I have an algae/bacteria problem in my 300TD wagon's fuel tank that I've been putting off for a while now. However the sludge has reached the "constantly plugging filters" level and I have to do something about it.
So, I've decided to remove the tank and have it professionally cleaned. Thing is, the wagon also has an expansion tank. Should I have it cleaned too?
Also, is this expansion thank just a hollow thing or does it have some sort of filter inside that I should be aware of?

Did you try Biobor diesel biocide, or StarTron diesel fuel treatment yet?
Both available at marine supply houses or online.

A bit easier than removing a tank.
I keep both in the trunk and use as needed.

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78 W116 300SD 'Desert Rose' new as of 01/26/2014
79 W116 300SD 'Stormcloud' RIP 04/11/2022

Not trying to criticize, but why didn't you try and fix the problem when it wasn't bad? If you're clogging filters it got too far and that is just neglect.

Try biobor and finish it off with startron. Startron is good when it's at the beginning stages, but wjen you're clogging filters it's time for biobor.

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Only diesels in this driveway.
2005 E320 CDI 243k Black/Black
2008 Chevy 3500HD Duramax 340k
2004 Chevy 2500HD Duramax 220k

I have a good half inch of sludge on the tank (I took a peek thru the fuel sender hole). Do you think it will do the job with that much contamination? Wont it all just keep plugging filters? (I attached a pic of my fuel filter. Currently, if I clean it, it will be this dirty in about 10 miles or so.)

I know it is neglect.

Long story. The short version: the car belongs to my father. I spotted the problem nearly two years ago and told him about it. The reality is that he is quite old now, and doesn't have the same disposition that he once had. In the mean time, he stopped driving altogether. We are now sort of fixing the car as a father and son thing.

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Put a fuel clamp/vise grip on your rubber fuel feed line under the car near the tank.
If you drain the fuel from the tank and remove the strainer (with channel lock pliers, plumber's wrench, or socket) from the tank (clean the strainer while it is out),
you can hook a pressure wand to a garden hose and the drain for your hot water heater and pressure spray away most of the sludge using the fuel sender hole.
The hot water will kill alot of the live stuff, and the pressure of the spray will remove most of the clingy bits.

EDIT: If the water heater hasn't been drained in a while, make sure the water heater tank has been well flushed and that no sediment comes out of the drain hose before you use the hot water to flush out your fuel tank.

Then, treat the relatively clean fuel tank with Biobor and/or StarTron when you fill up with fresh truck stop station diesel fuel from the 5 galllon jug that you filled beforehand.
Let the mixture sit overnight.

Then run the car and you shouldn't get too many clogged filters.
Keep adding StarTron on each fillup until you no longer see debris in the primary filter.

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78 W116 300SD 'Desert Rose' new as of 01/26/2014
79 W116 300SD 'Stormcloud' RIP 04/11/2022

Bacteria live on the boundary between oil and water. They consume the water as they grow, then die and drop to the bottom. What you see as sludge is already dead. The problem isn't how to clean the tank, it's where to buy your fuel. Since the problem is chronic, it means your fuel source has moisture in their tanks. If you weren't getting a steady feed of fresh water, the biological infestation would be self-limiting.

Ok, I just came back from trying to drain the tank of most of the bad fuel and I noticed a piece of an old fuel sender unit dropped inside the tank, so I will have to take the tank out anyway. At least I found the source of this really annoying clunking sound!
So back to my original question: Do I have to clean the expansion tank too?

Ok... wagon.

If your tank is plastic, find if it's cracked...

If so, get a metal one.

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 190D 2.5Turbo
1987 190D 2.5-5SPEED!!!

1987 300TD
1987 300TD
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!