Mariner 50HP overheat alarm

McGurk · #1 May 12th, 2003, 11:21 AM

I get a HighTemp alarm only at WOT. New water pump. Lot's of water in the telltale stream. I opened the head and exhaust covers and water passages are clean. However there is no thermostat installed in the engine. Should there be one? Its a 1988 Mariner 50 HP 3cyl. S/N OB248535. Could no thermostat be the problem.

jim dozier · #2 May 12th, 2003, 11:35 AM

Oooohhh! Deja Vu! I have a 87 Suzuki DT85 with a very similar history. I started with a new water pump and thermostat, but occaisionally my temp alarm would sound at WOT. I replaced a head gasket with a new factory design and removed a lot of blockages in the head from corrosion. I felt sure that was the cause. Now the "new" thermostat got stuck and I tried running it without one which helped for a while (winter the water was cold) but this forum convinced me to put it back in and when the water warmed up the alarm started sounding again. I removed the exhaust cover and cleaned out some blockages there and installed a temp gauge. Still the alarm sounded at about 190-200 degrees F. Aaahhh, but checking with the dealer I found that the sensor isn't supposed to cry until 237 degrees F. So I replaced the sensor and no more alarm. Hope this story helps.

PS does your engine have a pressure relief valve in the head?

McGurk · #3 May 12th, 2003, 11:54 AM

No pressure relief valve in the head or anywhere else on this engine as far as I can tell. The temp sensor is OK and set for 240 degrees. I tested in boiling water and is stayed open circuit. Had to get it near the flame to get it to switch. I don't know who, why or when the thermostat was removed, but I think is should be used. No info about this in the engine manual other than how to test it on engines so equipted.

jim dozier · #4 May 12th, 2003, 01:11 PM

Oooohhh! Deja Vu Vu again! I tried testing my sensor in a pot of boiling water also. But since water boils at 212 F mine would not trigger unless I stuck it on something hotter. The catch here was that it was supposed to go off at 237 F. and was going off somewhere between 212 F. and 237 F. Anyway I had already ordered the sensor since I didn't know what else to do and when I replaced it no more problem. My engine cylinder head runs about 195-205 F (as near as I can determine from the number on the temp gage) at WOT and a slightly oversensitive sensor was all it took I guess.

I am guessing that your thermostat is missing because a prior owner ran into this before. I would put one back in and try to find what component of your cooling system isn't doing what it is supposed to do.

BillP · #5 May 12th, 2003, 04:26 PM

Was it doing this before you replaced the impeller? If not you missed something on assembly or the impeller rubber may slipping on the hub. This hub slipping deal happened to me on a 140 Johnson. You didn't tweak the timing lately did ya?

I know some cars will run hot without a tstat but never heard of an outboard doing it. If you continue to run without the tstat make sure you decarb often because piston crown crud builds up fast on cold engines...then the crud holds heat and burns little holes in the crown.

McGurk · #6 May 13th, 2003, 07:07 AM

The WOT temp alarm problem came first. I rebuilt the water pump twice so I don't think that I missed anything like an impeller slipping, but anything is possible. I've ordered a new temp sensor and will try that next.

BillP, What is your thinking on the timing?. Could too far advanced cause this overheat issue? I'm not convinced that the engine is really going overtemp but without a gauge there is no way to know. The engine runs fine.