Less than 12 gals to fill up - PeachParts Mercedes-Benz Forum

fruitcakesa · #1 06-03-2009, 05:10 PM

I decided to test out my newly flushed out fuel system to see how accurate the guage is.
So I filled the tank with D2, put 4 gals of Biodiesel in the trunk and started driving .
At mile 357 with slightly less than 1/4 tank showing on the guage, it lost power and started slowing down.
I pulled over, poured in the BD, primed it, started and drove to the gas station to check mileage.
I managed to squeeze in 7.75 gals of D2 along with the aforementioned 4gals of BD make 11.75 gals. 30mpg, not bad but where is the extra 5 gals that the tank should have in it?
I had pulled both the strainer and sending unit and cleaned all the gunk from both so I know that they are not restricting fuel flow.
When I opened the fuel cap, I heard a whoosh from the tank.
Would a plugged vent line cause the whoosh at the cap and also prevent total fuel usage?
I did not attempt to prime the engine after opening the cap before adding the BD so I do not know if there was still fuel in the tank.
Any ideas?

pawoSD · #2 06-03-2009, 05:29 PM

Sounds like you might have a collapsed fuel tank due to the aforementioned "whoosh" (of vacuum in the tank).....limiting its capacity. Check the tank physically, it is behind the rear wall of the trunk.

fruitcakesa · #3 06-03-2009, 06:42 PM

I pulled the tank when I got the car and really cleaned out the inside. It was not really gunky and the strainer was clean although the sending unit was gummed up . It was normal shaped.

snookwhaler · #4 06-03-2009, 07:02 PM

Not sure if the 240D has the same tank as the 300D but when I ever fill mine up and it is down near the "R", it will take 17 to 18 gallons.

I recently pulled my strainer after running B20 for a couple months and found it to be spotless. It also looked like it was the first time it had ever been out of there.

Before I pulled the strainer I ran the car down to the "R" on the gauge. To my surprise I found that there was an additional 4 gallons of fuel to drain out!

I not sure about the "woosh". Some sort of vent line must be plugged or something?

pawoSD · #5 06-03-2009, 07:24 PM

Quote:

Originally Posted by fruitcakesa

I pulled the tank when I got the car and really cleaned out the inside. It was not really gunky and the strainer was clean although the sending unit was gummed up . It was normal shaped.

Interesting....the tank should hold close to 21-22 gallons when completely empty.....have you tried venting the tank once its near empty to allow the fuel to flow more readily? Perhaps the low-power issue was due to too much vacuum in the tank and not from low-fuel. Clean out your tank vent line, as it is plugged.

fruitcakesa · #6 06-03-2009, 09:56 PM

I have not looked at the vent line yet but I guess I will as there doesn't seem to be anything else that would cause this.

pawoSD · #7 06-03-2009, 10:05 PM

There is a check valve in that line that can get plugged up/fail....

2.5Turbo · #8 06-04-2009, 12:11 AM

You say you were flushing the fuel system...did it involve disconnecting fuel lines? If so you could have swapped the supply and return lines, if I remember correctly the return line hooks up further up the tank and would run dry first.

pawoSD · #9 06-04-2009, 12:13 AM

Quote:

Originally Posted by 2.5Turbo

You say you were flushing the fuel system...did it involve disconnecting fuel lines? If so you could have swapped the supply and return lines, if I remember correctly the return line hooks up further up the tank and would run dry first.

They both connect in the same place....the return is actually lower considering there is no screen attached to it, so it would probably actually be the last to run dry.

racebannon · #10 06-04-2009, 08:44 AM

If the vent is plugged, wouldn't you create a vacuum that gets stronger as the tank empties? You could try your experiment again, but loosen the fuel filler cap when the fuel gauge shows about 1/2 (to hopefully prevent diesel from spilling out) - you won't be creating a vacuum that way.

biopete · #11 06-04-2009, 08:56 AM

I collapsed my tank on my 85 300D. It now only holds about 12 gallons. Those mud packing spiders clogged my vent. On a long interstate drive i notice the gas cap impossible to get off almost. The fuel gauge did not work anymore. Pull the two screws from the first aid kit box and look down and see if your tank collapsed. I'm planning on seeing if i can blow it out with compressed air.

charmalu · #12 06-04-2009, 12:25 PM

Quote:

Originally Posted by biopete

I collapsed my tank on my 85 300D. It now only holds about 12 gallons. Those mud packing spiders clogged my vent. On a long interstate drive i notice the gas cap impossible to get off almost. The fuel gauge did not work anymore. Pull the two screws from the first aid kit box and look down and see if your tank collapsed. I'm planning on seeing if i can blow it out with compressed air.

Your best bet is to replace the tank.

I would say PNP would be a good place to get one, but here in Socialist, Kommie, Enviro, left Wing, looser, feel Good, Kali., they poke holes in them.

rendering them useless.

Charlie