oxygen sensor

[BillKerler]

There are many C280's running with original oxygen sensors with mileage way over 150K. The sensors don't give a CEL and so they continue to be used. I have a story to tell.
My mileage dropped from 24 mpg down towards 20 mpg. The car seemed okay...no complaints except low fuel mileage.
The oxygen sensor before the cat has a huge role in determining the air/fuel ratio. The sensor AFTER the cat really only checks to verify the cat is doing its job.
I took the sensor before the cat out of the car using a crow's foot offset oxygen sensor socket. I took it to my work bench and put it in a vice. Pin #4 of the plug is + voltage output and pin 3 is - voltage output. I hooked these up to a digital voltmeter set on a 2000 millivolt range. Take a propane torch and heat the tip of the sensor...put the tip of the blue flame right on the sensor. It will take anywhere from 15 to 25 seconds for a voltage output to register on the meter. It should read as high as 1000 millivolts. When you take the heat away, the voltage should drop to zero within 1 second. Very fast response.
An oxygen sensor is a chemical electrical generator...just like the lead acid battery in your trunk is a chemical electrical generator. With the sensor, zirconium oxide on a ceramic base when heated to 662 degrees F will generate an electrical signal in the presence of oxygen...the more oxygen the LESS the output...100 millivolts is a signal to the car's computer that there is way too much air in the exhaust and so fuel injectors, get to work and dump in more fuel...pronto.
450 millivolts for an output is a perfect air/fuel type output signal from the sensor.
Mine was putting out 250 millivolts tops and worse yet, when I took the propane torch away, it took over 30 seconds for the voltage to decay down to zero...sluggish, very sluggish.
So my original sensor was telling the car computer that more fuel was needed...and when more fuel was added, the sensor did not quickly respond further confusing the car computer...well then dump in more fuel says the computer.
My spark plugs told the story...light fluffy black coating on the base. The tip was okay but yes there is evidence of over rich.
I bought a new Denso sensor (they look better built than Bosch to me) and did a bench test first. Right up to 950 millivolts within 18 seconds of flame and drops to zero instantly when the heat is taken away (the flame burns up oxygen in the air around the sensor...that's why you do this test with a propane torch instead of putting the sensor in an oven to test it at 662 degrees F).
Does the car drive better...honestly, the difference is slight. But the fuel mileage is right back up towards 25 mpg for normal driving. Pollution into the environment has to be less too.
A 20%+ fuel consumption improvement will definitely pay for the sensor over the next 100K of driving (100K miles is the sensors useful life expectancy).

[97C28O]

I had a oxygen sensor code come up and the car ran funky for a little bit. One day I had to drive it out cause i had no other car and the car ran fine after a while and the light dissapeared. I had already ordered the sensor but didnt replace it. Do you think its better to replace it if the car is running fine? I dont see much fuel consumption (as this car already eats a lot)..

[JRE320]

Originally posted by 97C28O
I had a oxygen sensor code come up and the car ran funky for a little bit. One day I had to drive it out cause i had no other car and the car ran fine after a while and the light dissapeared. I had already ordered the sensor but didnt replace it. Do you think its better to replace it if the car is running fine? I dont see much fuel consumption (as this car already eats a lot)..

IMO you better replace it. ;) Since you got it.

[BillKerler]

Your '97 is still an inline 6 with 1 sensor before the cat and 1 after the cat, right? If your car has over 100K miles on the sensor in there now, yes it should be replaced.
I hear many people say their 280 is a gas hog. Before the W202 I had the W113 ('79 280E) and let me tell you, with CIS fuel injection and no oxygen sensor feedback, that car was a pig on fuel (like 13 mpg city and 19 highway).
But the W202 has always been fuel efficient for me...near 24 to 25 mpg and I live in the Sierra foothills of Calif...up and down, nothing is flat.
If your car is hogging fuel, sure make certain the ignition works strong (plugs, wires), the air is clean and metered right, and make sure the sensor is good plus check your vacuum lines and intake manifold gasket or injector seals...check for vacuum leaks.
If you ever suspect injector seals or gaskets, spray WD-40 around the sealing area with the car idling...if the idle speed REDUCES a little and yet sounds STRONGER, then you found a leak (WD-40 acts like a very short term sealant).
Your car should idle in neutral at 600 rpm when warm...if it idles at a higher speed...you have a vacuum leak somewhere. Air gets into combustion which did not go through the MAF sensor but got detected by the oxygen sensor and the computer was told to increase fuel to compensate thus raising your idle speed higher.
When you take the oxygen sensor out of the car, check it with the propane torch to verify it really is worn out. Then there will be no question.

[97C28O]

This is now interesting.. I don't know where exactly those manifolds are since I don't know much about this motor. I know more of my VW haha. I know that my oil pan gasket is leaking very very slow, there's grim all over the bottom of the pan near the bolt, it is not leaking out from the bolt. I cleaned that up with some break cleaner. There's no oil driping from the car at the least..

I mght have to do some maintence check on the 202, it has 68,XXX miles. There are some issues i've been experiencing as well, like having the shifter in D but it is either in Park or neutral... If I move the shift a little it would pop into D. And I'm not sure if there is any major problems but at idle sometimes the car would vibrate at a mellow pace...

[BillKerler]

I did a thread today on vacuum leaks...look at it...that block manifold vacuum leak I described cured a lot of my transmission problems. On my '94 C280, there seems to be transmission vacuum lines as part of the overall operation and the leak in my vacuum manifold block was the culprit.
Your '97 has transmission issues...transmission fluid needs to be changed regardless of what anybody says. If you have ever seen the inside of an automatic transmission it is a maze of fine passage ways with springs and ball bearing check valves...yuck. Clean fluid is a must. Most problems go away with clean fluid filled up to the brim properly.

[xvvvz]

You need a pretty big propane torch to do this, right? The one the size of a small fire extinguisher? I would need to buy one for another electronic project and it would be great if I could get by with one of the smaller ones that put out a small flame and use a small canister but I don't think that would be large enough for the O2 test.

[97C28O]

Originally posted by BillKerler
I did a thread today on vacuum leaks...look at it...that block manifold vacuum leak I described cured a lot of my transmission problems. On my '94 C280, there seems to be transmission vacuum lines as part of the overall operation and the leak in my vacuum manifold block was the culprit.
Your '97 has transmission issues...transmission fluid needs to be changed regardless of what anybody says. If you have ever seen the inside of an automatic transmission it is a maze of fine passage ways with springs and ball bearing check valves...yuck. Clean fluid is a must. Most problems go away with clean fluid filled up to the brim properly.

The transmission fluid was done about 5k miles ago. When it was first done, it drove great. The tech also said something about my shifter bushing causing my shifter to be loose.. I remember creating a thread on here but its been so long. I will take a look at your thread.

[BillKerler]

You are right, the propane torch I use is the size of a small fire extinguisher. The flame needs to cover the oxygen sensor tip. The tip is about 7/8" diameter by 1 1/2" long. It will glow red during the test.

[BillKerler]

If your transmission shifter bushing is a problem forgive me, I have never had that problem.
My Porsche 911 has serious issues with shifter bushings and it is of upmost importance. Soft nylon bushings from the factory wear out quickly and there are aftermarket stronger bushings that are bullet proof.
But on my Mercedes, no, bushings have never been a problem.
(I bought my Porsche aftermarket bushings from Performance Products in the LA area who sells for Mercedes and Porsche high performance gadgets)

[strictlyspeakin]

Lucky you. I have had to replace the bushings twice.

Thanks for the thread Bill. I am going to replace my 02 sensors. I've been thinking that my car was running rich. Fuel performance isn't quite where I thought it should be, except on long high runs eg. 300 mile trips.

[BillKerler]

One very important point...after you cange oxygen sensors, please disconnect the negative cable on your battery in the trunk for 10 minutes. This will clear all the volatile memory inside your computer involving your misbehaving oxygen sensor prior to replacement. Start your car off fresh...anytime you make a major change to the sensors involved with ignition or air/fuel ratio, clear out your memory in the car's computer by disconnecting the battery.

[Sulaco]

That's a good tip, Bill.

I've read threads where people had their car act really strange when they replaced MAF/o2 sensors without resetting their computer (terrible idle/acceleration for days while it re-learned ratios).

I just need to buy a new positive battery cable. I can't seem to find one anywhere. I broke mine and I think it's causing some weird issues. It's still attached, though it's broken at the point where it wraps around the battery post.

[97C28O]

I changed mine out and noticed increased power delivery. I may have to go and disconnect the battery and see if the car runs smoother after a new calibration.