Fuel system tank cleaning
Dirty Tank - cleaning
If you have to remove your fuel tank and despite it being in good cosmetic condition, it has tar-like deposits in the bottom, these can easily be removed when tank is empty by using caustic soda which is cheap to buy. Let it sit for a day, ocassionally agitaing the tank to ensure all areas are covered. Rinse four times with clean water then wash with liquid detergent and water (laundry gel is fine) and finish by rinsing five or six times with clean water. You can dry it rapidly with a heat gun but ONLY use the tank filler hole. If you use a heat gun via the sender hole, you could melt the plastic exit pipe inside the tank. Diesel tanks are prone to deposits which can loosen and clog filter, so a clean tank on refit is essential.
Warning: Caustic soda can burn hands so wear gloves as well as eye protection and dispose of it sensibly.