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Mercruiser 305 Overheating Problem

D

Deuce

New member
I bought a boat I believe has sat in a garage for some time. I know that a service center winterized the boat in the fall and drained and started it in the spring but it was not used through a few summers because of illness. Anyway I went fishing boat runs great after trolling at idle for a bit the temp runs up to 260* as soon as I give it just a little throttle cools down quickly . Any suggestions on where to start correcting this problem? Thank You
 
You mention that the boat sat for a duration. Most any sea water pump impellers that have not been used for a year or so, will need to be replaced.
When a rubber blade/fin type impeller has been at rest within the pump body, three/four blades have been stationary against the impeller "cam" (or for the Alpha, eccentrically positioned)!
This causes the rubber to eventually take a "set" and to loose resiliency. They will generally not recover from this. A replacement is imminent.
I'm going to say that is your place to start!

You may also want to look for any burned components...... rubber exhaust couplers, hoses, etc. If any impeller blade pieces are missing (when you remove the old impeller), these pieces will need to be retrieved.

A 260* over-heat is not good! Keep a very close eye on things when you try operating again. Check the engine oil periodically for any signs of moisture/water.

Also note that water must be present before starting the engine.... even for a very short duration. Water is the lubrication and coolant for the impeller.
Bottom line..... no matter what the hour meter says, it's ole Father Time that kills our impellers..... particularly when NOT in use! If you plan to lay your boat up for any long duration, at least remove the impeller.... or plan to replace it before running the engine again!
 
raw water impeller first, t-stat second, might be frozen half closed, good idea to change anyway.

Post after you run the boat after changing those 2 things if still doing it then we can go from there.
 
First this is a Mercruiser 305 with alpha 1 drive with fresh water cooling. I have never started the boat without the drive full in water not to say it never happened before I bought the boat. It sounds like the impeller may be the issue like I said the boat has been stored. The guy I bought the boat from had it serviced spring and fall I looked at all the receipts and did not find any work done to the cooling system, impeller... The bellows were changed last fall. I know on my outboard engine I have to change the impeller every few years. Is an I/O drive the same as an outboard impeller? I thought a I/O was cooled by the water pump on the engine. I guess its time for a service manual right off the start.
Thank you for the help
 
IMO here..... the Alpha drive impeller is more prone to the rubber taking this set than many other types of impeller pumps..... again, IMOO.
Compare and you may see why I say this. This topic has been discussed many times across forums.
Not a bad pump, you simply must be more vigilant on changing them.

Out Board impellers typically have longer blades, and will survive longer when not being used! Not necessarily fair to compare the two.

So I'd say start with an impeller change.... maybe an entire new pump (once in a while Merc will offer a deal on these).

Check your E/G mixture level just as you would with your car/truck after an over-heat. And like mentioned, check to see if anything was burned.
 
alpha 1 gen 1 and large merc outboards of the same years all used the same impeller.

a short stiff finned impeller, high pressure low volume,

the newer alpha 1 gen II uses a longer soft finned impeller very similar to johnson/evinrude outboards. High volume, low pressure

The older style will take a hsrd shape if not used for a extended period of time.

when ever a merc I/O over heats when increase in rpms occurrs it is 90% the time the impeller....................especially when it has been sitting for a long period of time.

Dont waiste your time even thinking about it

REPLACE IT!
 
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