jumpy fuel gauge. - PeachParts Mercedes-Benz Forum

IVIuzzy · #1 09-21-2008, 10:08 PM

i recently purchased an 81 300d that has sat for a few years. Ive gotten the major work done, now i can deal with the details. The fuel gauge when it gets around a half tank or below, just waves all over between half and empty.

also when i fill it, it takes about 5 minutes for the gauge to go all the way to full.

Is this just crap built up on the float (im assuming there is one)?
do i need to take the tank apart?
Can i add something to the fuel to clean the screen and float?

dieseldiehard · #2 09-22-2008, 07:27 PM

This could be a broken wire but I'll place my bets on ALGAE

because you said the car sat up for a long time. Thats a recipe for algae
It can really crud up the resistance wires and theres a small bleed hole that is probably plugged. The only way to find out is to remove the sender and open it up. The cover is held on by a spanner nut at the bottom, one can use a set of needle nose pliers to grab it in the 2 notches provided and turn off.

The sender consists of two thin resistance wires and a float the runs up and down making contact to them. If you find black stringy stuff in it its a sign there is algae in the tank and dose of biocide will be necessary to clean the tank out (and some new filters in the process). Clean those thin wires very carefully using brake cleaner or Acetone or Gasoline (my order of preference). A Q-tip might be a good thing but just barely touch the wires else they break and you are going to be looking for another sender (not a bad idea if there is a junk yard nearby!)

If you take the sender out and try to repair it, be sure to use a dab of loctite on the bottom nut that holds the aluminum cover on, otherwise it may come off and roll around in the tank making abnoxious noises and almost impossible to get out without dropping the fuel tank. Don't ask me how I know

81300sd · #3 09-22-2008, 10:41 PM

mine does the same thing, but it bounces quickly like its a short, or bad ground. Tried cleaning all the grounds behind the cluster and that didn't work. Temp guage does it every once in a while also.

My car also sat for about 2 years before I got it.

2.5Turbo · #4 09-22-2008, 11:32 PM

Mine did that between 1/4 and R when I got the car. Tried using commercial bio once and it hasn't done it since. Guess all the gunk that was on the sender came off and went straight into the filter I had to change

bosshog383 · #5 09-23-2008, 01:07 AM

My 84 300sd did the same thing when I bought it, also had sat for several years. After a few tanks of fuel, problem cleared itself up.

dieselnutnj · #6 09-24-2008, 12:15 AM

remove sending unit from tank. large amount of crap builds up in the bottom of the sending unit. To avoid the bouncing guage on teh road.. there is only a tiny little hole at the top and bottom of sending unit to limit fuel in and out thus keeping guage still. The very fine wires in the sending unit that the float slides on get gunky too. I've cleaned and fixed about 2 dozen units. Also.. you'll find your reserve light begins to work again.

mobetta · #7 09-24-2008, 12:27 AM

I just did one this afternoon. took all of 6 minutes once out of car.

IVIuzzy · #8 09-24-2008, 04:29 PM

i just tried some biocide before i take the sender out (i am afraid i may break the sender). if that doesnt work, i will try to take the sender out.
I bought 5 pre filters at car quest. im assuming i will need to make some changes.

If i change the filter every 200 miles? or is that too soon? should i wait til it clogs? i want to prevent being stuck on the side of a road doing it, since i normally drive at night.

turbobenz · #9 09-24-2008, 04:58 PM

This happened to me on my first 300d. It ended up being a sender and I had no money so the car ended up being totaled with the problem. Mine would become useless under half a tank

daw_two · #10 09-24-2008, 07:16 PM

Mine is definitely a short in the dash --- a good "TV smack" to the top of the dash above the instrument cluster and the fuel gauge reads correctly. And the burnt out light indicator comes on ---- if my lights are on.

IVIuzzy · #11 09-29-2008, 07:16 PM

biocide did the trick, i got the Power Surge brand at my local peterbuilt shop. it ran me 18 dollars for a quart.
a "shock treatment" for 20 gallons is 1 ounce. i put a little over 1 ounce by accident. i changed 3 pre filters during this process.

i am on my 2nd tank and i will change the both preliminary and secondary filter tomorrow, gauge went all the way to empty this time and the reserve light worked. no jumping at all. it looked like it thought about it once or twice though.