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1984 Mercedes 300D Euro Spec
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Discussion Starter #1
My fuel gauge for the past few days has showed I only have a 1/4 of a tank left so I went to fill it up today. But it only held 10.4 gallons. I had put about 11-12 gallons in it about 2 weeks ago to fill it up and the gauge that time showed about half a tank. Both times after I filled it up the needle went to full. Could this be a problem with the sending unit or a voltage problem?
 

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1983 300D
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1,248 Posts
I would check the sending unit in the tank. These typically get full of sludge and the float inside it gets stuck. So a good cleaning should help if that is the case. Also, there are three or two fine wires that float rides on and if one or two breaks would give you an incorrect reading.





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1981 300D with a funny tranny
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965 Posts
My fuel gauge for the past few days has showed I only have a 1/4 of a tank left so I went to fill it up today. But it only held 10.4 gallons. I had put about 11-12 gallons in it about 2 weeks ago to fill it up and the gauge that time showed about half a tank. Both times after I filled it up the needle went to full. Could this be a problem with the sending unit or a voltage problem?
you can access the floating unit, it's under the first aid kit. unscrew that and it should be there. unplug the wires unscrew the cylinder and use some brake fluid to get the goo off.
I was going to do it, but the biodiesel cleans everything, so it ended up fixing itself. Also if you are adding injector cleaner like 'kleen' or 'lucas' it should be slowly cleaning that too. Assuming your car is diesel.

good luck.
 

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`81 W123 280E, `83 W126 280SE, `85 W126 280SE, `91 W126 500SE, `96 W202 C180T, `99 W202 C220CDI
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511 Posts
It is a common problem...You must have to find something like this:
 

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1981 240D
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185 Posts
Depending on how you use your car, how quickly you consume fuel, the quality of your fuel, and your local climate, it is safe to say your diesel tank probably has an algae problem. In addition to sediments at the bottom of your tank, the tank walls and sending unit probably have a healthy coat of algae. I recently removed my tank, cleaned it with shout gel (the gel coats better and laundry soaps are engineered not to foam as much as say dishsoap). and then I went over it again with a very weak muriatic acid (pool acid) solution (approximately a 1:200 mix) and then rinsed it again. I dumped as much of the water out as I could (all but about 1/4 cup which wouldn't come out due to the shape of the tank), then I put about 2 cups of diesel in and swished it around to coat the inside of the tank. I did this so I could leave the tank open and in direct sunlight to allow all of the water to evaporate without the whole inside of my newly cleaned tank rusting. I left it out for a few hours. While the tank was drying, I replaced all my fuel hoses, fuel filters, injector return lines, and injectors/heat shields. After about 5 hours the tank was completely dry and free of rust. I wish I would have taken some photos, but it was getting dark outside by this point. I reinstalled the tank with a brand new sending unit, o-ring, strainer, fitted hose, and filler neck grommet. I put 5 gallons of diesel in, primed it, and drove it to the gas station. My car runs marvelously! The gauge reads the correct amount of fuel and it doesn't bounce all over the place. I should mention that I got the idea for this from Russell at DieselGiant, I changed a few things to suit my needs. What I have written here is exactly how I did it, it may not follow Russell's procedure exactly, but I was too lazy to drive to a car wash and my wife would not allow me to use her blow-dryer to dry my tank. Also, my neighbor is a plumber, so I had free access to muriatic acid. I am not recommending you do this without knowing how to use acid properly and not go blind, kill yourself, or melt your flesh off the bone. I did this same exact procedure on my Jeep a few years ago with a muriatic acid solution of about 1:50 and it ate holes through my tank, so BE CAREFUL! You will be suprised how much goop is at the bottom of your tank. I got about half a 5 gallon bucket's worth. It is disgusting. Just think about how much of that crap has gone through your poor engine! Anyway, it is imperitive to have a functioning gauge because it is embarrassing to be pumping your hand pump on the side of the road!
 

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1977 300D, 1980 450SL, Genuine Buddy 125cc Scooter
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252 Posts
HAH I ran out of gas today for this same reason. Here I thought I had solved the problem by draining it to change the hose underneath and dumping deiselkleen in it and allowing it to drain out the bottom.. 5 gallons of algae sludge? holy cow hope mine isnt that bad. .anyway I now know that 1/4 is actually empty.
 

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1981 240D
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HAH I ran out of gas today for this same reason. Here I thought I had solved the problem by draining it to change the hose underneath and dumping deiselkleen in it and allowing it to drain out the bottom.. 5 gallons of algae sludge? holy cow hope mine isnt that bad. .anyway I now know that 1/4 is actually empty.
Well, like I mentioned in my previous post, it was only about a half of a 5 gallon bucket, so 2-3 gallons. I was very suprised considering I have used diesel kleen on every fillup since I have owned the vehicle.
 
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