My 2nd O2 sensor is bad (the one farther to the rear, the less important one?), still, yes! I think I've been getting worse gas mileage since this occurred. Is this common? Seems like it would be because it has to do with the air/fuel mixture???
Also, on cold nights, like tonight (outside temp 45 F), with the heat on low, going 50 mph or so, the engine would not reach a temp of 80 C. I kept it in third gear to try and get the engine a little warmer, but it wouldn't work? I know that the engine temps on C's do go up a bit higher than 80 when conditions prevail, but what about lower? Anyway, my question is, is this normal, and could it be related to the bad 02 sensor? Thanks GUYZ.

Maybe you should check your thermostat as well as the O2 sensor you mentioned. A malfunctioning O2 sensor will certainly reduce gas mileage. Thermostats otday are designed to fail in the open position. This is good because the old way if your thermostat failed closed you would have no cooling at all and over heating is way worse than over cooling. It is also possible in cold weather that your engine temp will drop. I have noticed this, but mostly in sub-freezing weather, not much in balmy temperatures like you mentioned (we would kill for a day as high as 45F here). Especially when it is around 10F or lower the car will warm normally in city driving and get up to the 80 degree mark in just a little longer than normal. However, if you get on the freeway and get up to 60+mph all that 10Fdegree air rushing at your radiator drops the coolant temp pretty quick. You dash board temp indicator displays coolant temp only. What you might do before you test the thermostat is to see if there is a way you can partially cover your grill to keep this cold air out. This is done quite a lot up here, especially by truckers. They even have special custom padded grill covers. By keeping the cold air from the radiator you allow the coolant temp to rise much faster and be less affected by the outside temp. Of course make sure your air intake for the engine is not blocked, and most of these grill covers have some small flaps that can be opened (you have to get out of the car to do it) so that some air will get in. Blocking all the air from the radiator is bad as your temp will go up too much.

I can tell you that the C280 should not be below 80C. I have seen this car sit just over the 80 mark in 20F weather at 70+ MPH on the highway.

Someone once posted a way to get a reading of every sensor in the car through the climate control. To get a more accurate temp of the coolant, hold down the rest button for about 10 seconds with the car and climate control on. Then it will start flashing two numbers in the temp part of the display. The first number will be the number of the sensor, the second will be the actual reading. Each sensor is numbered in sequence. Use the blower + and - to cycle through to number 6 or 7? That should be the coolant temp. In the 97 C280, it is Fahrenheit, don't remember the exact number of what it should be, but it should make sense.

Ali
 

Wow, I had no idea you could do that!! But where is the reset button?? And I'll get the O2 sensor and see if that's the culprit. And is it bad for my car to run below 80C if I'm not driving it hard?

thanks

I said rest button, not reset button. More info is available at this thread.

http://www.peachparts.com/shopforum/showthread.php?threadid=9166

Driving the car with a lower than normal coolant temp will give you worse gas mileage and over an extended period of time can do more wear to the engine if it is not reaching full temp. I do not think these cars should really be below 80 at all with a properly functioning thermostat. Definitely replace the thermostat, it is an easy DIY job and is only about a $10 part.

[Edited by Ali Al-Chalabi on 02-15-2001 at 01:18 PM]

Ohh, sorry about that. Thanks ALI, you're the best!
That is the best info! When I get back into my car, I am gonna check it out!

DAVID

[Edited by Hottee18m on 02-15-2001 at 01:32 PM]

Ali or others...
 

Can you give me details on how to change the thermostat. Can you tell me exactly where it is and what tools I need, etc? Should I just go to local dealer (I'm going there tomorrow anyway) and get the part? I would like to do this myself if I can. Thanks. Today has been great for me! All this great info!!!

[Edited by Hottee18m on 02-15-2001 at 09:04 PM]

I have never done it on a Benz, but when I did change it on my BMW 325e, I spliced in a generic Bosch O2 sensor. The generic O2 sensor was $50 compared to $110 with the BMW logo on it. Same thing, just different connector.

Maybe someone can enlighten us as to whether a generic sensor can be used on the C280 and save some $$$.

Oh gosh Ali, I'm sorry about that, I meant the thermostat, as you recommended I change. I edited it to late!! Thanks and sorry again.

The thermostat is in the thermostat housing, where three big rubber coolant hoses come together next to the radiator towards the driver's side of the front of the engine block. It is held together by a few bolts (10mm maybe?). Drain some coolant out , open the housing, change the thermostat, then refill and bleed the system. Should not take that long, just make sure enough coolant is drained otherwise there will be a very big mess when you open the housing.

If your engine is not reaching operating temp, there is no other possible culprit besides the thermostat.