Yamaha/mariner 40 hp overheating

[anglosax]

HI all knowing gurus
I have an overheating problem with 1986 mariner/yamaha 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
over to you Redbolt

 

[tmryan]

Re: Yamaha/mariner 40 hp overheating

saw your post. had same problem with my motor.

finally fixed by taking off lower unit. put hose to water tube (use plastic hose adapter and fasten hose to water tube) this will allow you to run motor to determine if you have a restriction.

when I did this still did not get much water to thermostat but still got water pissing out--did not get much water out of two exhaust outleta on lower unit.

finally just got a stiff wire and jammed it up the water tube and worked it a while then put back the hose on the water tube--put water on full force, started motor and some sh-t come out of the exhaust

problem fixed

 

[Laser87]

Re: Yamaha/mariner 40 hp overheating

I have a 1988 ProV150 with an overheating issue all of a sudden. I ran it with the T'stats removed Sat. Oct. 2nd no water came out of the thermostat housings. Cranked water pressure up on garden hose to ear muffs and water started coming out the T'stat openings.

I will probe the water line from the pump to the engine and see if there is any debris up in there.