Evinrude Acting Up

My 250 etec ho, after lending it out, is apparently overheating.

Good stream at the indicator.
Pump and impellor only have about 5 hours.
On starting, temp on digital thermostat climbs to dangerous levels but alarm does not sound.
If I go WOT for 15 seconds, when I power down to cruise it will be fine until I stop. Until I reduce power it will indicate surprisingly cool. 132°f 65°c.

The first time I drove it after getting it back, I noticed nothing out of the ordinary until I glanced at the digital tach while it was displaying temp. 212°f 100°c! But no alarm. And no steam visible. Good stream at the indicator though. That was at slow cruise: 6 knots, 1100 rpm. Bottom not immaculate but better than other occasions when that speed/power did not cause trouble.

Possible issues:
-something loose inside the engine causing trouble sometimes, then not.
-baked pump from misuse by neighbours. Pump leaking badly and priming poorly w/o the blast of water.
-Faulty thermostat sending nonsensense signals. Although computer should have triggered the alarm at boiling point. And temp resting at boiling point seems unlikely coincidence
-Baked computer from baked pump no longer responding correctly to inputs. Although that also seems unlikely. Because then it probably wouldn't start at all.
-Something I didn't think of

 

Check all minus connections from battery to senders and whatnot else on engine for oxides and bad contact. What's the temperature in the indicator stream?

 

I will do those things.
Scraping bottom at the moment

 

What do these holes do? I had assumed pressure pickup for pitot tube speedo, but there are six. Barnacles inside reducing flow. Now picked out.

 

Additional water pickups iirc; feeding water when engine is operated in raised position (or when side ports are picking aerated water, as on a classical seasled.....).

 

Cooling water intakes, per water flow diagram on pg 184 of the manual
Well damn.

 

Yup. They will be sterilized in a minute. Hopefully not too much crap inside out of reach

 

Well heck. I think it is my own darn fault. I could not force the impellor inside the cup while dry, and I had all the products on hand because installing new pump, impellor, and prop. I put a thin coating of marine grease in the cup to get it in. Maybe the marine grease has attacked the impellor.

 

Next time try warming the impeller up in a pot of hot water, obviously not too hot.
Water soluble grease ( damp bar-soap ) can also be used with less chance of damage to the rubber.

Like barnacles in the lower cooling water intakes!!
You're going to want to consult the manual for cleaning those out.
Those engines are meant to be stored raised, out of the water.
Barnacles should not be growing there...

EDIT: Let me guess, the aft deck impedes the engine from tilting all the way up so the six holes are left submerged in sea water.

 

No, but good guess. The engine is timmed all the way up, but it is as low as possible on the transom, and made 5" longer.

 

We had an overheating issue with a brand new Mastercraft in 2013 ish. It was a demo and the unit had maybe 10 hours on it when we bought it from the dealer.
Pulled the impellor and it was toast. ah ha, fixed it!! but then:
Over heat, take it to the dealer, overheat, take it to the dealer overheat, take it to the dealer. Major blow up, take it to the dealer. Four times.
Finally, a non Ilmore dealer had a look at it and diagnosed that perhaps some pieces of the first damaged impeller had worked its way up to the thermostat.
A couple days later, he presented the boat back to me, with a small ziplock of rubber impeller pieces. Never an issue since.
I am not sure if there is someway of tracing the line to see if you have rubber impeller pieces within the cooling loop

 

Are you sure it is not the sending unit or the gauge? An non-contact infrared thermometer is relatively cheap.

 

Definitely need one. A good tool to have.

I do suspect something loose inside.

 

In most cases an on-off alarm should sound first time you turn the key on, then cutting off when engine is running ok. How about yours, have you ever heard it? If yes, then it should be regarded ok. If that is the case, then there has not been serious overheating (the alarm has the highest setting), but random disturbance of the flow around the wet ends of the variable resistance sensors. The telltale indicates reasonable pump delivery, so pump itself not primary suspect.

I have had experiences like Barry, both with an outboard, and with a (locally) very common diesel, which has the oil cooler for the transmission first in the chain after the sea-water pump. Its pipes are smaller than both the other heat exchangers, so everything sticks there, and it is hidden behind a lot of other paraphernalia (of course...).

So my take on this is: for the moment being, don't worry too much about the plugs showing this or that, unless there is one that differs radically from the others, or there is a local discoloration of the paint or signs of leakage/corrosion along the sealing plane edges. Check for debris in the thermostat housings, in the ECM/fuel cooler and in the pressure relief valve. Also check water pump inlet screens; they are fine mesh and with organic mud in the water, which dries to a hard cake in the air, the don't let water through. Then there is only flow through the front holes.

If nothing is found then, you got to remove the lower housing. Then, try to backflush the system from top down. And when you are at it, attach a spray deflector to the nose of the leg extension. And when you leave the boat for a couple of days, put the gear housing and prop in a plasic bag. That will save your heart (unless you forget to remove it next time you start...).

 

Thinking about it one more turn, my favourite sinner is the pressure relief valve. Its basic function is to ensure that there is a minimum system pressure to prevent vapour lock and internal cavitation around the cylinders, analog to the function of the valve in the radiator lid in your car.

If it is sticking slightly open, or reacting sluggishly, the thermostat(s) don't have authority over the temp control any more. When running at low load, normally the thermostats move towards closing and the water is dumped via the thermostat bypass (acting as a resistance), which will keep the system pressure ok. But if it is leaking, even slightly, the flow can take two routes, leading to a reduced system pressure and random vapour locking at low speed. When you hit the throttle for a moment, the flow (and consequently system pressure) through thermostats and PRV increases and the vapour(/air) pockets are cleared, but the system pressure is not under control. If the valve is sticking, the high speed resistance will be too high to allow adequate cooling again; there may be some sweet spot where there is a balance (kind of).