Change It, ’Stat

BONEHEAD BLUNDER: “Saving” money

THE BONEHEAD: Generic Shortsighted Guy


WHAT HAPPENED: I can hardly count the number of mechanical things that straightened themselves to attention when I went to get my 5# maul from the truck. One that didn’t, though, was the thermostat. It is simple and it is amaz- ing. Yet, it’s one service item that I constantly battle over at replacement time. “OK, sir, it’ll be $78 for the pump kit, about one and a half hours to heat the bolts out and replace the impeller and housing, and the service call — total about $225.”

“Let’s do it.”

“I’d also like to change the thermostat while I’m doing the job.” “How much?”

“If I can heat the bolts out without breaking them, it should take about 30 minutes of labor.”

“How much for the thermostat?” “About $44.”

“Are you nuts? Forget the thermostat.”

I never was a salesman, and I stumble as I try to explain a thermostat’s rele- vance to the powerplant. He battles me. I give up too easily. A week later, the engine dies of piston seizure, and, of course, it’s my fault. I’m a better teacher to someone in front of me than I am an explainer to someone stuck in traffic two hours away.

To me, it’s simple. Tag along as I try to bring both ends of an outboard together. Let’s start with the water-pump impeller, initiator of a low-pressure, high-volume cooling system. It’s offset in the housing in order to draw water through the gearcase pickup screens. It spins at flywheel speed, so what it  sends northward is directly related to engine rpm. The water goes up through a pickup tube to the base of the powerhead,
so larger engines are fitted with what are known as “pop-off valves,” which open to allow even more water to exit at higher rpm and temperatures.

Here are some things I’ve seen with thermostats:

•  Salt may build up at the base, blocking water discharge, allowing overheating.

•  The side brackets of the assembly

The thermostat is simple and amazing.
which has an open cooling system, meaning what goes into it drains right out if nothing is being pushed into it. There we’ll take a break.

The powerhead is comprised of as much aluminum as possible, but for longevity, things like cylinder sleeves are steel or space-age composite. At rest, a cold aluminum piston and a cold sleeve are essentially the same cold temperature,
can corrode open, either encouraging overheating and further damage to the powerhead, or excess cooling, leading to:

•  piston seizure because the sleeve didn’t expand and/or

•  reduced compression and combus- tion, leading to

•  cold oil that won’t lubricate, and an obvious clue of an excessive amount

and there is no coolant in the system.

The moment the engine starts, piston rings start transferring heat from the pistons to the cylinder sleeves, relieving the piston’s combustion heat of

about 1,000 degrees F, and the piston starts to expand. Because steel doesn’t expand at the same rate as aluminum, piston/cylinder tolerance becomes dangerously tight until the sleeve starts to warm and expand. To aid in that expansion, the cooling water from the lower unit encircles the sleeves within the block and is held as one more heat extractor by the thermostat, allowing the cylinders to expand safely with the pistons, avoiding piston seizure. When the water temperature rises to the predetermined thermostat opening temp, the thermostat opens, allowing warm water to discharge and cool water to enter the cooling chambers. It appears to be crude, but it’s quite effective, and inexpensive to manufacture. As I noted earlier, water volume is related to rpm,
of oil showing on the dipstick. This

may be water due to temperature variances, also called “making oil,” or gas due to non-combustion, called by the same misnomer.

LESSON LEARNED: If I lived in a saltwater area — and I do — I’d encourage an owner to change his water-pump impeller every two to three years, and the thermostat at the same time. What you do is up to you, but I really feel it’s cheaper in the long run this way. Class dismissed. BW

Bonehead Blunders

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