Yamaha 90hp Thermostat replacement help/advice needed

[ollyjscott]

So I'm going through some basic checks on my new to me 1989ish Yam 90hp 2 stroke and I noticed on a test run that the coolant pee stream is a little cold for my liking and when I touch the cylinder head after 20 mins of running at 4000rpm its not really that hot. Its not cold, just warm / 80-85 degrees I would guess.

First question - is this normal? Keep in mind sea water temps here in Norway are currently around 55 degrees so pretty cold, therefore the pee stream will be correspondingly cooler.

I suspect a thermostat that is stuck open. I have ordered one but when I came to take off the thermostat the first bolt cracked off nice and easy but the second bolt didn't feel so good so I have let it soak in some WD overnight. I'm a little worried about breaking off a bolt and having a bigger issue on my hands than I started with (I might not even have an issue)

Has anyone had any problems with these bolts before? What is the solution here? Heat the bolt up? Do I risk damaging the power head if I apply too much heat to this area?

See picture - top left bolt backs out easy, bottom right is the one I'm having issues with

Help and advice appreciated!

[robert graham]

Take it nice and easy and use the WD-40, work it back and forth and hopefully all bolts will come out without breaking....afterwards, wire brush the threads and apply grease to them to help prevent future issues. Your thermostat may be needing cleaning or replacement. Also check/clean/replace that pressure control valve/spring/grommet....a good cleaning may be all that's required on these....You may probably need a new gasket under that cover....

[boscoe99]

Yams are known as cold pissers. That is, water from the tell does not go through the block or the thermostat.

[ollyjscott]

Ok will do - I'll try a little heat in them too but just a mild amount.

On this model am I supposed to see cooling water exit from the exhaust ports at the rear of the shaft? I seem to remember my 40hp Yamaha did this but I don't see water exit the shaft on this 90 hp, only through the the pee hole. At the same time he motor is definitely not overheating, running too cool if anything

[99yam40]

If you are referring to the idle relief exhaust ports at the back top of midsection, I have not seen much more than a mist of water come out of them on my C40 and a C60 while idling sitting deep in the water.
I would think a 90 would be the same

[TownsendsFJR1300]

Wd 40 isn't the best penetrate to use. PB Blaster (my preferred) or Liquid wrench penetrate would work better.

Some (not alot) heat to the block (heat gun) wouldn't hurt and some gentle tapping to the head of the bolt generally helps shock the stuck bolt.

The engines run cooler as salt water run them heated up to say automobile temps(say 205F) would tend to cause the salt to cook and passages to clog up with salt..

[ollyjscott]

So after 24 hours of penetrant soaking, heat application and extremely careful back and forth action on the lower right bolt = snappity snap! The bolt broke off right about where the treads start. So I am now left with a thermostat cover that is held on with 3 bolts. After some further reading it seems that I might not have an over cooling issue at all so I am going to run it up and see if it holds as is.

Living in Norway, the labour rates at boat shops and machine shops run at around 160 bucks and hour so to be honest I'm reluctant to touch any more bolts on this motor. It came with the boat and was therefore relatively cheap used purchase. If it holds I am really in the mind to just run this baby until she dies a death. The labour to pull the head if that is required (which will almost certainly involve more broken off bolts from what I read of old Yams) will be greater than just buying a second hand late 90s motor if/when this one fails. That's one thought on my mind right now.

I would attempt a fix myslef but I'm concerned that it could turn nightmareish when trying to hand drill an SS bolt with soft surrounding aluminium. Especially when that soft surrounding aluminium is a cylinder head with water channels etc. Do you think the motor could be orientated into position in a machine shop mill to accurately drill this out WITHOUT having to remove the power head? Helicoil of course could fix the issue.

Essentially, boating season is here and I'm hoping this will hold with 3 bolts for the 4-5 weeks of light use that I intend. I will consider finding a solution to fix this when time allows in the winter.

Any thoughts or recommendations?

[TownsendsFJR1300]

I don't believe that bolt is stainless steel(just treated).

And yes, a machine shop can remove it W/O pulling the power head.

They could probably remove the other three W/O issue while there..

[rodbolt17]

not the first one I ever drilled and heli coiled or retapped to 1/4x20.
with the head and water jacket still in place.