How to remove varnish from degraded gasoline with Sea Foam Motor Treatment [SOLVED]

I’m working on getting one of my 1973-1974 Mercury Capris (V6, 2.6 and 2.8 liters) running again. They’ve been sitting for years. One’s an organ donor, the other is more likely to be resurrect-able. I’ve been told that if cars sit that long, I’m going to have to flush out all the varnish or whatever in the fuel system. Sea Foam has been recommended (not sure which product) to attack the fouling. Today for practice I took the (in-trunk) gas tank out of the worse of the two cars. Still holding about 3 gallons of really foul-smelling, yellow gas from… 15 years ago??? No external rust, no leaks — it’s in a dry, protected place in the trunk behind the rear seats. There was some sediment, like rust or maybe little bits of tar; and a tarlike substance on the in-tank fuel strainer on the pickup, and the pickup tube was clogged. Lacquer thinner dissolved the gunk on the strainer. Mineral spirits didn’t touch it. Poked welding rod through the pickup tube and scrubbed it as best I could with pipe cleaner and a parts washer.

I cleaned those up, but I suspect that’s just the tip of the gunky iceberg. How do you recommend proceeding? I figured send the tank out to be boiled. But, what about the lines from the tank to the pump? And, probably less critical, pump to carb? Will your products do anything to clear out the lines?


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Jim D.
4 years ago

Thanks for looking us up Peter. First and obvious is your fuel is degraded and worthless – remove all you can from the fuel system though I would not expect any fluid fuel to remain anywhere in the system but in the tank.  What I would do: Once your tank has been removed and having examined the fuel system parts (fuel lines, filter, pump, etc), you can use a can of Sea Foam Spray to clean from your intake end back to where your fuel line connects to the tank. Disconnect the fuel line somewhere before the carburetor end and start spraying Sea Foam Spray (reverse, gravity) so the fluid flows backward to where the tank is disconnected. Once you see the fluid coming out, plug the tank connection end and continue to spray/fill the fuel system passageways. Once you think you filled the much of the fuel lines with fluid, plug the intake and let everything sit for 2 or 3 days to soak, then drain. That’s our solution for cleaning any remnant varnish from your Mercury’s fuel passageways.

Once you have the engine running, start with small amount of fuel in the tank mixed with a high cleaning concentration of High Mileage. A can per gallon of fresh gas would do well to continue cleaning the carburetor circuits. At some point after you get things humming, do a Sea Foam Spray treatment down the carburetor throat to clean and lubricate the upper engine. And finally, don’t forget to add a half can of High Mileage every oil interval to clean crankcase residues!