Merc 5.0 Heating Issue

[tehlib01]

Hi All,

I have a Merc 5.0 TKS in my Maxum 1900 SR3 with an Alpha one. The boat originally had a 4.3 in it which was later replaced with the 5.0. I am getting an intermittent alarm going off which I believe is a heat issue but I am debating if it is electrical or something else. I have narrowed it down to the heat / electrical side and its not the oil pressure or gear oil as it seems to go off when I high idle for a few seconds and the temp gauge ever so slightly drops.


The running temp seems to be always riding just over the 175 degree mark, likely around the 180 to 185 range. I was pulling a tube and the alarm would go off. If I stopped and did a high idle the temp would drop ever so slightly and the alarm would go off. It never seems to go any higher than what I have described. Oil pressure is always about at what it is on the gauge.

I had the boat in for some service and they did a water pump kit on it (small rubber impeller), new t-stat and replaced the gimball bearing and bellows as there was a small leak. I asked them to check the heating issue and they couldn't figure anything out. They said the used an infrared gun and the temp would climb, then I feel the boat sat and they just never looked at it again so I went and got it back. They also said they ran it without a T-Stat and same issue. They also said there are no blockages in the risers etc.

I had it out again later in the year and I ran it fast and hard around lake and the alarm never went off once. However I shut it down, turned it on and the alarm would sound. I would then increase the RPM at idle a bit, the temp would have a quick drop in the gauge, alarm would go off and then the temp would come back up a bit and no alarm. This last time I took it out, the temperature outside was definitely a bit cooler which makes, where as the previous time it was a very arm summer day.

I feel the alarm is riding right on the edge of the threshold however I have no idea where to go from here. I am unsure if maybe electrical connections need to be cleaned? I asked the shop to replace the temp sensor and they didn't feel it was required as they saw the higher temp with an infrared gun. Only other issue I can think as well is an old battery maybe and low voltage causing issue?

Sorry for the long post, just trying to figure this out.

[mmwjr]

Under Documents of this site you will find Mercruiser manual #24 for SBC, go to the trouble shooting guide for cooling and find the flow test and perform it. This test has you disconnect the supply hose to the thermostat housing and capture the water over a minute to verify the flow rate is correct. If this test fails you have a blockage between this spot and the water pickup. If it passes you may have a weak engine circulating pump or blockage after the thermostat. Could be a flapper came off blocking the exhaust. The alarm sensor should not trigger until around 200F so if the IR readings are only 175F the sensor is bad.

[tehlib01]

Ok, I will take a look here shortly. Can this be done with the garden hose attachement? I know when they changed the flapper impeller out it wasn't missing any flaps, but who knows if it was missing one previously and if it had been done prior.

[mmwjr]

Yes this can be done with a hose. I was referring to an exhaust flapper broken off and not an impeller vane.