Why is my 350 Mercruiser over heating?

Question

My 350 Mercruiser is over heating, everything is new including all pumps. I do not have a thremostat in place but cars run cooler with out them. If my thermostat is the problem why? Also once I start it it steadly climbes to temps that are very high.

Do you think that a thermostat makes that much diffrence in boats please explain why or possible salutions. Thanks in advance.

Answer 1

A thermostat is important to use because when the engine was designed ,it was designed to run at a certain temperature for optimum fuel economy and other mechanical issues that could make this discussion lengthy.But anyhow if you are sure your raw water pump actually works then most likely you may have clogged risers / manifolds.
When you test for raw water do not use muffs as you are feeding water under pressure to your engine . You want to suck it up .
If you can find a drum and make sure your lower part of the drive is completely submerged at the pickup holes run it this way while garden hose is feeding barrel.Then run engine for awhile and make sure it gets to regular temperature and you have good exhaust water fliow coming out of back of boat.
Do not put it in gear as the water will get thrown out from the spinning prop.This method will allow you to know whether it sucking at pump.
If you have in board motor put a 5 gallon bucket in bilge area and put the intake hose in it while feeding water and do the same but you have to kep an eye out because inboard pumps will suck bucket empty real fast.l
If you can confirm good flow either way but poor exhaust flow then take risers off manifolds and look at water passage ways they are most likely clogged with rust.

Answer 2

First answer looks good. I just wanted to mention. On the newer vehicles people may run them without a thermostat. As the first answer mentions, this does not allow the engine to reach proper operating temperature and the computer stays in a "Closed Loop", basically it thinks the engine is cold and keeps the choke on (In a sense) running rich. So, for a $5 part a person is burning a lot more in fuel.

Answer 3

you need to check the transom water hoses for kinking or leaks pull the hose off from the transom to recirc pump or on non lower unit fresh water pump systems after the pump and run the motor you should have good head pressure if not its in water supply not the engine if you pull the t stat out and run in cold water the engine will never reach proper operating temp remember this is an open type cooling system

Answer 4

if your engine has some age on it, the exhaust manifolds and risers could be very clogged with rust. check it out, you might be surprised. if the water where you operate is 70 degrees or more, you can probably get away without a t-stat.