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I just purchased a 1993 with 122k miles. Blueish smoke on start up and louder than usual tick. I was going to seafoam the tank. When taking fuel filter out the fuel was black. I filled with seafoam and truck ran great until old fuel was used again. Truck seems to have a slight miss. Did previous owner use oil with diesel? Truck runs alittle better every time. Tryn to get fuel cleared up before I take it in and try and advance timing or do injectors.
Welcome aboard!
Sounds like you have a biological growing in the system (algae). It will only get worse unless properly treated. It's gonna get messy, so prepare for that. If the tank was filled with a mix of #2 and engine oil, it would be a little darker, but not "black". Algae will be strong coffee black, and stain the filter element black.
Seafoam is good to help prevent biological growth, but won't help get rid of it once it gets to the point you are seeing.
Do not run the engine until the fuel system has been flushed thoroughly with fresh fuel, and fresh fuel in the tank is treated.
If you don't have a full day or a weekend to deal with this, consider taking the truck to a shop that can do a flush and treatment. It isn't costly or difficult to do it yourself, but does require a lot of time. If you don't have comprehensive insurance that will cover the shop, it can be costly.
First, stock up on filters. Once the critters start dying off, they will plug it up real quick, and often for a while. Even after flushing, enough of the algae will remain in the system to cause havoc for a while, but won't be too bad if you flush well and treat. Purchase enough biocide to treat the fuel tank size (truck stops/shops carry it). You will also have to dispose of the contaminated fuel. Ask a truck stop/shop about that (they will collect it from you). They may have containers to loan, or advise the best method.
Drain/remove the tank and clean it thoroughly (you don't have to drop the tank, but you'll spend a LOT more time and money on filter replacements). Flush the fuel lines from the tank to the fuel filter, and the return line from the engine valley to the tank (easy while the tank is out). Flush the fuel filter assy. Install a clear line at the return line, anywhere between the injector return tee and the tank, and route it to a container (do not allow return fuel to return to the tank until it runs clear).
Once everything is flushed and back together (except for the return line), fill the filter manager with fresh fuel, and at least 5 gallons in the tank. Add prescribed amount of biocide to the tank. Start and idle the engine. Watch the return line fuel. Once it runs clear (fresh fuel), with no floaters going by, you can reconnect it to the tank return. Restore everything to running condition, and drive it. Once it seems to run "normal", fill the tank with fresh fuel and the prescribed dose of biocide (repeat this for 2-3 tanks) Be prepared to change the fuel filter at any time. Driving down the highway, it can go from running well to DRT in just a few miles. Change the filter soon after performance begins to decline.