mariner 40 overheating

[anglosax]

I have an overheating problem with 1986 mariner 40 hp serial 6E9 [ mariner 2 cyl 2 carb built in japan by yamaha] We have good flow from teltale but overheats in around 10 mins at half throttle. [Steam & way too hot to touch] Checked t/stat and PRV seems ok so tried to run with T/stat cover removed to check flow - no flow at T/stat exit hole and exhaust pressure at PRV seating. Something not right here - exhaust in the cooling passageways ??? Should there be water flow at open t/stat housing ??
What is the normal flow of cooling water around the engine from the impeller pump? prevoius owners hinted at cooling problems and I suspect some blocked passageways in the head/jacket - How best to clean out - muriatic acid ?
Is there a t?stat bypass on this model?? - how to pinpoint problem??
thanks to all on this forum who spend time to help

 

[Faztbullet]

Re: mariner 40 overheating

Should there be water flow at open t/stat housing ??

Yes...

Is there a t?stat bypass on this model??

Yes...it the valve and spring under cover next to t-stat
Have you installed a new impeller as this one sounds weak? This would be my first repair on this engine. Was engine operated in salt or brackish water? If so the inner exhaust cover may be rotted and exhaust pressure is pressurizing the water cooling system, also a defective head gasket will do the same.

 

[anglosax]

Re: mariner 40 overheating

Fastzbullet
The impeller is original and unknown age so agree it needs to be replaced but telltale stream is very strong - I cant stop it by putting finger over the hole.
Previous owner [now deceased] had exhaust cover off so we know there was problem there - inner exhaust housing looks clean and corrosion free. We are going towards head gasket but it show no other symtoms ie white smoke at start up and runs generally well. If there is exhaust in the cooling passage where are the possible leaks/breaks other than H/G?
I wiil check cooling with pressure hose to see if there is flow

 

[Faztbullet]

Re: mariner 40 overheating

It will not show white smoke if gasket is leaking slightly....

 

[tmryan]

Re: mariner 40 overheating

did you find out the reason for overheat??