Motor too low causes backed up exhaust?

[johnyrude200]

I’m trying to figure out why I am having an overheat situation in my ’81 35, SS OMC. First off, the water tube guide and grommet are fine. I replace the impeller and thermostat, there are no clogs in the telltale or cylinder T-stat cover (I’ve had all of those apart).

I took the exhaust bypass cover off, and it appears that the gaskets and stainless steel baffle are OK, I can’t see any warping.

The motor pumps fine in neutral and forward at idle, but anything much above 1/4 throttle, the water slowly dies off and the motor goes into an overheat. Throw it back in neutral, and water pumping resumes and it cools properly (steady 138-145°).

In neutral, I observed intermittent exhaust coming out through what I believe is the prop. When there is no bubbles coming out, then the motor starts to run rough, then exhaust comes out, and it smooths out for a few seconds, repeat.

Is is possible I have it sitting too low in the test tank. The water level is high enough where two holes on the aft exhaust housing are submerged, and the water is high enough to be at the bottom of the transome bracket just in front of the swivel housing.

Could this be a situation where the exhaust can’t push it’s way out through the prop due to high water, and then is working it’s way into the tube with 2 o-rings where the driveshaft goes into at the bottom of the exhaust housing?

[fleetwin]

Is this the same engine we were discussing in a recent post??
Well, having an engine too deep in the tank, or on a boat with a low transom surely increases exhaust back pressure and can adversely affect low speed running quality. I don’t really think that pent up exhaust is sneaking into the water pump though (unless the pump isn’t sealing properly).
On the other hand, we all know how an test tank can affect running quality, and create air/exhaust bubbles in the tank which may get ingested into the cooling water intakes creating a cooling problem.
Have you run the engine on a boat to rule out test tank headaches?
So, you are saying the engine will cool just fine if left in neutral, even if revved up slightly, but will stop pumping once put in gear and the throttle is advanced a bit. Do I have it right??
Once test tank issues have been ruled out, I am still thinking that there is a sealing problem with the water pump/shift rod oring.
The engine will cool fine if left in neutral because the pump is immersed in water completely. Once in gear, with some throttle applied, the water level drops below the water pump allowing air to be drawn into the water pump.

[chris-p]

Yeah, try her on a boat. Without a large test tank, and test wheel, you will get too much turbulence above high idle in gear to prove anything.

[frankr]

Sounds like air getting into the water pump inlet side. I suppose you have the gaskets above and below the stainless steel water pump plate, right?

[1946zephyr]

The wear plate for the impeller has to have a good seal on the gearcase, or exhaust gases will greatly effect the water flow. I recently had this issue on an old 18 and found that the wear plate was not sitting flat on it’s seat correctly. I cleaned both surfaces really good and applied a little sealer and problem solved. It pumps like it should.

[johnyrude200]

I agree with everyone.

Fleetwin, you are correct. Neutral it’s pumps plenty, anything much above high idle in gear, water starts to get sporadic, than stops. Shift into neutral and voila, water pumping is restored. If the water supply stops again I’ll look at that shift rod seal. That would be pretty annoying if that is all it was.

Chris P – I use test props on anything over 8hp otherwise I’ll get soaked…fast!

FrankR – this does have gaskets on the water pump plate.

Good thing this motor has an overheat alarm on it. I was running it the first time at WOT in my 275 gallon test tank with a test prop (wearing ear protection), and that alarm sounded. Shut it down in time to prevent damage, but it had climbed up to 210° in a matter of seconds! Shut it down, started back up with water pumping, temps dropped back to norm quickly.

I tore the exhaust cover off and didn’t really see anything that indicated exhaust making it into the water inlet to stop water flow with this 2nd 35hp motor (which coincidentally is an ’81). The stainless steel baffle appears OK. I am going to button up the motor tonight with new gaskets to be safe (originals seemed to be just fine, peeled off in one piece and were still pliable/strong), run it in the test tank, and if it overheats/stops water flow again, mount it on my boat tomorrow and try it on the river. If it overheats again, well then we know its a faulty seal in the impeller housing or something related to that.

This is my first short shaft 35 I’ve run in a test tank, but I have run into overheat problems due to air bubbles getting into the water inlet at the gearcase with motors as small as 3hp. With the smaller ones, I usually just turn the motor to get the water swirling and the cooling system keeps up fine at WOT.

I will say it’s getting old lugging a 135lb motor everytime I have to tear it down…I’ll be installing a winch system soon…a lot more of these motors to work on this off-season for next year!

[fleetwin]

I’m still confused, is this the same engine you were dealing with in previous posts? Any signs of previous overheat problems?
Post some pictures if you can

[johnyrude200]

This is the pan, exhaust housing, and gearcase from that other motor, with a different powerhead. The other powerhead got transplanted to another pan/leg/gearcase and will be converted to manual start since the electric start orifice on the front of the crankcase was not salvageable.

So I buttoned up the exhaust bypass with 2 new gaskets and still have the same issue. Neutral, pumps plenty of water, in gear, anything much above idle and the water gets shut off. As soon as I shift into gear and the RPMs drop, water pumping drops off too, HOWEVER, when I rev up to RPMs similar to what it has in neutral/not under load, the water pumping basically dies off quickly.

I pulled the motor from the test tank and will pull the gearcase off tomorrow morning to look at the gaskets/o-rings. If all looks good (which I’m pretty sure they are), I’ll mount the motor on my test boat and give it a spin.

This is an odd one to me. I know that the powerhead is OK, I had all of the water jacket apart and there are no clogs anywhere. Thermostat is OK, new impeller. There is something going on either with

A) air bubbles in the test tank getting sucked into the water intake screens on the gearcase
B) somewhere leaking exhaust pressure from the powerhead finding it’s way into the impeller housing
C) faulty seal(s) in the gearcase or driveshaft water tube.

I will do my best to eliminate each of these 3 causes tomorrow. Hope I can get this resolved by end of day because I need to get on my next project…falling behind!!!!

[fleetwin]

OK, so the same results with a different powerhead. And, you mention that the engine stops pumping when revved in neutral, correct? So, that pretty much rules out test tank turbulence.
Something in that water pump isn’t properly sealed….

[johnyrude200]

I just got back from running the motor on the test boat and it ran like TOPS. So apparently this test tank is letting air get sucked into the water intake and giving me false positives. No overheating, if anything the motor ran cold (105-110°), even after WOT for 2-3mins non-stop.

I think I hit 35mph on my 14′ duratech boat…DAMN YOU WINTER!!!! Love boating.

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