95' 40hp magnum overheat alarm at idle

[Kyle C]

Hello, I've got a 95 Mariner Magnum 40hp 4 cyl. that the overheat alarm goes off after about 5 mins. of idleing. History: I took the boat out about 2 months ago for a 3 mile no wake run and a 1.5 mile w.o.t. run each way, no problem. Last time I took the boat out the alarm came on after the 3 the mile no wake run but the motor was not untouchable (hot) and the tell tale was strong as it always has been (very strong, but very warm). After limping it back to the boat ramp, I decided that the alarm was bad so I replaced the harness (in case of a short) and bought a new key switch with a built in alarm. I had previously replaced the oil sender and the oil module. Also, I bought a Faria temp sender and hooked it up on the exhaust cover (only place I could find). I ran the motor today for about 5 mins and the alarm came on again (steady alarm). The motor is not so hot that I can't touch it for a few seconds and the temp gauge reads 100 degrees. I did test the temp sender with a heat gun so I know it works. I've also found this model motor doesn't have a thermostat, if that is true how does it regulate temperature? The only thing I haven't replaced is the factory temp sender, I'd like to test it but don't know what temperature the alarm should go off at. Any help is appreciated. Thanks, Kyle

 

[CharlieB]

Re: 95' 40hp magnum overheat alarm at idle

How old is the water pump and impeller?

An impeller can take a 'set' as it ages, still moving water but not enough at an idle to properly cool the motor until the RPM increase.

Pull the lower unit off and inspect the pump, plan on replacing the impeller at least, if the stainless cup and lower plate have a 'groove' worn into them then replace them also.

 

[Kyle C]

Re: 95' 40hp magnum overheat alarm at idle

Sorry, I forgot to mention that the impeller and housing are less than 3 months old and have been in constant use.

 

[CharlieB]

Re: 95' 40hp magnum overheat alarm at idle

Most outboard operate around 140 and alarm near 160

Have you checked the 'sticky' posts at the top of this forum, one of them has links to service manual, you may be able to download your manual, it will have all the info and details how to check each problem.

 

[Kyle C]

Re: 95' 40hp magnum overheat alarm at idle

I have the Mercury/Mariner technicians handbook for this motor and it gives no information on the cooling specifications.

 

[Kyle C]

Re: 95' 40hp magnum overheat alarm at idle

Alright, I dropped the lower unit to see if anything might have gotten stuck in the impeller area and confirmed that the impeller is in perfect condition. I guess what I'd really like to know is how the motor regulates temperature. Does it have a poppet valve somewhere?

 

[Kyle C]

Re: 95' 40hp magnum overheat alarm at idle

So nobody knows how this motor regulates temperature? Okay. New development. I left the key in the acc. position while engine was not running and the alarm went off after about 5 min. Could it be a short or something? Thanks, Kyle

 

[CoffeeHound]

Re: 95' 40hp magnum overheat alarm at idle

So nobody knows how this motor regulates temperature? Okay. New development. I left the key in the acc. position while engine was not running and the alarm went off after about 5 min. Could it be a short or something? Thanks, Kyle

I dont know the answer either,, but willing to bet if you can find a parts breakdown for your power head you will see if there is a thermostat or pop-off valve there some place.

 

[Faztbullet]

Re: 95' 40hp magnum overheat alarm at idle

So nobody knows how this motor regulates temperature?

It is regulated thru restrictions in the block and exhaust plate. Unplug the oil module and see if alarms goes off as the solid alarm is hot and oil level on that model..

 

[Kyle C]

Re: 95' 40hp magnum overheat alarm at idle

Awesome, thanks for the response Faztbullet! Yes, the alarm does stop when I disconnect the oil warning module. I thought it was an intermittent alarm for the oil. I replaced the oil warning module and the oil pressure switch about 4 months ago, so why would that be the problem? Also, the water coming out of the tell-tale is quite hot (hold my hand under for about 6 sconds @ start up). Is it supposed to be that hot? Again, any and all help is much appreciated!