any doubt, buy them from MB.
New plugs should come with the correct gap.
Fit them and torque them correctly.
If the engine still missfires, it is not the plugs.
Next place to look is the coil-packs and plug leads.
Then the engine management system which sets the mixture and the timing.
You may need a diagnostic tool ... Possibly STAR.
But - Get the basics correct first.

Sorry I should clear up, two new ones were put in while it was at the garage getting a gearbox rebuild and I wanted to do them all at once so waited a few weeks before doing them all myself at once. But yes will begin to dive into fixing the issue

 

[grober]

Might be worth doing a compression test on all the cylinders [ n3 esp] - these engines can be sensitive to oil quality

 

[aliharry]

Might be worth doing a compression test on all the cylinders [ n3 esp] - these engines can be sensitive to oil quality

I think that’s the best port of call next, I actually have pictures of the other plugs that are carbon fouled I believe. Does this help diagnosis ?

 

[Numair Aidroos]

Hi all,

I’ve recently replaced the spark plugs in my 2006 CLK 500, I’ll attach some pictures below but they are all quite dry and brown. Is this normal? Or could this be lean fuelling. I should add two of the plugs are only 2000 miles old (cylinder 3) and are being replaced due to wanting to replace the others which were not done. However even the new ones looked just as old.

if so what could cause this. I should add the car does have a rough idle and misfires under load / harsh acceleration. It always says cylinder 3 misfire however cylinder 3 had brand new plugs, coil pack and leads to no avail which makes me think the car is throwing cylinder 3 when it is in fact a broader issue.

Any help would be appreciated and here are the pics below. Thank you

God willing I've owned several gasoline cars. Never a Mercedes-Benz petrol though. However this formation and discoloration is fairly normal on all types of gasoline cars burning fuels efficiently. Nothing to worry about. Try switching the coil packs for cylinder 3, new doesn't always mean perfect. There could be any other underlying issue causing bank 3 error. Fuel injectors, valves for cyl 3 intake or exhaust, could be a slip leak where fuel is leaking past the piston (there would be fuel dilution in engine oil). You'll have to run various tests to hit the right diagnosis.