I run my F150 every month instead of fogging because I have no time to get it in the water over the summer. However, I don't fog because (1) I like to have the boat ready for a last minute trip, (2) Fogging is a PITA, (3) I like to charge the batteries with the motor, (4) I enjoy sitting in the fiberglass yard monument while quaffing a beverage and smoking a cigar, (5) Running the motor flushes out the mud daubers and other insects that inevitably build nests in every orifice. Number 4 above gives me a general feeling of false class superiority and satisfies the urge to be on the water when I might otherwise go postal from work stress.
I use a large rectangular 60 gallon Rubbermaid trash can filled with water to run the motor. If you have access to one of these plastic cans it will slip under the motor when tilted, then tilt back to vertical. Place a support under the can to elevate it so that water will cover the upper intake slots and the water pump impeller will also be submerged. Run motor and trickle enough water from the hose to replace the amounts lost from the indicator stream. Occasionally monitor the temperature of the water in the can to ensure that it is not getting too warm.
I can usually run my motor for fifteen minutes easily using this process and the water never gets warmer than 80 degrees. This is my method and it works for me. Simply drain the can with a short siphon hose after you're finished and remove the can. During the winter I am on the water at least every other week and running the motor on the trailer is not necessary.
When new I ran the motor on muffs a couple of times but they never really sealed well and the indicator stream did not seem strong enough. When I replaced the water pump impeller last fall I noticed the plastic housing had a couple of areas in it that appeared to have heated enough to melt slightly. I'm going to go out on a limb with this blanket statement: When running the motor without enough water for proper cooling the plastic water pump housing is going to take a beating and QUICK! I would think 30 seconds might be all it would take. As far as the impeller blades, they appear to be fairly robust. Mine were in great shape after 400 hours - Still replaced it though.
If you crank up after a long interval and your indicator stream does not work, check the flush port. If there is strong water there you are okay. A mud dauber has blocked the hose to the stream port. Get a long piece of stiff wire and rout out the hose, disconnect it if necessary. Be sure to check if the lower unit is submerged enough to suck in water for cooling. The long slots must be under water, not just the high speed pickups. In winter sometimes the indicator stream hose will have frozen water trapped in it. Again, check the flush port, then wait for the motor to warm up the hoses for proper indicator stream operation. Always drain the flush port before reattaching so it does not freeze as well.
Whenever I find the indicator stream not functioning I shut down the motor and problem solve. I never take the chance on overheating the water pump and destroying the housing. I only continue to run the motor after I verify that there is indeed water being pulled into the pump.
Probably more than anybody ever wanted to know/read about this stuff!
