burning serious gas

[benzo23]

Hey guys, its been awhile since I've been on but think I have a serious problem...so far on this recent fill up I topped it off and I have exactly 1/4 of the tank left and I've only gone 170 miles....I can usually get around 350-375 out of a tank...my driving habbits have been the same as before conservative with some highway cruising speeds on the bridge, things I've checked so far...air filter looked clean and I knocked any sand or dirt out which there was barely any, plugs are good, cleaned the maf with crc maf cleaner and that looks good, changed the gas filter about 5k ago and ran like a champ after that.. I've checked under the car to see if I can find any gas leaks seaping from the line and they all were dry...the car performs like normal and I see so decline or increase in performance, all I know is that its only been 170 miles since I filled up and now I have to put another 65 bucks in... Any suggestions would be great..thanks guys...doug

[sensoryx]

You sure its not a problem with the gas meter itself?

[benzo23]

i just got off the phone with the guy who usually does the work on it and he said these cars are bad for having the sensor in the gas tank go back so even tho it says ive only got 1/4 left it still might be over a half...im thinking its either that or maybe i had an o2 sensor go bad...but i would of thought that would have thrown a light since its an emmisions issue..

[xvvvz]

Time to go fill up your tank (put the gas in at slow speed to make sure it really fills up) and thus see how accurate your fuel gauge is reading.

[benzo23]

ok this is what i did, i looked in the owners manual and saw that our tank is exactly 16.4 gallons so i went up to the station and put in 4.1 gallons which is exactly 1/4 a tank...and when i did the needle moved exactly 1/4 of at needle...so i now doubt its the sensor in the tank i am now a believeer that it is prob the o2 sensor, and if so then which one? cause i know there are 2, one behind the cat and one before the cat...i did find where it plugs in to under the hood i prob spent 20 min looking for it and then like an idiot saw it right next to that thing that says "battery in trunk", now if i do replace it is it better to get it from the top or get it from the bottom of the car?, and who knows it might not be the sensor...any more ideas?

[timmyj51]

I seriously doubt its the 02 sensor. Can't imagine how that
affects the gas gauge. As your mechanic said, the fuel sender
units in these MBs are notoriously bad. The one in my old
190E was bad and the one on my C220 is also bad. Don't
see why you even need to do anything, though. Just flip your
trip odometer to zero when you fill up and keep track of
your gas that way. This is what I've been doing for years.
For what it's worth, MB put out a TSB on this problem with the C class and
said it's often caused by a pinch or break in the fuel sender
unit wire where it threads under the back seat. One of
these days I might have that checked with my car.

[benzo23]

but see i put in a quarter of gas and the needle moved a quarter like it should so i doubt its the sender unit...

[xvvvz]

That right there indicates that your guage is not accurate, doesn't it? You didn't have a totally empty tank, put in a full 1/4 tank and it now just reads 1/4 tank. At any rate, this wasn't really what I would have recommended. You should have put a full tank in and measured. Maybe a full tank wasn't in the budget, so I understand, but I wouldn't put too much weight behind your results.

Measuring by putting in partial tanks and then judging by how much the needle falls is hardly ever accurate either. The needles don't fall at a linear rate through the range from full to empty on most cars. I know mine doesn't and my Lexus, Hondas and VW didn't either. You need to fill up and know that it is really full (fill it slow, then use the same '1 extra squeeze' or whatever), drive while tracking mileage, then use the same filling technique to fill again. Then take the gallons reading from the pump and divide the mileage by that.

[OzC36]

Fuel consumption is dead easy to check by recording miles run against amount of fuel pumped from the bowser to refill the tank. Gauge errors on our cars are notorious.

If you find it is not a gauge error, then;

Either the fuel is going through the engine mostly unburnt or leaking before it goes through the engine.

If fuel is going through the engine unburnt, then you will have BLACK smoke (not blue smoke which is engine oil) from the exhaust (have a friend drive behind you to observe).

If it is not going through the engine then it likely that you have a gas leak.

With the engine running, check under the hood and especially under the car for fuel leaks. Look and smell. You should see no signs of wetness or smell any raw fuel at all for a normal functioning car.

Don't forget to check the area just inboard of the right rear wheel where the fuel pump and fuel filter is located. This is a known area for leaks.

Report back your results for further guidance.

[benzo23]

Ok well I checked all of the above and found nothing, no gas leaks, smells or anything coming either from the engine bay or under the rear of the car where the filter is, I drove up and down our major road with my friend behind me and he saw no smoke of any kind...the needle moves at a standard rate as it shoud its just that I'm getting about 13.8 mpg based on my math of how much gas I have on 16.4 gallon tank...I'm stumped

[OzC36]

Mmmmm...interesting...thinking, thinking...

In the meantime take a close look at your air filter for clogging or any other possible source of blockage in the air intake system and intake trunking.

[benzo23]

Well got home and checked the filter, completely clean knocked it along the wall and hardly anything came out, and checked all the pipes up the the mass airflow and all clean..this is really strange, do u think if I unplug my o2 sensor and let it maybe reset?...I know its a stupid idea but who knows...