Johnson 40hp overheats at LOW speeds

[eshan]

The engine starts up fine but an alarm ("beep") goes off after its been running for a couple of minutes on a low speed. The indicator jet (‘pee’) however, continues as a steady stream. Its possible to stop the alarm by cooling the engine block directly with a wet cloth. When running at high speeds though the alarm does NOT sound (i.e. the engine does not seem to overheat) and I can run continuously without a problem until I slow down, when almost immediately the alarm goes off again.<br /><br />I've been told that the engine block head needs to opened up and ‘cleaned’ and that the cooling system impeller needs to be replaced. It seems that opening up the engine block would be a big operation and would mean having to replace several gaskets as well.<br /><br />I could use some feedback on what exactly could be wrong and what the most economical solution might be as I would need to purchase all replacement parts from abroad which may be quite expensive for me.<br /><br />Cheers!

 

[Walker]

Re: Johnson 40hp overheats at LOW speeds

Could very well need a new impellor. Does your motor have a thermostat? If it does it could be bad and only letting water thru via the bypass with higher RPMs.

 

[ob]

Re: Johnson 40hp overheats at LOW speeds

Esh , If your waterpump has not been rebuilt in the last two years consider it as needing attention.Your symptoms point toward weak waterflow due to either worn out impelller or impeller cup and or faulty thermostat assy as Walker mentions and at higher speeds relieves through bypass.I recommend changing waterpump and thermostat before you need things like pistons and powerheads.

 

[wilde1j]

Re: Johnson 40hp overheats at LOW speeds

Esh, if you're running in saltwater without flushing with freshwater after every use, you probably have some salt buildup in the water passages. As ob suggested, you need to rebuild the water pump and if your motor uses a thermostat (Vernitherm) you need to check that as well. With the thermostat removed, you should be able to see if the water passages are clear. Salt buildup often has the appearance of wet sand.<br /><br />Good luck!

 

[BillP]

Re: Johnson 40hp overheats at LOW speeds

Slow speed overheating is a typical symptom of a worn or stiff waterpump impeller.

 

[eshan]

Re: Johnson 40hp overheats at LOW speeds

Cheers guys! much appreciated...<br /><br />From what u guys have said it doesn't seem practical to open up the head block and clean it out etc. OR do u think that might still b a good idea? (I will check the pump first though)

 

[ob]

Re: Johnson 40hp overheats at LOW speeds

Esh , While lower unit is off and thermostat housing removed you could backflush block and let run out drain from pump discharge tube.There are a number of products you could use as a scale breaker to flush through jackets.I'm sure some of the guys will have tips as to what they prefer.

 

[Walker]

Re: Johnson 40hp overheats at LOW speeds

I have used CLR in my test barrel, but I found that 50/50 vinegar and water works real well. I run the motor til it reaches operating temp then shut it off and allow to cool. I repeat this over 8 to 10 times and the motors have always flushed out real good.