89 Merc 100hp overheat horn @ trolling speed only

[danpcro]

My 89 merc 100hp outboard has been running perfect for the last 2 years. The last 2 times on fishing trips the overheat horn has sounded after approx. 15 minutes at trolling speed. There is no horn on the way to and from the fishing area when the engine is at a higer speed and rpm. The load in the boat was light on one trip and heavy on the next trip. Once the horn sounded I shut the engine down for approx 1/2 hour to cool and then it ran fine all the way back to port. Has anyone had and fixed this problem?

 

[cowboyup]

Re: 89 Merc 100hp overheat horn @ trolling speed only

I have(had..just sold my rig this weekend) the exact engine with warning horn issues. First question is what sound did it make...a prolonged beep (indicative of overheat) or short intermittent beeps (oil issue)? On mine, I disconnected the black wire to the temp sensor (it's actually terminated at a terminal block) and started it up. It kept a solid beep. I then disconnected the two blue wires (bullet connectors) from the oil reservoir. It kept the long beep. I then cleaned the terminals (all connections with the warning module, temp sensor and oil leads. It worked fine in the yard. But the #$^ alarm went off again during the test run with my buyer. I ended up buying a new warning module for him. Works fine now. Good Luck.

 

[danpcro]

Re: 89 Merc 100hp overheat horn @ trolling speed only

It is a prolonged overheat beep. I think the engine is overheating but only at low rpms. Any suggestions on a likely cause?

 

[KCLOST]

Re: 89 Merc 100hp overheat horn @ trolling speed only

Poppet Valve.... If you have one.... If it is stuck open at lower rpms it will dump water out of the block causing an overheat conditions at lower rpms. It should remain closed up to around 3000 rpms.. At higher rpms there will be enough water to keep it at the correct operating temp.

 

[KCLOST]

Re: 89 Merc 100hp overheat horn @ trolling speed only

Yours should be located at the top of the cylinder cover.

 

[danpcro]

Re: 89 Merc 100hp overheat horn @ trolling speed only

That sounds like the problem. I pulled the boat out of the water today and am going to work on it this week. I'll start with the poppet valve, which is located under the same cap and just below the thermostat. Thanks. I'll let you guys know what I find.

 

[danpcro]

Re: 89 Merc 100hp overheat horn @ trolling speed only

The poppet valve, thermostat, and water pump all look to be in good shape. Could the oil pump be failing and only pumping at high rpms?