Johnson 85esl74b 85hp Thermostat/water pump question

[kippstakes]

Got my boat back from the shop a few weeks ago and have been working to figure out an evident cooling system problem since (on their instruction).
SO, I have installed a new impeller, which I don't think was the problem. So that's covered.
Additionally, the boat has a completely new pump kit, so that shouldn't be a problem.
Basically, the pump is fine.
My model has two tubes leading from the pump up to the motor (water jacket/thermostat). After inspecting the pump itself, it looks like one tube is an output, pushing water toward the motor, and the other is a return, bringing water from the motor and out of the boot vents. I tested for clogs/irregularities the other day by spraying a jet of water up the two tubes, one at a time, toward the motor. The strange thing is that water moved freely into the jacket thru the output tube (the tube that would deliver water to the jacket), but water could not get to the motor through the return tube, like it was clogged or closed.
Is this how they are engineered? The vernatherm (thermostat) should be holding its valve closed til the motor warms up, but is its purpose to let water OUT of the jacket instead of into it?
According to the shop, my boat would pump water correctly until it ran awhile (warmed up?), then it would stop pumping and overheat. Very odd.

Any ideas what might be going on here? What steps I need to take?
It would be great if the vernatherm didn't need to be removed, since the thermo housing might as well be welded onto the motor.

Thank you

 

[Woodonglass]

Have you Checked or replaced the VernaTherm. Mine was stuck. I cleaned and oiled it and re-installed it and everything worked fine afterwards. Are you absolutely sure that you got the tubes inserted properly when you re-installed the lower unit??

 

[F_R]

OK, kipps, you are making some good guesses, but they are faulty. First let me say I do not know why your motor is overheating. But I certainly can tell you how it is supposed to work:

That motor has a thermostat-controlled recirculating cooling system. The water pump takes lake water and pumps it up to the powerhead. After circulating through the powerhead, the water is presented to the thermostat (vernatherm). The thermostat checks the temperature and decides if it is warm enough or not. If it is not warm enough, a bit is discharged to cool the exhaust, but most of it goes through a check valve and returns to the water pump, which sends it back up for another pass through the powerhead. The water keeps going around this way, till the thermostat decides it is warm enough. When warm enough, the 'stat opens and the water is discharged and the pump sends up some more cool lake water and the whole business repeats. It is NOT a simple matter of the 'stat being open or closed. The temperature check and thermostat position is going on constantly, with the thermostat opening or closing as necessary to maintain ideal temperature. At slow speeds and cold lake water it may never open at all, while at full throttle it may open and stay open till you slow down.

Remember the check valve mentioned above? Water cannot go through it backwards. That is why you can't force water UP the DOWN (return) pipe.

 

[kippstakes]

Understood. Thank you for explaining it. It looks like I just need to find a way to get that vernatherm out and check/replace it.
Thanks again! Think I'm on the right track now.