Plug condition indicates Rich Mixture

[timreid]

At 76,000 miles I changed spark plugs on my 400E, and the old plugs condition indicated a rich fuel mixture, so I decided to change oxygen sensor.
Yesterday I started detailing my car, and I decided to pull a couple of plugs for inspection and it is still indicating a rich fuel mixture (92,000mi). I would like to learn/correct this problem, but I’m not sure what to do, or how to hunt down this problem, need special tools, or is this one for the dealer?
Thanks, enjoy your drive, timreid

[manny]

Some questions that come to mind are:

1. Are the plugs the correct heat range ?
2. Do you do a lot of short trip driving ?
3. Are you experiencing any check engine lights ?
Anything else you can tell us, regardless of how trivial ? We thrive on trivia.

[timreid]

Hi manny,
1. A mixture of driving, small percentage of city (can't really say city, I live in a small town) mostly highway, engine always reaches operating temperature. Driving style would be on the fast side.
2. The plugs are factory recommened Bosch Super F8DC4.
3. No engine lights
4. Use mostly Amoco 93 octane gasoline, gas milage has drop a little (2 mpg), I thought do to age?
5. Engine does not start hard, no misfiring.

Thanks timreid

[Duke2.6]

Tell us why you think the plugs indicates a rich condition - photo if possible. If you are passing emission tests, I doubt if there is a problem.

Plugs will always accumulate deposits, typically black on the metal shell when running unleaded fuel. What really counts is the deposits/color of the porcelain insulator.

Duke

Duke is right on the money here. Unless the plug is just fluffy black EVERYWHERE, it is probably not a rich condition.

If you need peace of mind, have it checked. It doesn't take long to connect a PC with O2 trending software and check it.

Good luck,

[timreid]

"What really counts is the deposits/color of the porcelain insulator. Duke"

Sorry should have described the plugs, Duke your right on the money, the porcelain insulator is black, (not oily) but the dry carbon fouling look. I thought changing O2 sensor would have corrected the problem, a wrong guess on my part.
I need to get set up so I can send pictures.

Thanks timreid

[Duke2.6]

The insulator should be clean - off white to tan, maybe with some tiny reddish brown specs. If the insulators are black that is an indication of a rich mixture or too cold a plug or short trip driving that doesn't allow the plug insulator to warm up sufficiently to burn off deposits, but from your driving habit description this doesn't sound like the problem.

As suggested a check of O2 sensor performance is a good place to start. If you live in an area that requires emission testing, a mixture rich enough to leave carbon deposits on the insulators will likely bust an emission test. If the car is running okay you could wait 'till the next required test, or have a "training test" done to see how it's performing.

Duke

[timreid]

Decided to have the dealer tech inspect the plugs. He agreed the plugs indicated a rich fuel mixture, he check the O2 sensors and it tested faulty, replaced sensor.
I driven about 1000 miles on new plugs, O2 sensor, I pulled the plugs to test my compression, and the plugs still indicate a rich fuel mixture. On my forum question about compression, I noted a friend “Ludwig from Germany told me to try the Bosch F9dc0 plugs. I’m using Bosch F8dc4 that the owner manual indicates to use, but it also calls for F9dc0. Ludwig said MB went to the F8dc4 to improve the idle, however it’s a colder plug and may indicate a rich fuel system and the f9dc0 will burn the fuel better.” I’m trying these plugs, but is this the correct thing to do, the removed f8dc4 plugs are totally black?
Enjoy your drive, timreid

[manny]

timreid

You have nothing to lose by trying a slightly hotter plug.

On some vehicles, I may ask people to test the coolant/aircharge temperature sensor, just to be sure the values are in spec.'s, as an errant reading may tell the ECU to enrichen the fuel mixture to compensate for a " cold " engine.

However, it is only speculation on my part, that this would apply to an M-B.