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Engine Overheat Troubleshooting

If you start your engine while sitting at the dock and you let the engine warm up and you notice that your temp steadily climbs and climbs, you might have a bad thermostat.

Usually, when first started, it is a good idea to watch the gauges and monitor the temp. Watch the temp gauge as the engine warms up. The temp will usually climb just a little above the normal operating temp and then it will drop down a little when the thermostat opens up. That is normal and it confirms that the therm is opening OK.
If your thermostat is sticking, replace it. NEVER run the engine without a thermostat installed or you will cause severe engine damage.

Buy a new Thermostat Kit

Excessive overheating can ruin your engine.
If your engine is overheating turn it off and do not run it until you have fixed the problem.

Your engine can overheat for a variety of reasons.
It can be a temporary problem such as weed or debris wrapped around your lower unit. It can also be from a more serious reason such as a bad water pump or clogged exhaust castings.

Overheating is a problem which can be diagnosed through a process of elimination. The best way to tackle such a problem is to first consider the obvious.

For example, if you have just run aground on a sand bar and now you are overheating, chances are good you have sucked up some sand and possibly ruined your water pump impellar. Not only will you have to rebuild your water pump but you might have to backflush the cooling system to remove the sand.

Another common typical example that we see all the time is the customer who has just started using his boat after it has been sitting for a year or more. "It was running fine for the first 1/2 of the day" they usually say. We usually find that the rubber water pump impellar blades have turned brittle from sitting. After the impellar was used for a couple of hours it crumbled to bits and the engine overheated. If your boat has been sitting for a year without being run, you should rebuild the water pump.

One more common occurrence worth mentioning happens when a customer runs the engine out of the water without an adequate water supply. All it takes is 45 seconds of running dry and the impellar is ruined. Make sure you have a good set of flushett muffs that won't fall off while you are running on the hose.

The water pump might be fine but it's a good place to start. The pump should be replaced each season. If you are overheating and you haven't replaced the pump for a while, do it. Replacing the pump will "kill two birds with one stone". Not only could it correct the entire problem but it will also get some scheduled maintenance out of the way. Even if it doesnt correct the problem at least you can rule it out as being suspect.

Symtoms of a Bad Water Pump

Sometimes overheating symptoms can vary. Usually when you have a bad water pump impellar the engine will overheat at both low speed and high speed. If you are under way and see your temp gauge too hot and you pull back to idle at slow speed and the temp stay hot or gets even hotter after idling for a while chances are good you have a bad water pump impellar (provided the water intake is clear of debris).

If your engine temp is OK at idle and slow speeds but when you get on plane you notice that the temp slowely keeps creeping up and up, you might have clogged and/or rusted exhaust manifold or riser water passages. This is called a High Speed Overheat. Usually the engine will slowley cool back down after you come off plane and cruise slow for a while.

Boats can explode. Moving parts can rip off your fingers. Hot exhaust can burn you to the bone and props can slice you up. One spark near a battery can cause an explosion. You can never be too careful. Remove the battery from the boat before working with fuel. Always disconnect the batteries negative terminal first. Clean up any spilt fuel and let ALL of the fumes dissipate before installing the battery and starting the motor. Pulleys and Belts can grab your clothes and hair. Don't wear loose clothing and keep your hair up under a hat. Always wear safety glasses. Be smart.
Change your Alpha Gear Lube
Change your Bravo Gear Lube
Why does my drive lube have white streaks?
Why does my drive lube have gray mush?
Replace Your Alpha One Water Pump
Replace your Alpha Gen II Water Pump
Rebuild your Bravo Raw Water Pump

Troubleshoot & Replace your Alpha Shift Cable
Troubleshoot & Replace your Bravo Shift Cable

Replace your Alpha One Bellows & Gimbal Bearing
Replace your Alpha Gen II Bellows & Gimbal bearing
Replace your Bravo Bellows & Gimbal Bearing
Why does my drive "Growl" when I turn?
Water is leaking into my boat from the transom

Replacing your Trim Sender Switches
My Trim doesn't work but my Trailer Switch does
My Trim Gauge is Pegged

Rebuild your Alpha One Upper Driveshaft Housing Assembly
Rebuild your Alpha One Lower Gear Housing Assembly

Check your Engine Alignment
Check for Excessive Steering Play
Troubleshoot an Overheating Problem
Should I remove my thermostat?
Why do I overheat when going fast?
Troubleshoot a No-Crank Condition
Troubleshoot a No-Start Condition
My Engine Backfires and/or Surges and has Lack of Power
What is an Anti-Syphon Valve?
My Engine blows Steam from the Exhaust
Why do I hear a "clakity" sound while at idle?
Exhaust Manifolds...IMPORTANT Information
How do I Winterize my engine?

What is Galvanic Corrosion?
I have a Bravo III Galvanic Corrosion Problem
What is a Mercathode?
All about Zinc Anodes
How do I check my carburetor?
What carburetor do I have? A Rochester or a Mercarb?
I have water in my fuel
My fuel smells like varnish
My fuel is old
My engine has been sitting for several months without being run.

I have a Mercruiser 470, 485, 488, 190, 3.7L Engine
I have coolant dripping from the front of my engine
Can I install a regular Alternator instead of the Stator-Rotor?

Converting an OMC to a Mercruiser

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