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Old 08-01-2002, 04:22 AM
Posts: n/a
Question Bad Oxygen Sensor?

My car is smoking. (1987 420SEL 99,000 miles) I can drive it around for about 45 min before white smoke starts coming out of the tail pipe. I asked a mechanicly inclined freind and he said it might be the O2 sensor.
His theory is that the O2 sensor is on it's way out but not bad enough to trip the little light on the dash. He claims the car starts smoking after 45 min becouse the sensor gets hot, sends a bad signal to the motor, and makes the engine run way to rich. I have noticed bad fuel economy but to be honest I haven't had the car to long and I'm not sure what to expect. Is this possible?
I know it may be the head gasket but if it was wouldn't it smoke all the time? I hope it's not.....I don't wanna spend the money. WHO DOES?! WHAT IS GOIN ON?
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Old 08-01-2002, 07:12 AM
Registered User
Join Date: Jul 2002
Location: Trenton, NJ
Posts: 248
I'll agree with your friend that your problem is the O2 sensor. I just went through a period of about a month on my '87 560SEL with extremely bad mpg (7 vs 17) and after replacing plugs, wires, cap, and rotor, I finally got to the O2 sensor and things seem to have improved dramatically.

There's still a little sign of roughness and the smell of rich exhaust but not the clouds of smoke that would follow me and the smell of someone pouring gasolene on the ground that I had had. I have an appointment next week with a local shop that has been recommended to make the remaining fine adjustments that I don't have the tools to do.

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Old 08-01-2002, 10:38 AM
pmizell's Avatar
Benz Zealot
Join Date: Jun 2001
Location: Houston, Texas
Posts: 838
Black smoke would indicate that your engine is running rich, not white.

Is your car running rough? White smoke would indicate water could be entering your combustion due to a blown head gasket.

Check your oil dipstick for any signs of coolant in the oil and go from there.

I wouldn't drive your car under these conditions until you find out what's going on -- serious damage could result if you're getting coolant in the oil (hydrolock, etc)

Good luck
'01 E430, Sport 72,000 mi
'98 C280, 126,500 mi
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Old 08-01-2002, 10:57 AM
Join Date: Mar 2000
Location: Gainesville FL
Posts: 6,844
The v8s almost never blow white smoke due to coolant problems. Its almost always is oil burning due to valve seal problems. The O2 sensor can't make a properly set up car run rich enough to smoke and if it did it would be black. Huge amounts of fuel can smke blue but won't run on those cylinders.
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician
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Old 08-01-2002, 11:15 PM
Posts: n/a
Thanks for all the help. I really don't think it's a blown head gasket. The car is running smooth, even when it starts to smoke. I checked the oil and there are no bubbles. Wouldn't the car smoke all the time if the gasket was blown?

I tryed this trick where you take the cap off the radiator in the morning when the motor is cold just to release any pressure. Put the cap back on, start the car, run it for 30 sec to a min. Stop the motor and take the cap off to see if a huge amount of pressure has built up. I didn't.
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