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  #1  
Old 04-03-2003, 09:19 PM
slowlane
 
Join Date: Aug 2002
Posts: 198
Plug condition indicates Rich Mixture

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 Iím 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
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timreid
1992 400E
euro lights
500E sway bars
210 16in wheels
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  #2  
Old 04-03-2003, 10:02 PM
Registered User
 
Join Date: Feb 2003
Location: Ontario, Canada
Posts: 3,160
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.
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  #3  
Old 04-03-2003, 10:34 PM
slowlane
 
Join Date: Aug 2002
Posts: 198
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
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timreid
1992 400E
euro lights
500E sway bars
210 16in wheels
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  #4  
Old 04-03-2003, 11:26 PM
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Join Date: Mar 2003
Location: Southern California
Posts: 2,036
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
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  #5  
Old 04-04-2003, 08:20 AM
LarryBible
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Posts: n/a
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,
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  #6  
Old 04-04-2003, 09:46 AM
slowlane
 
Join Date: Aug 2002
Posts: 198
"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
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timreid
1992 400E
euro lights
500E sway bars
210 16in wheels
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  #7  
Old 04-04-2003, 12:04 PM
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Join Date: Mar 2003
Location: Southern California
Posts: 2,036
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
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  #8  
Old 05-26-2003, 01:36 PM
slowlane
 
Join Date: Aug 2002
Posts: 198
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 ďLudwig from Germany told me to try the Bosch F9dc0 plugs. Iím 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 itís a colder plug and may indicate a rich fuel system and the f9dc0 will burn the fuel better.Ē Iím trying these plugs, but is this the correct thing to do, the removed f8dc4 plugs are totally black?
Enjoy your drive, timreid
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1992 400E
euro lights
500E sway bars
210 16in wheels
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  #9  
Old 05-26-2003, 04:07 PM
Registered User
 
Join Date: Feb 2003
Location: Ontario, Canada
Posts: 3,160
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.
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