Anyone who has owned an outboard for more than a year or two has almost certainly heard this intrusively annoying sound at least once. It’s highly unpleasant blare is designed to wake the dead, but necessary for the health of the engine. The hot horn is part of a warning circuit that detects too much heat in the powerhead due to the partial or complete loss of cooling water. The other part of the circuit is designed to slow the engine down immediately, which it does by shutting down a cylinder. This insistent message cannot be ignored; one minute you’re happily zipping along; then the horn goes off and you’re immediately almost dead in the water. The good side of all this is that if the overheat circuit failed to do its job your outboard’s light aluminum powerhead would be cooked beyond repair in just a matter of a few minutes. So you learn to live with it.

Whenever I get a hot horn my usual procedure is to quickly yank the throttle back to the idle position, and as soon as the revs slow sufficiently, shift into reverse so the prop can clear the obstructing weeds or plastic bag away from the cooling water intake. At the same time I carefully watch the telltale stream of water from the top of the exhaust housing that indicates the water pump is doing its job. If it is, I shift back into neutral and let the engine run at idle for a minute or two. The powerhead cools much faster with water pumping through it than with the engine shut down. If all is well the hot horn is automatically silenced and you can throttle up safely in a minute or so.

Last summer my four-year-old 4-stroke Merc 60 started to sound the hot horn every time I throttled up after clearing the launch ramp area. But the telltale squirt of water from the upper exhaust housing never faltered, nor could I visually detect any loss of volume. The first time it happened I took the rig back to the dealer, who replaced the defective (frozen shut) thermostat. This seemed to cure the problem for a few trips. (A tip: on most engines the thermostat can be replaced in a matter of minutes if you take the trouble to learn where it is and also carry a spare —they’re usually $20).

But then a month later the same problem occurred again. I immediately installed the spare thermostat, but this did not solve the problem.(Note: 4-stroke outboards do not run well without a thermostat; operating without it is at best a temporary emergency thing). So back I went to the dealer, who pulled the water pump and found a pinhole leak in the gasket.

Guess what! The next trip out, I got the hot horn again before the boat traveled 100 yards on plane. Figuring the overheat sensor must be failing, I bought a new one and installed it myself (took about 5 minutes). Back at the launch ramp I found out for the umpteenth time I still had an overheating problem.

That’s when I got the bad news. During a head-scratching session the mechanic mentioned that he had observed this problem once before; the cause being a swollen rubber grommet where the water passage from the pump in the lower unit connects with the powerhead. It’s a problem that can develop over a period of years with engines operated under abnormally hot conditions, and Florida Bay with its 90 plus degree water in midsummer certainly qualifies. The cure: completely remove the powerhead and replace the four dollar grommet. The painful news: the labor for this, along with the necessary new gaskets, runs about $1,000!

Fortunately for me, I had bought an extended warranty from Merc, which was still valid when the problem began. So, even though it technically expired just days before those repairs were completed, Merc graciously honored the warranty.

The next trip out, the engine ran like brand new. And still continues to do so.
Since I do a lot of my fishing in very shallow water, I’ve always been timely about my due diligence when it comes to scheduled maintenance, and that includes regularly replacing the water pump. The only other repair that engine has required over its four years of hard saltwater use has been to replace the voltage regulator. That certainly speaks well for durability!

The two best investments I’ve made over the life of the engine were the extended warranty and a detailed service manual. The manual says where everything is located on the engine, making it easy to replace such accessible parts as the thermostat, heat sensor, and voltage regulator. And that not only saves the cost of a mechanic’s services, but also time spent hauling the boat to the dealer and back home again.