2007 Mercury 115ELPT4S

by Matt Smoak
(Pawleys Island, SC, USA)

Hey everyone, I have a little bit of aggravating troubleshooting. I have a 2007 Mercury 115 ELPT4S on a 24’ Bentley Pontoon. My serial number is 1B534635. My hours on the engine are roughly around 120-130. So here’s the issue….

We took out the boat for its first maiden voyage this summer. Everything was operating great! Then it happened…that long single buzzer at WOT. Pulled the throttle back, buzzing stopped. Went to try full throttle again and now I’m in safe mode. Ok…so it’s overheating. We nurse the boat back and go home. Checked the water intake, nothing blocking it, checked the tell-tale, it’s flowing like it should. Pulled the thermostat and stuck it in boiling water, only to see no change. Ordered a new thermostat, checked it in boiling water, it worked. While installing the new thermostat, I also changed the impeller with a new Mercury impeller kit. The old impeller still looked new (I religiously change them every season.) Anyway, took the boat out again yesterday and at WOT, the buzzing started and back into safe mode. I then pulled the temperature sensor and checked the Ohms after the engine cooled….I got 7k ohms. So I then stuck the sensor in cold water and the Ohms went to 11-12K Ohms, then hot water, the Ohms dropped to 4-5K Ohms. I don’t know the tolerances for the sensor….I’m tempted to ho ahead and replace it since it’s about $40. I figured the only other options were a clogged tell-tale, or head gasket. I traced the tubing for the tell-tale and disconnected and blew compressed air through all the fittings, no debris was observed. I also want to add I changed the oil when I did the thermostat and impeller. No milky oil was expelled. I just checked the compression and on the top (2) cylinders, I got 195 and the bottom (2) cylinders, I got 200.

My questions are…what else could cause the overheating?
Is the compression in specs?

Just replaced the temperature coolant sensor (what Mercury calls it) on the starboard side as well as I replaced the exhaust temperature sensor on the port side. Anyone have any ideas?

Also, I have looked in my Selocs manual and did not locate anything about a poppet valve for my motor. Does this motor have one and if so, where is it?