My 5.9 developed a coolant leak and it seems that it was coming from the water pump. With the engine running you could see coolant being slung off of the belt down by the water pump. Turns out the problem was not with the water pump....

Turns out the upper radiator hose was bad, but not obviously. When it gets old (200K miles), it begins to swell under pressure. Unfortunately, this hose is close enough to an idler pulley and it will make contact with the pulley and wear through. When I initially squeezed it, while troubleshooting the leak, I did not hear the leak or see any coolant come out. When the engine is cool, there appears to be enough clearance between the bottom of the hose and the top of the idler pulley. However, once that coolant gets hot the hose swells and makes contact.

To check on your truck, feel the underside of the hose at the point where it crosses over the idler pulley. If it feels anything but smooth, it very well may be starting to make contact with the idler pulley. Also, if the hose looks swollen (especially with the engine hot), I'd bet that it's close to failure. The new hose can be had for ~$15. I replaced both upper and lower to be sure.

Good luck. HTH

 

i sure hope you don't wait 200K miles to change radiator hoses. it's recommended that you have your cooling system flushed and fluid changed every 2 years even when running the new ELC propylene (?) glycol coolant. my local radiator shop has seen many thousand dollar repairs due to people buying into the 5-year 100K mile change intervals of the orange dexcool crap. even that coolant should be changed every 2 years. along with a new thermostat, upper and lower hoses should be changed since they usually come off for that kind of maintenance. don't wait 200K miles to change hoses.