Recently my 1995 E320 wagon has experienced a major gas mileage drop. On the 19 gallon tank, we usually get about 400-450 miles on the tank. About 24-26 MPG. We are now getting around 250-300 miles on the tank, or 15-18 mpg. No Check Engine light has illuminated, but maybe the bulb is out? (It doesn't illuminate briefly on startup, either; should it?) There is no gas smell, and no visible leaks. My guess is the single O2 sensor before the cat. Is the mileage change this bad? The car smells like it is running rich from the exhaust, and it starts up now kind of like a carburetted car getting too much gas. Before I pay 120-150 dollars on a new O2 sensor, could it be the wiring harness sending incorrect engine temp reading to engine? How can I determine? Harness appears O.K., car still runs fine, all electrical O.k., just bad gas mileage, and a smellier exhaust. Thanks in advance. Car now has just turned 60,000 miles.

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2002 Mercedes-Benz E320 4Matic
1995 Mercedes-Benz E320 Wagon
2005 Land Rover LR3 V8 SE
1999 Audi A8 4.2L quattro

The Engine MIL should illuminate with the key on, as well it should extinguish when the engine runs. No MIL suggests that the ECU is not completing the circuit. It can be a few different items, with your vintage I would suggest a new OverVoltageProtection relay as a starting point. They were known to be an issue, not necessarily for this complaint but worthy of the investment.

The same thing happened with my 96 SL320, I had an inspection, once I got the car back from the dealer, the mileage dropped dramatically and I now smell gas when I start the car in the morning. The MIL does come on when the switch is turned on, but then goes out when the engine is running. What is an OVP relay and how much do they cost? Wouldn't this stuff show up on an ODB II car during inspection? I am really surprised at my decrease in mileage, the car runs wonderfully, maybe they adjusted my fuel type? because I was getting the MIL occasionally and they said it was because of bad gas? I always put 93 + in the car. If anyone has any ideas let me know.
Brian

DO you have a dmm and know how to use ???
If yes.. you can easily test the O2 sensor .

The car is OBD1 and EFI , so this is a quick test...

Under the passenger side rug is the connector for the O2 sens..
The green wire is a seperate plug and is the sensor wire [ the others are heater]
Unplug the connector and hook up the meter [volts/dc..] to the wire on the sensor side..
Under the hood is the fuel pressure regulator.
Start the car , get it warm, and then disconnect the vac line at the pressure regulator. This should instantly make the engine run rich and the sensor v. will go up to around .8-.9v.
[ keep you finger on the vac line end so you do not create a vac. leak when doing this test or you will be creating a lean condition]

Now , put the line back on and this time disconnect a vac line
[ on this engine , an easy on is the line that goes to the SWOs in front of the engine]
This will make the engine go lean and the meter should drop to low volts [ .2-.3v] .. as you put your thumb on/off this line , you should see the meter respond fairly quickly to these changes..

This is just a primary, easy O2 sensor test.. a scope is best, but for diyer, it works.
All you are doing is manually creating rich/lean conditions to the engine and seeing [ with the meter] if the sensor is responding to your engine mixture changes...
As you have disconnected the sensor from the ECU, it does not
have and closed loop feedback , so the ecu can not correct your manual mix....
You are the ECU...
It is also advisable to see if any gas comes out from the reg vac line when doing this test, as that is also a possible to your condition.[ bad/leaking diaphram causing high fuel pressure and raw gas going back to engine through vac line.- Common problem on EFI 104s]

Check both....for starters

Troverman:
This is like a bad case of deja-vu for me. I have a 1995 wagon and when it turned 60K the exact same thing happened. Your car may have been factory fitted with the same self-destruct mechanism that mine has!
Seriously, I did not know what the problem was but I needed to replace my HFM wiring harness anyway. The insulation on the wires leading to the top of the thermostat was cracked a bit, and I figured that if I changed it out I might save the car from being damaged by a bad harness. Unfortunately, preventative maintenance for this car. When I was finished, the mpg was up to 24/19 and the smell had disappeared.....a nice surprise. I checked the rest of the harness for deterioration. There were sections of the harness (sections that made more contact with hot engine than the rest of the harness) in which the insulation was so brittle that it just shattered when the wire was bent slightly.
Two different part numbers for the 1995 model, depending on date of manufacture. Mine was the more expensive one at $720. I think the other was about $100 less. Not too difficult do-it-yourself job. My thinking was that replacing the harness may lessen any collateral damage if done before it really needs to be replaced (i.e. when the car breaks down). And it will need replacing, it's just a matter of time.
Brian

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Brian
1995 E320 wagon

Check the codes from pin 8.

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Zafar
94 E320 58000 Miles

Sorry to reply so late and thanks all for the suggestions. Calling all parts stores everyone said (including MB dealer) they could not get the OEM Bosch O2 sensor for weeks on end as the factory was out. Consulted independent mechanic (MB Master) about possibility of O2 sensor and he said "wellllll..." Took a gamble and bought a Bosch "Universal" O2 sensor which according to Bosch is the same sensor as the MB OEM with a universal connector. Approx $100. Fairly simple job of splicing connectors together (included!). Had to use vice grips to get the old sensor out, plus make sure to let the car run so the exhaust is hot. (This was the key) In order to get the new connector to fit, you must do the electrical connections under the rug first, and then push the O2 sensor through the hole in the trans tunnel, or else the connector will not fit. (Trial & error in the rain) It turns out this solved the problem, and on a recent trip the car averaged about 24 MPG, including a few city miles. The smell is also gone. Actually, when running the car to get the exhaust hot for the original O2 sensor removal, the sensor was unplugged and the car actually ran better with NO O2 sensor than with the original! So its fine again...and the "universal" Bosch is about $50 cheaper. I did think of the wiring harness, and the insulation on the wires which go to the thermostat are cracking a little.... I think its possible either the Check Eng. light bulb is out or someone unplugged it to pass inspection, because it has not worked since our purchase of the car @ 39,000 miles. The car has always run perfectly, though, even with the bad O2 sensor. Thanks all, again.

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2002 Mercedes-Benz E320 4Matic
1995 Mercedes-Benz E320 Wagon
2005 Land Rover LR3 V8 SE
1999 Audi A8 4.2L quattro