Could you please look at my spark plugs?
 

I just pulled my plugs on my 97 SL500 for the first time, and I don't like what I see. The car's been running fine, though I've been ignoring some minor uneven idling. But now that I see the plugs, I can see why.
Also, though it doesn't show in these pics, the center electrodes on the 2 plugs I've pulled look like melting ice-cream cones. On both, the electrodes have worn down, much more so toward the ground electrode direction. (It's like a slope going down toward the ground electrode side.)

Plus, the back side of the ground electrode looks powdery, and it's yellow-greenish.

The wet carbon fouling could be because I've been running 91/87 fuel mixture the past two tanks (before that I've been strictly 91), because the ceramic is still fairly clean. But the overall plug is rusty, and I definitely need new plugs.

Could anyone provide some insigtful diagnosis

The rust color on the insulator is normal for a very old plug. They do that usually after they've been take past their normal interval. Have a look in the supressor to check if there's any corrosion in there.

The plug tip is very worn, they've gone too far past their normal interval (i think it's 60,000km/40,000mi from memory). The ceramic looks about the color i'd expect for the 119 v8, you don't want it too white (too lean)

Which ignition system do you have? twin coil? direct ignition/8 coil packs? If you have twin distributors then check the distributor car and rotor for wear and replace if required.

Check your suppessors for correct resistance (with direct igniton) or check the whole igniton lead (inc coil leads) for resistance (with twin distributors).

This is a good step forward to finding your rough idle. (mechanical or efi fuel system btw?)

Direct Ignition
 

I have direct ignition. How can I check the suppressors for the right resistance? Which multi-meter from what vendor would you recommend for general testing of cars?

I think the car runs great given the eroded electrode. But new plugs would do be wonders. As far as the fouling characteristics, I don't know whether the 91/87 mix has retarded the timing too much to leave a residue, so I better put in 91 fuel and see how the plugs look after a week. But checking the suppressors would be a good thing, too. I just hope my valve seats aren't going south....

How long have the plugs been in their? Are they original? They just look like an old plug that needs to be changed.

Btw running a lower octane won't hurt anything but it is false economy. You engines computer will just adjust the timing so it won't ping but your power and mileage will go down.

No, not original
 

No, I don't think they're original. The car has nearly 140K on it, and it was starmarked at about 40K or so. But I think the plugs are 50K or older, judging from the oxidation on it.

I think I'll not worry about the electrode eroding, since the plugs are obviously very old. But why the wet carbon fouling is a mystery, since the insulator is obviously free of it. It's probably due to the 91/87 octane cocktail--the timing was retarded, and the combustion isn't complete due to the mix. I'm going to put in 91 gas and change the K&N air filters with Mann paper (I don't want to risk my MAF) and put in new plugs at the same time. If I still get wet fouling, then I'll worry.

Btw, how do I get Bosch from the internet (like Fastlane) and guarentee that I'll get the German ones? :)

"Btw, how do I get Bosch from the internet (like Fastlane) and guarentee that I'll get the German ones?"

Why not just order from Fastlane.

If you coat the ceramic and the inside of the spark plug boot with dielectric grease, you will greatly help eliminate the rust problem.

The threads on your plugs look almost like new. Did you wire brush the threads? or the threads could have been coated with anti-seize when they were replaced before.

Too Much Motor Oil
 

Oh, I didn't realize Fastlane would always send the German Bosch....

Btw, I think I found out why I've got the wet carbon fouling. I changed my oil a dew days ago, but I think I have about 9.25-9.5 Qts. of oil in there (the capacity is 8.5). I'm going to drain the car tomorrow of 1/2 Qt. and drive on the free way for a bit. I'll reinspect the plugs then and post pics again....

If the above is the case, have I caused harm to my engine? :(

Pull the supressors off the coil packs and check the resistance of the supressor. It should be around 2000ohms (it'll be written on the side of the supressor if they're genuine.) Any that vary greatly from the norm should be replaced. don't expect them to read right on 2000ohms, they will vary and still be ok.

MB plugs are almost always black on the edges of the shell these days.

Those plugs are just worn out, there is nothing else wrong with them, and the gap is now wide enough to give preformance problems, to say nothing of the condition of the center electrode surface!

Non-platinum electrode plugs really need to be replaced at about 30,000 miles, I think, or any time the center electrode begins to round off. The sharp edge of the electrode produces much better spark than a rounded, rough surface, and ignition preformance will decline.

Platinum tip electrode plugs will last much longer, but are only available as resistor plugs and you must adjust the resistance of the entire secondary circuit to compensate -- different suppressors, or different wire -- so that the totat resistance is the same as for non-resistor plugs. May be easy (on my older cars I just substituted non-resistor plug wires), or may be a real pain.

NGK plugs were very well in MB engines, too.

Peter

send me the entire set of plugs. mark them by engine location.

i would be happy to read them.

i would be cutting out the insulator core from the shell, however.

tell me, can you spin an insulator core in its shell?

appears as if you might be able to.

Drained overfilled oil....
 

Thanks guys. I'll check the suppressors next weekend if I get a multimeter. And I'll order NGK's for this car. And I'll send Albert my plugs....

Update: I drained about 1 Qt. of oil, and the oil level is now below the max level on the dip stick. It's drizzling in LA, so I couldn't do much else. But next week, when change the plugs with new NGK's, I'll take pics of the old Bosch plugs and see if the change in the oil level and a tank of 91 octane gas will burn off the new wet carbon deposits. And, by then, I should have new Mann air filters to replace my dirty K&Ns (I don't want to risk over-oiling) and a new fuel filter, so after a while, I'll see how the NGK's are doing....

Cheers! :D