Time to Get Diagnostic

By: Grid Michal

QuestionI have a mid-1990s 18-foot Grady-White with a Yamaha 115 two-stroke. It is pristine with minimal hours, always kept under cover. Last fall I launched to go fly-fishing in the upper reaches of a river. As I accelerated, the boat came up on plane nicely, then the alarm came on and the engine rpm cut back. The engine was definitely pumping out the visible-discharge hole. Eventually the alarm stopped; I hauled the boat and took it to a local shop where they replaced the thermostats and pronounced it fixed. I re-launched it, came up on plane, went about 200 yards and the engine cut back again. Three of my friends said they had the same problem, which was resolved when they removed and cleaned the screen in something held on by four screws. They cleaned the screen and had no further problems. Do you think this will cure my problem as well?

AnswerI’d love to do a Johnny Carson “Carnac” routine here and keep it short and sweet, but it ain’t gonna happen. It’ll be more like Dr. Phil, unfortunately. Let’s go back to your original statement about minimal hours: Let’s say the boat is a ’95 and — like most owners say — you have “fewer than 100 hours” on the engine. Twenty years into 100 hours is about five hours of running time a year, which does nothing positive for the engine.

First, are there lights flashing on your tachometer? You have problem-lighting for water and oil, so that would be a great place to start. Let’s say the lights are not working, which would be unusual, but not unheard of. Since we’re dealing with a boat/engine combination, and the problem could be in either place, the first thing I’d do is “divide and conquer.” Remove the fuel feed closest to the engine from the boat and hook up a remote six-gallon tank. If the problem goes away, you’ve found your gremlin is probably in the boat tank’s anti-siphon valve, which has restricted fuel flow to the engine, causing it to run lean, pistons heating from lack of fuel, and setting off the alarm.

With today’s fuels, my next inclination would be to look to the carburetors as the problem, but let’s take it a step or two further. The alarm will engage for either a lubrication problem or a cooling problem, so let’s look at fuel and oil.

Oil: Is there enough in the external tank to prevent the alarm from sounding? Is the oil transferring to the on-engine reservoir, and is it full? Have any hoses come disconnected that would prevent oil from getting to the cylinders and lubricating them?

Fuel: Does squeezing the primer bulb when the problem occurs make a difference (you’ll need someone to help with that)? If the engine picks up rpm, you probably have a faulty fuel pump. Have you tried engaging the choke/primer with the ignition key when the engine drops rpm? If it picks up rpm for a moment, the problem is in the carburetors not delivering the fuel lubricant/coolant to the cylinders. That will account for the rpm drop and the alarm sounding.

If the light/alarm is for cooling, it could be because it’s not getting enough water through the impeller, which, if you haven’t changed it in 20 years, would be a good place to start. Did you add a transducer between the time there was no problem and when the problem occurred? Make sure it isn’t interrupting water flow to the gearcase. You didn’t say the alarm went off after you had the thermostats replaced.

It could also be because the powerhead isn’t discharging enough water. Thermostats will often cure the problem, or cleaning/replacing the pop-off valve, which aids in the discharge of heated water. Neither will affect what you say you saw, which was that the engine was pumping.

Summarizing, you’ll have to cure this with an on-the-water test, but you can do some of the diagnostics to save that expense. If your instrument warning lights work, trust them and use them as part of the diagnostic process. You should also deal with the shop that charged you for not fixing the problem. Wouldn’t you hold them to the grindstone and not pay them if it were your car?


Your email address will not be published. Required fields are marked *