I cleaned my fuel system, replaced the hoses and filters. Before that my fuel was a dark blue and kinda dark gray. I found a silvery paste in my tank when I cleaned it which I'm still clueless about after asking this forum. After that I put clean diesel in and it showed a clear yellow color in my primary filter. Great.

Well now my fuel is slowly beginning to turn into a blue color and constantly getting dark, turning back into the old color of diesel that I had. Does anyone know what's going on??

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1983 300D turbo diesel

shot in the dark, but it could be that someone used a tank lining product that is now coming off?

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2015 GTI
1991 350SDL with a clean body and a bunch of miles.
1984 190D with an AMG 3.6

i've seen that silvery paste in high mileage gasoline gas tanks before; i came to the conclusion (might not be correct) that the paste is from the gas pump nozzles; every time you put the nozzle in the filler hole there are some flakes (silver metal) coming off the nozzle and over time they accumulate in the bottom of the tank. since they're always suspended in gasoline (or diesel) it kinda makes that silver paste.

I'd say the blue or dark gray would be a fungus issue. Have you dumped biobor or startron in there at all? Can't hurt to give it a try and it'll kill whatever is in there anyways.

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TC
Current stable:
- 2004 Mazda RALLYWANKEL
- 2007 Saturn sky redline
- 2004 Explorer...under surgery.

Past: 135i, GTI, 300E, 300SD, 300SD, Stealth

X2
You have an infection.
Post again when you have treated it !

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Grumpy Old Diesel Owners Club group

I no longer question authority, I annoy authority. More effect, less effort....

1967 230-6 auto parts car. rust bucket.
1980 300D now parts car 800k miles
1984 300D 500k miles
1987 250td 160k miles English import
2001 jeep turbo diesel 130k miles
1998 jeep tdi ~ followed me home. Needs a turbo.
1968 Ford F750 truck. 6-354 diesel conversion.
Other toys ~J.D.,Cat & GM ~ mainly earth moving

I'm not sure you have an issue. Road diesel is dyed blue and off road diesel is dyed red so I'm thinking you may be fine. A photo of your prefilter might help. A few little black specks is normal, but if it's filling up the filter then you've got fungus issues.

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The primer pump could also be leaking engine oil into the fuel system, turning the fuel blackish.

I guess I'll get some startron just in case

There is nothing collecting in my filter that's why I didn't think it was a fungus thing.

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1983 300D turbo diesel

try some studjuice?

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

a fungus among us? startron is the cheapest and easiest thing to try. fyi my diesel fuel as viewed through a transparent pre-filter is a golden color. i've never seen red, blue, green, or any other colors.

That is Close; it is the little O-ring in the Fuel Supply/Lift Pump that goes bad. When that happens some Members have had the Oil inside of the Fuel Injection Pump get sucked into and mixed with the Fuel inside of the Fuel Supply/Lift Pump.

If your Injector Nozzles are extremely worn or the Spring inside is broken (that would cause a miss) it is also possible for combustion gasses to get into the returning Fuel.

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84 300D, 82 Volvo 244Gl Diesel

I've seen it greenish (sort of like mountain dew) but I think that was due to the winter additive. The only time is has actually been dark was when I had some fungus in the tank. Startron took care of it immediately.

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TC
Current stable:
- 2004 Mazda RALLYWANKEL
- 2007 Saturn sky redline
- 2004 Explorer...under surgery.

Past: 135i, GTI, 300E, 300SD, 300SD, Stealth

You know what oil sounds about right. When I changed my last filter it had a thin black coating on it. So did the tank strainer. No clumps at all, and the holes weren't clogged, just a thin black film. Do you know the part number for that?

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1983 300D turbo diesel

made me laugh. I've been working with commercial generators for lots longer than I have been playing with 123s. First time I saw the fuel filter in my 123 with the amber fuel I had to stop and think why the fuel was a funny color. I'm used to seeing red fuel in the gen sets....

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The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech

2-stroke oil turns fuel blue and other fuel additives. Otherwise, straight diesel is yellow.

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85SD 240K & stopped counting painted, putting bac together. 84SD 180,000. sold to a neighbor and member here but I forget his handle. The 84 is much improved from when I had it. 85TD beginning to repair to DD status. Lots of stuff to do.