Hey all, so I've been having some problems with my tank sender/fuel tank since I got the car. It's really odd...

First off the fuel gauge lies to me, the needle will now get to the "R" and once it's there the car is literally running out of gas, no reserve light comes on, no warning.

Then on top of that, I filled up my tank today which should be like 17 gallons and it only took 13 gallons to fill it up, when it was getting close to the "R" which means it was almost out of gas... What's going on here????

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2007 BMW 328XI

Well first make sure it low fuel bulb is working in the cluster by swapping it with a known good one.

Sender might be dirty and needs to be cleaned.

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2016 Monsoon Gray Audi Allroad - 21k
2008 Black Mercedes E350 4Matic Sport - 131k
2014 Jeep Wranger Unlimited Sahara - 62k
2003 Gray Mercedes ML350 - 122k

The low fuel bulb is working, when I took out the sender a couple of weeks ago for cleaning I moved it up and down to make sure it was working properly, I found that when I tilted it as though it was empty it tripped the reserve light which did come on, but when it's in the actual tank it doesn't work properly.

I don't understand why only 13 gallons filled it up when it was getting real close to where it runs out of gas.. Maybe it has the wrong sized tank or something? LOL

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2007 BMW 328XI

Three gallons worth in the Bottom of your Tank?

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'84 300SD sold
124.128

If it means anything, in over 3 years, I've never put in more than 16 gallons. I ran on the light for a good 50 miles on the freeway in the middle on 10 going East to AZ and it took the most I ever filled (16). On my average trips, the light comes on and I fill up ~14-14.5gal

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2016 Monsoon Gray Audi Allroad - 21k
2008 Black Mercedes E350 4Matic Sport - 131k
2014 Jeep Wranger Unlimited Sahara - 62k
2003 Gray Mercedes ML350 - 122k

I had a similar experience in my 400E about 4 years ago. I was getting very odd fuel level readings that a new sender unit did not fix. I could only fill up with about 12 gallons when completely empty. I lived with it like this for a couple months, and then I started to smell gas.

As it turned out, the fuel tank had actually collapsed in on itself. It had collapsed enough to reduce the volume substantially, and also cause the level sender to come into contact with the gas tank walls (hence the weird readings).

The source of the problem? I started by looking at the check valve that vents the gas tank. This was working fine. I replaced it anyways since it was <$5. I then checked to see if the vent line running to the driver's side front fender was clogged. It was fine. This line runs to the charcoal canister, which filters the nastiness out of the gas fumes. My charcoal canister was clogged, oddly. I replaced it, put in a used gas tank from a 300E, and I have had no problems since.

This is an unusual problem. But, it's always something to look into. Pull back the carpeting in your trunk and see if your tank is dented-in anywhere. Those Bosch fuel pumps are POWERFUL!

good luck!

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'85 500SEL, 245,000 miles (sold)
'92 400E, 295,000 miles (donated)
'84 VW GTI (work in progress)
'10 VW GTI (daily)
'72 Chevy C10

Also on the back of the gage cluster is an adjustment (that is soldered) so you can adjust and if you search benzworld you can find putting dielectric compound on it will fix it because while in fuel the resistance of the weak connection varies its reading sometimes also making the gage jump or read incorrect.

The adjustment potentiometer on the gauge itself is designed to tweak the same gauge mechanism for use in cars with different fuel tank senders, such as the station wagon and cabriolet. It's a factory VDO setting, not something you screw with to make your gauge read more accurately. The standard sedan tank & sender should read approx 86 ohms with an empty tank, and the gauge needle should point to "empty" with an 86 ohm signal. The gauges are sensitive, a couple ohm difference make the needle move visibly (i.e., an 80 ohm signal on the standard gauge should indicate about 1/8 tank). Details & specs are in the factory service manual at this link.

The reserve light is a simple on/off switch in the sender. The indicator bulb is checked when you turn the ignition on, it will glow dimly, showing that the bulb is good. Make sure the sender is the correct one, although it's unlikely that someone replaced it with the wrong one. You can read the part number off the plastic when the sender is removed. Standard senders are white, the 500E-specific sender is bright yellow, and I'm not sure if the wagon/cabrio senders are a different color or not.

A couple of my 124's have the senders & gauges working near perfectly, meaning if I fill up immediately when the reserve light turns on it will take approx 16 gallons, and if I drive about 2 gallons past the reserve light (miles traveled varies based on the MPG), it will take about 18 gallons to fill up on the standard 18.5 gallon tank. However I've also had one car where the reserve light would not trigger until the 17 gallon point, which always made me nervous; and I had another that would trigger 3 gallons too soon (and no, the tank wasn't collapsed) despite taking ~18 gallons if I watched the trip odometer instead of the gauge. New senders and gauges did not correct the odd readings. I never did solve the mystery and chalked it up to manufacturing tolerances.

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Dave
Boise, ID
Check out my website photos, documents, and movies!

I didnt say mess with it but if you go to benzworld they get loose for lack of a better word and people have got a compund that when poured on said component the fuel gage reads right.

I think what he was saying is that the potentiometer can get dirty and the connection intermittent. If you are careful to set it back exactly where it is now, exercising the potentiometer could resolve the intermittent connection if that is in fact your problem. But the bottom line is that it is set in a certain position for a reason. Trim pots are susceptible to getting 'crackly' from lack of being turned and dirt just like regular pots.

Also, check the ground connections back there......

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-tp


1990 300SE "Corinne"- 145k daily driver - street modified differential - PARTING OUT OR SELLING SOON - PORTLAND OR. AREA - PM ME FOR DETAILS
1988 560SEL "Gunther"- 190K passes anything except a gas station
1997 S420 - 265k just bought it with a rebuilt trans. Lovely condition

my 1990 300TE does the same thing...it will shut the pump off, if you are doing it automacally, at around 3/4 tank level. Seen it many times....the fuel gauge, though, should reflect this. You think you've filled the car up and are pleased with the price only to get in the car and discover it's only 3/4 full. And the fuel gauge is VERY non-linear...so it sounds like standard Mercedes issues...good luck

here's another thing...you must understand that these cars are designed for a more mature audience. By nature Mercedes is going to calibrate the gauges to err on the side on of senility, they don't want their valued customers running out of fuel because the customer simply didn't look at the fuel gauge. My 300TE will have the needle in red with no telltale ever on the dash...and the telltale works. My 1988 BMW is spot on with the fuel gauge, telltale and even trip computer..if this ***** says you have 10 miles to go you better believe it....

The MB fuel gauge on the 124 is linear if working properly. I've owned enough of them over the past 15 years to know...

I like how in all of my cars(MBZ) that I've had that from the hash mark after 1/2 to the R line it takes like 30 miles and then its R

rather annoying.. I am getting 360+/- 10 miles to full tank in my cabrio... From R instanly on to full is like 15 gallons when the manual says the tank is like 21 gallons.. weird?

Hey all thanks for the replies! Sorry I haven't been on to comment back, our internet is down...

Pretty much I just want to get it so that when I'm getting low the reserve light comes on, I hate not knowing when it's going to run out of gas, it seems like it's always a different spot when it starts to run dry, no warning, no light. It just starts sputtering. When I took out the sender to clean it, it helped a very tiny bit by at least getting to the "R" when it was about to stall.

It just strikes me as odd that when is was REAL close to running out of gas it only took 13 gallons. My tank vent valve is missing, broke off before I bought the car and figured out it was the culprit for my fume problems I was having with it. That's going to be ordered friday, I'm wondering if I should get a sender from a junkyard car, just to see if it's that maybe? The actual gauge could also probably calibrated as well. I want to take the tank out but one bolt is stuck and won't come off, of course -_- lol

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2007 BMW 328XI