350 MAG MPI over heating

Handymans342

I agree with Ras

raybo3

How old are the manifolds and raisers???????

Handymans342

My boat was running warm because it was sucking air from the strainer top

Dream_Inn

One thing for sure, if it is drawing in air, it can significantly affect the amount of water it's pulling because it may lose prime.  It only takes a small leak for that to happen.  It's the same reason when you have wear in the plate on the inside of the pump. That small wear can cause that loss of prime.  It happened to me when I replaced my impeller this year.  The old impeller was worn enough to match the grooves on the plate, still allowing suction.  The new one lost prime, thus I had to buy a new pump.

rasbury

oh I sure went through that Dream......I know that story well! It takes very little to lose that pressure......

Black_Diamond

Yes, it was the blue plug on the water pump itself. 

Handymans342

Gee BD, my wife could have figured that one out

rasbury

easy Handy.......I'm going to check my plugs!

PartsMan

how many plugs on the merc 350?

Dream_Inn

PM, it depends on if you're closed cooling or not.  You will have two on the seawater pump, one on thermostat, then if you have the one drain system, one on the bottom of it.  But, if it doesn't then you'd have a couple on the engine block.

PartsMan

I have closed cooling. I thought there were 5 but I only could find 4 

pmerklinger

Noticed on my 320 (bravo 3) that the starboard temp would rise slightly when under load,not enough to set off the alarm. When I pulled the boat out for storage I noticed that the lower end water intake has the bottom three holes plugged with zebra muscles . Might be something to looked at. Good ole Lake Erie.  Pedro

Michael T

@pmerklinger, that's interesting. I'm having my boat hauled-out for the winter this week. I'll be sure to look at the drive intakes while we are removing them to check the drive alignments.

Black_Diamond

Well, took the boat out for a run before haulout this morning, same problem.  Overheats in probably less than a minute under load, cools right off at idle.  Obviously not the raw water pump nor the winterizing drain plug.

I'll pull the thermostat out and check it in a pot of hot water, but I don't think that's  the problem.

I'll pull the exhaust at look at the flapper valve and go from there...

Michael T

My money is on a restriction in water hoses either before or after the thermostat. Before thermostat spongy hose that collapses or restriction, after the thermostat obstruction in hose, like a burned flapper. Fingers crossed for an inexpensive fix.

raybo3

How old are the risers and manifolds???????? 

Black_Diamond

Risers and manifolds are original (2003).  All fresh water usage though.

raybo3

Yeah BD but still 12 years old. Maybe you should atleast check them...

Black_Diamond

I'll pull them and check the shutters too. Only so many things it can be!

raybo3

Good luck bro....

Dream_Inn

BD, 12 years is a long time no matter if it's fresh water or not.  If the risers have never been pulled off and checked, then it is definitely a possibility that is the issue.  Good luck and let us know what you find.  BTW, you hadn't made it back down to my area this year for a ride on the Chesapeake!

SanityCheck

Google "bravoitis"

rasbury

I'd start with that thermostat, we always jump to the worst scenario and often the most simple- been laud up all winter- runs fine at idle but maybe not moving enough water at higher rpm?

Black_Diamond

The HotRod 9.9 is awesome!  My AB rib flies through the water. Fun!!