Engine Overheating - Troubleshooting Guide

Throttle back to idle as soon as the alarm sounds. If the gauge keeps climbing or you see steam, shut the engine off once the boat is safe (anchored or clear of danger). Do not restart a very hot engine until it has cooled; restarting at high temperature can cause serious damage.

• Outboard: look at the small telltale stream from the side of the cowling. It should be strong and steady (on some engines it only shows above idle).
• Sterndrive/inboard: look for water coming out with the exhaust.
• Strong flow and a normal sound point to a gauge or sensor issue (go to step 9).
• Weak, spitting, steamy or no flow means a water supply problem. Keep going.

A weak or missing telltale with a normal gauge is often just a blocked outlet. Push a piece of wire or a small zip tie into the telltale hole while the engine idles. If a strong stream returns and the temperature is normal, that was it. Insects and debris often block it during storage.

Tilt the outboard or drive up and look at the water intake grates on the gearcase. Plastic bags, weeds and mud are common. Clear them by hand with the engine off. On the water, briefly shifting into reverse can sometimes blow a bag off the intake.

If you ran in shallow water or stirred up sand or mud, the intakes or pump may be packed with sediment.

If your boat has a raw-water seacock and strainer, make sure the seacock is fully open. With the engine off, close the seacock, open the strainer, clean out weeds, shells and debris, and reassemble with the gasket seated. Reopen the seacock.

Open the engine hatch (after the engine is off and the bilge is sniffed). A broken or slipping belt can stop the seawater pump or the coolant circulating pump on some engines. Look for hoses that have popped off, split or are leaking. If water is pouring into the bilge, see the safety line above.

Only after the engine has cooled: open the expansion tank cap slowly and check the level. Low coolant with no obvious leak can point to a leak inside the heat exchanger or engine; that is a shop job.

If the intakes are clear and flow is still weak, the water pump impeller is the most likely cause, especially if it has not been replaced in two or more years or the engine ever ran without water. A slow rise in temperature over weeks is a classic sign of a worn impeller.

Alarm only at high rpm? If the engine stays cool at idle but the alarm sounds at cruise or wide open throttle, the pump is moving enough water for idle but not for full load. A tired impeller, a partly blocked intake or sediment in the passages are the usual causes. Replacing it on an outboard means dropping the lower unit, which is not an on-water job. See the impeller guide.

If water flow looks good but the gauge reads high, the problem may be electrical: a bad sender, a loose ground or a voltage change at the dash. Some gauges also shift when you turn on lights. Feel (carefully) or use an infrared thermometer to compare the engine's actual temperature with what the gauge shows.

Water pressure gauge reads zero at idle? That alone is not proof of a problem. Yamaha explains that its outboards use a high-volume, low-pressure cooling design, so the pressure reading can be zero at idle (most sensors only start reading at a few psi), and the meaningful pressure check is at wide open throttle. Your manual or dealer has the minimum figure for your engine.

A thermostat that sticks closed blocks water flow and causes overheating; one stuck open makes the engine run too cool. In salt water, corrosion and deposits make thermostats and pressure relief (poppet) valves stick. If water flow looks normal at idle but the engine runs hot, have the thermostat checked or replaced.

If temperatures have slowly crept up over seasons, salt, scale or corrosion may be restricting cooling passages, especially in engines used in salt water without flushing. Flushing with fresh water after every saltwater trip helps prevent this. Yamaha notes that salt water left inside can crystallize on the hottest surfaces (above roughly 170°F) and form deposits that restrict flow, and that combustion by-products of ethanol fuel can concentrate on hot spots in the exhaust and eat into the metal. Clearing heavy buildup is a shop job.

A heavily fouled bottom, a wrong prop, or something wrapped around the prop makes the engine work harder and run hotter. If you are also losing speed, check the prop for line or weeds with the engine off and the key removed.

If water flow is partly restored and the temperature is normal, return slowly at low rpm and watch the gauge. If there is little or no water coming out, do not run the engine: call for a tow. Have the cause fixed before the next trip, and check the impeller even if you found a blockage, because overheating often damages it.