Force 125hp overheating above 2500 RPM

[21BaylinerCC]

The overheat buzzer on my 1988 125hp outboard has been going off at RPM's around 2500 and above.
I've read over numerous threads that cover the topic, but haven't found any solutions to my problem.

Following the Clymer manual, I synced the carbs and set the idle, making sure idle is around 900 RPMs. Before this, it idled at 2000 RPM and overheated only a handful of times (still not sure why). I figured by setting the correct idle, I'd fix the problem.

Took it out on the bay and it idled in forward gear at 700 RPM, same with reverse at idle. When i got out of the channel, i throttled up to around 2500-3000 RPM, and the overheat buzzer went off after about 1 min.

Took the boat back in, changed the thermostat (since it hasn't been changed in +/-5 years I've owned it), and, of course, the impeller.

Took the boat back out the next day, and still overheating at the same RPMs.
Water is flowing out of the pee tube/exhaust, water doesn't boil off the head/block. Ran it with the engine cowling on and off. No signs of leakage from the head gasket.
I can run it in forward gear around 1000-1200 RPM, no overheating after 15 mins.

Any ideas?

 

[Nordin]

As you have changed the impeller the next step is to try to run without the thermostate and check if the buzzer goes off.

Have you checked what temperature the head has when buzzer goes off?
An cheap infrared gauge (gun model) is a good tool in the tool box.
The sensor should close the circuit and send an alarm at 160-170 dgr F (71-77dgr C).

The sender could be bad and close to early.
If the sender is good there maybe be a blockage in the cooling system.

 

[jerryjerry05]

What did the thermo look like?? Pics??
The impeller, did it come out with all the vanes intact? Pics??

IF??? there is any buildup on the thermostat it can mean the cooling passages are blocked.
Follow Nordin's suggestion and get a heat gun.
I got one, eBay $16

If your handy, you can attach leads to the thermo switch and see if it's defective.

As it overheats at higher rpm's ??
I'd say the pump and wear plate might need changing?? Pics??

The pics below, 40hp Yam that was NEVER flushed after being used exclusively in salt water.
The cooling passages were about 60% blocked yet never set off the alarm.
The sending unit was up near the top in the cooler area.

 

[21BaylinerCC]

I'll get an infrared thermometer and check the temp of the engine when i take it out again. I have a new Thermoswitch on order, so i will also replace that.
When i removed the thermostat to replace it, i flushed water through the opening and water flowed freely through the pee tube and the motor leg. I also flushed water up through the intake tube when i removed the lower unit to change the impeller. Again, steady flow of water.

Jerryjerry05- Thermostat did not look too bad - some corrosion, but not too much. I ALWAYS flush with freshwater after running it. Thermostat was not nearly as corroded as the pic you attached.
Old impeller had all fins, no cracks, and the wear plate is still smooth, without any grooves.

I have not checked the timing yet, which i realize could be a problem, but would that cause the engine to overheat? what about adjusting the idle mixture screw?

 

[jerryjerry05]

If the timing linkage has never broken, or you haven't "adjusted" it (just a little)
then the timing should be ok.
They don't jump time like a car engine.
The air screw is for low speed operation and shouldn't affect anything other than the running at low speed.

If there was any corrosion? it could signify there is a build up in the lower passages?

Do the heat gun test and see if it's really overheating?

 

[Jiggz]

Try running it first without the t-stat and see if it still overheats. Note that with a cold engine, upon start normally there should be no water coming out of the exhaust spout or the mid leg relief holes until the t-stat opens. If water runs out to the exhaust spout as soon as you start a cold engine, it's an indication the relief valve is stuck open or the t-stat is stuck open.