I have a 1996 F-250 with a 351 that is overheating. I have replaced the thermostat and the radiator cap. I am wondering if anyone has any ideas on what to do next. Symptoms:
1. Whenever I start it up I hear water rushing behind the dash area. This may be unrelated but seems odd.
2. It was overheating whenever I drove it very far, but I replaced the radiator cap and that seemed to stop it. I think it was just getting low on fluid because the fluid was coming out at the cap. That seems to have stopped now with the new cap. However, now it will overheat if I let it idle in place too long. The fluid gets really hot and gushes/boils out out of the coolant fill tank that is to the right of the radiator cap. It seems like the fluid is getting so hot it has to escape somewhere and the coolant fill tank is where it goes now that i have a new radiator cap. I drove it four hours last week and it didn't overheat so long as I was moving, but when I let it sit and idle it would get hot quickly.
3. It always has the running hot smell, even when the gauge shows it is not running hot yet.
4. I have noticed some junk in the coolant fill tank and wonder if I need to do a flush. Do you think there is a clog inside the radiator that is not allowing the fluid to fully circulate? Is the water pump not circulating the water?

Thanks in advance for any help/insight into this problem.