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Old 10-29-2000, 03:43 PM
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My '89 300E with 116k mi just had its head rebuilt because of a cracked head gasket and a warped head. I can't recall how much was milled off the head but my trustworthy mechanic tells me it's minimal compared to other MBs he has seen.

After putting about 300 mi on the car, I checked the plugs and found that all 6 have a little bit of black carbon build-up (like a coat of gloss black paint) and the plug threads are soaked in black oil. The car runs like a champ and doesn't seem to burn oil, not enough to notice anyway. Also, I have records of oil changes every 3k miles since the car was new from the previous owner.

Is this carbon build up normal? What does this say about the condition of the engine. Thanks in advance.
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Old 10-29-2000, 04:29 PM
engatwork's Avatar
Join Date: May 2000
Location: Soperton, Ga. USA
Posts: 11,234
talk to your mechanic. You may just need to go to the next hotter plug.
good luck
'95 E320
'97 Honda CRV
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Old 10-29-2000, 06:05 PM
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Join Date: Mar 1999
Location: Suwanee, GA, USA
Posts: 4,712
Give it time. I would let the mechanic know, but I wouldn't be alarmed yet. I would drive the car a little more.

Be sure to use only the specified plugs and only copper core plugs (no platinum).

Donnie Drummonds
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Old 10-29-2000, 08:51 PM
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I must respectfully disagree with the "hotter plug" advice. Changing heat ranges on plugs from oem is a risky proposition. A long time ago I put a hole in a piston in a racing engine doing this. The problem is that they can raise combustion temperature. Has to do with the size (height) of the plug and how quickly it dissipates heat. If you choose too hot, you can indeed bring the piston surface to beyond the melting point of the metal, at high loads or speeds. You have to get up pretty early in order to beat the designer at his game. I have relearned this lesson over and over. You really should stay with the OEM heat range. The solution to excessive oil/carbon might have nothing to do with the top end (how about rings....)

Benzmac: I'd be curious as to why you have a problem with the Platinum plugs? Thanks.


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Old 10-30-2000, 09:16 PM
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I spoke to my mechanic. He said that the rings are fine and the oil on the plug threads are anti-seize compound (i feel really stupid). The carbon, he thinks, is from not driving the car long enough for it to reach normal operating temp (i only drive 8 mi a day - one way) OR it could also be the injectors. I'm using Bosch Platinum +4s but he doesn't see any problems with those. He said a couple of his customers claim to be getting better mileage with them.

I'll change back to normal copper core to be safe and start using injector cleaners. I feel much better. Thanks for all the advice.

89 300E

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