How can water get to the diesel tank from the radiator reservoir?

fuse?had oil? don't think so how hot did it get check rad.make sure water in it ,if got really hot might be froze up chek battery also must have good battery

replace the faulty reservoir or radiator
what is happening is that the radiator cap is not raising the pressure in the cooling system because of a leak in the system
the excess water from heating is being pushed out probably onto the ground through the leak
when the cooling system cools down , water is being drawn back into the radiator
probably find a crack in the radiator tank that opens under pressure but seal off as the pressure is lost

Depends. Not usually because as engine heats and then cools when shutoff, reservoir should allow radiator to suck back what it needs. However, if you have just flushed system, or replaced water pump, you should fill at radiator as well. Bad radiator cap that prevents backflow can cause problems as well.

Yes, the radiator should be checked first for anti-freeze level. If it is low, top it off and pour remaining fluid into the reserve tank to the proper level. Re-check after driving and then you may just fill the reservoir only if you like.

If you are talking about the reservoir for the radiator, the radiator pulls coolant in from the tank when it is low.
If you are adding coolant - water every week and there are no external leaks, the engine is burning the coolant out the exhaust pipe.
Usually a leaking head gasket - intake gasket or cracked head.

Stop me if this has NOTHING to do with your problem (or if it's already been solved)...I don't know a lot about cars so...My Ford Taurus kept overflowing antifreeze from the reservoir cap. Car started to over heat and kept spilling"/dumping" antifreeze every time I turned the car off. The antifreeze reservoir had been replaced less than a year prior, and water pump, radiator, etc looked at. I got a free estimate that stated my water pump was NOT the issue, but I did need a new radiator and my reservoir tank needed to be replaced (again)....$800 parts and labor.... However...I noticed the overflow holes on the tank, and also that the orange liquid (antifreeze), was also spilled around the cap lid. I also noticed the CAP on the antifreeze reservoir would loosen when tightened completely- hence, not sealing correctly. So I went to the dealer, bought a new reservoir cap for $15, replaced it, and never had an issue after! I also found out that the CAP was a DEALER PART ONLY, and when the tank itself was replaced, the CAP was the OLD one! Problem fixed! Maybe it's similar? Or something "more serious".

3.0? you most likely overheated the engine, warping the heads.
Having the head gaskets replaced may fix this.
Get it looked over.

please rate, thx Coolant Recovery System NOTE: When the water thermostat (8575) is closed, there is no flow through the radiator coolant recovery reservoir (8A080).

Trapped air in the cooling system must be removed. A pressurized radiator coolant recovery reservoir system is used which continuously separates the air from the cooling system.

• When the water thermostat is open, coolant flows through the small hose from the top of the radiator outlet tank to the radiator coolant recovery reservoir.
• The radiator coolant recovery reservoir separates any trapped air from the coolant and replenishes the system through its radiator coolant recovery hose to the water thermostat housing.
• The radiator coolant recovery reservoir serves as the location for:
  • service fill
  • coolant expansion during warm-up
  • system pressurization from the pressure relief cap and
  • air separation during operation
• The radiator coolant recovery reservoir is designed to have approximately 0.5-1 liter (0.53-1.06 quarts) of air when cold to allow for coolant expansion.

yes, not having an overflow reservoir for your radiator will cause you to lose water. As your engine warms up the water expands and creates pressure in the cooling system. When the pressure reaches a certain point(usually about 15 psi.) the radiator releives the pressure by releasing water into the overflow tank. When the engine cools back down the water shrinks pulling water back into the radiator from the overflow. If there is no overflow, the everytime the engine warms up on a drive water will be lost and not recovered on cooldown.

You are talking about 2 different things here, YES the water pump should have been replaced with the timing belt. But the water tank you're talking about is it the radiator or a radiator reservoir. Either way they are not related to the timing job that was done. However I'm not by any means trying to blame the mechanic but sometimes the radiator can get damaged during a timing belt job is caution is NOT used when removing Crankshaft Pulley especially when using air tools. Good Luck contact if you need more help. Thanks for using FIX YA