Runs hot after2400rpm

[Eddie Ray]

Aq125a. 270OD. Hi again, boat stays at proper temp as long as I stay under 2400rpms or so? It can run at this speed and never gets hot,go past said rpms and she starts to get hot. Bring here back down ,and she cools right down?
I have resealed everything in the raw water system.new impeller ,flushed everything real good. When in the water and in neutral . There is good water flow out the exhaust,which is warm. How can I tell if the engine water pump is working properly? I have no leak in the closed system as it never looses any fluid. But I can't tell if the pump is working properly. It has a new thermostat,so that should not be a problem?
This is an old boat.but I have her looking good and have fixed most of her problems. Just a few little bugs to work out.
Thanks once again for any help.( I seem to say this a lot )LOL
Eddie

 

[HorizonblueDK]

Re: Runs hot after2400rpm

A possible cause of your problem is a corroded hose connection on the outdrive. I'm talking about the connection just below the universal joint bellow. They are prone to corrode and eventually there is a hole in it. In neutral and at slow speed, no problem, since the connection is submerged. But when you hit the throttle and get on plane, it is not surrounded by water anymore and will take a little air in. Then the engine will overheat.

A common failure on older aquamatic drives.

 

[Eddie Ray]

Re: Runs hot after2400rpm

Actually,I have already replaced that fitting. I thought that was my problem. But it didn't help.
If you have any other ideas let me know.
Thank you
Eddie

 

[Eddie Ray]

Re: Runs hot after2400rpm

Sitting here thinking about it,if the engine water pump was not working. It would probably over heat even at low rpm.
Will check the water pick up tube and those connections again?
Any help would be cool.
Thanks
Eddie

 

[Yungcow]

Re: Runs hot after2400rpm

Thermostat not able to open fully?

 

[diesel1960]

Re: Runs hot after2400rpm

To solve this problem( I pretty sure is low water pressure) I have installed on my boat low pressure gauge ,this let you see your water pressure at all range of RPM ,at full throttle you should get close to 10psi min.

 

[Eddie Ray]

Re: Runs hot after2400rpm

Have replaced the thermo? The gauge is a good idea. Was it a ***** to run? How did you tie it into the pressure line?
Thanks
Eddiw

 

[diesel1960]

Re: Runs hot after2400rpm

Use one of the water drains , I use one on the oil cooler , just install NPT tee or remove one of the drains and install nipple -size depends of the gauge nipple .

 

[diesel1960]

Re: Runs hot after2400rpm

I found automotive 2" gauge (low pressure/vacuum) and installed in my boat console,had a lot of experience with that issue.

 

[Yungcow]

Re: Runs hot after2400rpm

Centrifugal advance might be screwed up. That could make a lot of extra heat but you'd also see other negative symtoms.

 

[Eddie Ray]

Re: Runs hot after2400rpm

Centrifugil advance? What is that? Is that the timing? Anyway, please give me more info.
Thanks
Eddie

 

[Don S]

Re: Runs hot after2400rpm

You can check the advance with a timing light and in neutral. Just rev the engine and see if the timing advances to spec.

As far as your overheat. hook up a clear hose between your transom shield and raw water pump inlet tube. See if when running above 2400 rpm you see bubbles. Could be the seal between the lower unit and pivot tube is sucking air when it comes out of the water.

 

[Eddie Ray]

Re: Runs hot after2400rpm

You have read my mind? I was checking that out yesterday. Thought the same thing. If I run a line down to just below the boat bottom with a screen. To test that idea,will I hurt anything. Or must the water come up thru the od? I have thought about placing a brass water intake at the bottom of the boat and run the intake water line to that?
Thanks
Eddie

 

[diesel1960]

Re: Runs hot after2400rpm

Nothing will happen ,you will end up with another hole in your boat.
Will not work,your intake may create to much vacuum,and water pump will starve.
It take a lot of design and tests to make this work.

 

[Safisher71]

Re: Runs hot after2400rpm

Have you cleaned and descaled the closed loop cooling system heat exchanger? May have a few blocked tubes and therefore has lost the cooling surface area required for larger loads.

Cheers

 

[dennis461]

Re: Runs hot after2400rpm

Here is what I did on my boat to determine if the raw water flow was causing overheating.
My AQ131 has a copper tube from the heat exchanger (radiator) to the exhaust manifold.
Part way along, it has a drain plug.
Remove the drain plug.
Install a pipe thread to 1/4 inch hose adapter.
Connect a length of 1/4 CLEAR hose.
Run the hose down toward bilge, then back up along side the engine.
Run it over the stern toward the rear.
Clamp it along the way so it does not fly around.
Make sure you can see the hose from the cockpit (or bring a mate along for a ride).

Boat in water, engine running.
The water level in the hose will show if any raw water is flowing.
It should stay around the same height as the exhaust manifold.
WHY
Because at idle there is almost no pressure at that point.
Water is pretty much trickling through the exhaust and out the back.

NOW
Run up river.
Flow should increase in proportion with RPM's
And at 3000 rpm, should be running up above the stern out the hose at a steady flow.

On my boat, I'm convinced the raw water pump housing has degraded letting water slip past the impeller blades on the ends.
My raw water system is air tight (except for the ends) but would not draw enough water at high RPM's

 

[Safisher71]

Re: Runs hot after2400rpm

To determine if there is sufficient flow from your raw water pump. Disconnect the raw water feed pipe from the radiator and point it vertical, away from the engine. Start the motor and run in neutral, at 2000rpm + the water jet should be at least 4" high. This will determine if there is sufficient water delivery to the radiator. (obviously boat in the water)
If there is a good water column, then there must be a restriction in the heat exchanger or exhaust manifold jackets.
You can run the motor safely like this for a few seconds as the closed cooling will protect the block, Just the exhaust will not be cooled, so be careful.
It also has a strainer in the raw water route, is this clear and good.
There should be plenty of flow from the pump before the radiator and this should equal what is coming out the exhaust. If there is, then there could be a blockage in the sealed water side of the cooling system, requiring a block flush and radiator clean out.

By the look of the engine diagrams, it looks like the radiator can be disassembled and cleaned. Or remove it and take it to radiator cleaners for a good flush.




The raw water goes from your pump through the radiator tubes and then to the exhaust. If the radiator is blocked then the back pressure will cause the raw water pump to slip and insufficient water through the radiator tubes.