overheating? 40 hp Johnson

[wycliff]

I have a 1968 40 hp Johnson, When I started it this spring the engine felt hot to touch after about three minutes of running. So I replaced the water pump impeller & thermostat. I started the engine again and the engine still feels hot only after a few minutes of running. I then took out the thermostat cover and started the motor, water shoots out the thermostat housing. So I then removed the thermostat all together and restarted the engine. Still only after the boat is running for a few minutes the engine feels hot. <br /><br />Is this normal?? I used a 50 gal. barrel to submerge the engine. I believe the hot light doesn't work beacause of the age, is there any way of testing what the temperature of the engine cases? or did I miss something else? Or is every thing normal?

 

[Paul Moir]

Re: overheating? 40 hp Johnson

Welcome to iboats, wycliff!<br />The engine is supposed to get hot. It should reach around 160°F if the thermostat is working. At that temperature, it will be painful to leave your hand on the top of the top cylinder, but you should be able to hold your finger there for a few seconds. <br />Is the thermostat opening? When you reach temperature, the thermostat should open letting hot water out the large hole on the back of the exhaust housing. At idle, it'll be something of a spritz.<br />Make sure your water pump is submerged in the bucket. These older ones will not draw water up well at idle.

 

[alanR1]

Re: overheating? 40 hp Johnson

Does your motor have a tell tail? This is where the water that is pumped through the heads is squirted out. If you do have one it is usually located on the underside of the lower cowl. If you find it and water is squirting out try and put your hand under it for a bit. If it burns you immediately your thermostats aren't openning, it should be pretty warm but not too hot. If it isn't very hot you are probably ok, just worrying too much. Good luck.

 

[reeldutch]

Re: overheating? 40 hp Johnson

if you took out the tstats and the motor still runs hot then you have or a bad waterpump or the waterpassages are restricted.<br />check the waterflow restrictors.<br /><br />good luck

 

[wycliff]

Re: overheating? 40 hp Johnson

Thanks, for the replys, my motor doen't have a tell tail, the exhaust port only has mist of water coming out of it, I believe that the water exits the engine out the bottom by the propeller?<br /><br />Where are the waterflow restrictors located?<br />The thermostat was open after i removed it, therefore I think it is working fine. I did pour water down the thermostat housing when it was open and the water just ran out the tail. <br />Should more than just a mist come out the exhaust?

 

[reeldutch]

Re: overheating? 40 hp Johnson

Didnt you say you run the motor without the tstats?<br />if you run it without tstats it should run cold so you can put your hands on it forever.<br />if it stil gets hot that means that the water doesnt flow propper treu the water pasages and something is obstructing it.

 

[Paul Moir]

Re: overheating? 40 hp Johnson

I misread your first post. I didn't realize it warmed up so much without the thermostat in place.<br /><br />You're water pump has the two water tube system right? Are you sure you didn't get the tubes backwards? Were all the grommets and seals in good condition?<br />You say the thermostat was open when you removed it? It shouldn't be - it should close whenever it is cooler than 160°F.

 

[wycliff]

Re: overheating? 40 hp Johnson

The thermostat was open after I ran it. (I took it apart while it was hot) My water pump does have the two tube system, everything looked good when I took it apart. The head still gets hot.<br /><br />The only way I knew to check to see if the water pump was working was to take the thermostat cover off & thermostat out. I then started the motor for a few seconds and water would shoot out of the top of the engine. It appears to be working. <br />I would think if it had an obstructing in water passages their would be water in the thermostat housing? <br />With the thermostat cover off, I poured water down the ports and it seem to run directly out the tail, is this normal?<br />The only other thing I can think of is the hot water exiting the motor recirculating and coming back in? Is there another way to hook up a hose to the boat instead of using a barrel?<br />there is no side ports

 

[Paul Moir]

Re: overheating? 40 hp Johnson

I get it now about the thermostat. You can also put it in a pot of water on the stove and watch it open when it heats up and close when it cools off.<br />Without the thermostat in place, the water should come directly out the hole in the back. If you pour water down the opening where the the thermostat goes, water should pour out the hole in the back of the engine.<br /><br />The circuit works like this: Water is pumped up through one of the two tubes from the water pump and fills the cooling passages in the block. When the water is below 160°, the thermostat is closed, so there is nowhere for water to go except out through the hose on the intake side of the engine, near the head. Centred on the top cylinder. This hose carries water down to the bottom of the block, down through the other water tube and back into the water pump. <br />When the water heats up and the thermostat opens, water can then exit out through the much larger opening at the back of the engine. New, cool water is take into the system to make up for the hot water that exits the back.<br /><br />The head should get very hot, so that carbon burns off the spark plug ends. On my older engine, there's hardly any cooling passages in the head at all. A good place to test for temperature is the top of the thermostat housing. If your head alone is overheating, suspect an obstruction in the head gasket perforations that allow coolant through them into the head.<br /><br />I hope this helps!

 

[wycliff]

Re: overheating? 40 hp Johnson

Thanks for the update, I am going to remove the head this weekend and check for blockage, I do remember that the head gasket was replaced by a friend with limited knowledge about boats, maybe the new gasket he installed was for the wrong year, thus blocking the correct water passages. I did put the thermostat in hot water on the stove, it seems to be working properly. <br />The thermostat housing gets just as hot as the head, with or without the thermostat in place. <br />How can I check to see if the HOT light is working, somebody told me to ground the wire from the sensor and the light should turn on, is this correct???

 

[Paul Moir]

Re: overheating? 40 hp Johnson

Yes that is correct. <br />You could test the sending unit itself by heating it until it closes, checking it with an ohmmeter.<br />I forgot something; if you were getting good flow through the thermostat housing with the cover off, the head gasket cannot be completely blocked. On the 40hps, all water flow is directed through the heads.<br />Does your engine have compression relief valves in the head to aid starting? It's not possible that they are leaking exhaust gas?

 

[wycliff]

Re: overheating? 40 hp Johnson

I don't know if my motor has compression relief valve or not, What do I look for?? I don't believe they have evert been changed.<br />Also when It is cold and I try to start it, you have to give it a fair amount of throttle, would this have anything to do the compression relief valve not working properly?? I definitly has more compression when I increase the throttle to start the boat.<br />Thanks,<br />Cliff

 

[Paul Moir]

Re: overheating? 40 hp Johnson

I'm afraid I don't know a lot about the compression relief system. There should be two tall separate chambers sticking out the back of the head if you've got it. The valves are underneath those covers. And I think a linkage to the pull start. The idea is to open up two holes on the head to make pull starting easier.<br /><br />Hopefully someone who knows the system better will be along.