Johnson 6hp 1974 cooling problem

alussow · Nov 11, 2004

OK, my Johnson 6hp outboard (1974) runs for about 15min then sputters and dies. After checking fuel (it's new), filters (clean) and spark (great, regapped the plugs just in case), I figured the problem is overheating. I removed the lower unit, cleaned the water pump, replaced the impeller, and resealed everything. After remounting the lower unit, I removed the cap on the powerhead covering the thermostat and water inlet and outlet. My first shock was that a previous owner had removed the thermostat. My second shock was that NO water bubbled out of the inlet when I started the engine. Soooo, I redid the water pump again figuring that I had done something wrong. Same problem, no water circulating in the engine at idle even at high RPM's. What am I missing here? Water flows through the engine when I pour it through, so there's no physical blockage. Any thoughts?? AlexL

NathanY · Nov 11, 2004

Re: Johnson 6hp 1974 cooling problem

What shape was the impellor in when you took it out? If it was shredded, you might have a vane stuck in the passage in the water pump housing, or possibly in the pick-up tube.

Grant S · Nov 11, 2004

Re: Johnson 6hp 1974 cooling problem

Did you manage to get the water tube seated correctly in its O ring?

alussow · Nov 11, 2004

Re: Johnson 6hp 1974 cooling problem

The impeller I removed was in great shape. About two weeks before the prolem started, I ran the motor for a few hours with no problems. After reseating the lower unit the first time, I had my doubts about how the water tube was connecting with the pump. The second time around I replaced the grommet (washer) that fits the tube and made doubly sure that it was seated correctly. Still, I don't get water up the tube! Thanks for your thoughts, Alex L

steviecops · Nov 11, 2004

Re: Johnson 6hp 1974 cooling problem

Seems like there is a blockage somewhere. I'd have the powerhead off and make sure that the top end of the water tube is not blocked or whatever, then look at the water passages to see if there is anything in there. Maybe the passages were becoming clogged and that's why the previous owner took out the thermostat. If there is definately no blockage, then it does look like a water pump problem. Are you sure the impeller key is in place? Good luck Steve

Grant S · Nov 12, 2004

Re: Johnson 6hp 1974 cooling problem

In the past I have checked that the pump is working by spinning the shaft with a drill while the bottom of the leg is off and sitting in water.

Chinewalker · Nov 12, 2004

Re: Johnson 6hp 1974 cooling problem

You say you've poured water down through the top, but have you blown compressed air up through the water tube? There are multiple pathways for water to exit the motor, but only one way in... - Scott

Vic.S · Nov 13, 2004

Re: Johnson 6hp 1974 cooling problem

As Chinewalker suggests check that water will flow upwards through the pipe from the pump and into the engine and out through the thermostat housing. My problem, albeit with a later model, was corrosion around the grommet at the top squeezing the grommet over the top of the tube and cutting off the water flow. It's a powerhead off job to fix.

OBJ · Nov 13, 2004

Re: Johnson 6hp 1974 cooling problem

I'd like to ask a question about the water pump impeller. With the impeller removed from the housing, did the vanes of the impeller pop out straight or did the retain a curved position? If they retained a curved position, replace it. And make sure the impeller is on the key.

mfarish · Nov 13, 2004

Re: Johnson 6hp 1974 cooling problem

Take the side cover plate off and clean out the mud dauber's nest. I had one about a year ago that did the same thing. Had all of the water passages stopped up.

alussow · Nov 13, 2004

Re: Johnson 6hp 1974 cooling problem

Wow, thanks to all for the great responses. Here's where I am: I removed the lower unit again. This time I sprayed water up the intake pipe, and it goes enter the engine. I'm not sure how to evaluate if the flow through is totally free or not. I also attached a drill to the drive shaft, and ran the lower unit in a bucket of water. The pump pumps. I've previously removed the side cover plates, and all was clean in there. I did learn that the water pump is not original equipment. The current water pump has shorter blades than the original. Is it possible that everything works, but doesn't move enough water? Also, what's the bst way to clean out the upper reaches of the cooling system. Cheers, AlexL

steviecops · Nov 14, 2004

Re: Johnson 6hp 1974 cooling problem

If you're happy with the water pump, and the motor is still overheating, the next step I would take is to remove the powerhead to investigate. It's an easy job on that engine. While it's off I would take the head off and clean out what water passages I could, then use a new head gasket. Good luck. Steve

umblecumbuz · Nov 14, 2004

Re: Johnson 6hp 1974 cooling problem

Alex, Questions. In your first post, you said

...I redid the water pump again figuring that I had done something wrong. Same problem, no water circulating in the engine at idle even at high RPM's

How did you run the engine? With muffs? In a barrel? In the water? And had you replaced the 'stat at this point? Just trying to eliminate red herrings. Ciao

Snowdrggn · Nov 14, 2004

Re: Johnson 6hp 1974 cooling problem

AlexL, You stated in a post above that the water pump impeller that you installed has shorter blades than the original. Could you possibly have the wrong impeller installed? How much shorter are the impeller blades? I would suspect that the replacement impeller should be the same dimensionally as the one you were replacing, with the exception of the wear from usage... Cheers, Snow

OBJ · Nov 14, 2004

Re: Johnson 6hp 1974 cooling problem

The impeller should be a tight fit in the housing with the "blades" or "fins" curving along the walls of the housing. If they are not, you have the wrong impeller. When you assemble the the pump, put the impeller on the driveshaft, slide the hounsing down on top of the impeller and turn the drive shaft CLOCKWISE while pushing the housing over the impeller. A little soap solution on the housing walls will help out with this.

alussow · Nov 15, 2004

Re: Johnson 6hp 1974 cooling problem

The impeller that is currently in the unit is also in an appropriately sized water pump. It's just that the overall water pump appears to be smaller than the original. To test the engine, I ran it in a bucket of fresh water. The water was high enough to just cover the impeller. Unless there is a striking new revelation, I'm going to take the powerhead off. Wish me luck, I ain't very good at this. AlexL

alussow · Nov 16, 2004

Re: Johnson 6hp 1974 cooling problem

Hmmm, I popped off the powerhead and found a major blockage in the water tube. Just where the lower copper tube connects to the upper portion of the water tube about half way up the shaft is/was a grommet. The grommet was bent out of shape and there was an accumulation of salt/sand. I've cleaned those out, and will change out a few gaskets before replacing everything. I hope this works! Alex L

steviecops · Nov 16, 2004

Re: Johnson 6hp 1974 cooling problem

Well done Alex, sounds like you found the problem. While the powerhead is off, clean out as many passages as you can. I use an old throttle or brake cable and push it in and out of the holes. Then flush through with water or compressed air. Good luck. Steve

alussow · Nov 16, 2004

Re: Johnson 6hp 1974 cooling problem

Well, every thing has been cleaned, all gakets replaced, a few washers, grommets and one thermostat added, and she works like a charm! Thanks to everyone for your advice and support. Cheers, AlexL

OBJ · Nov 16, 2004

Re: Johnson 6hp 1974 cooling problem

Great work Alex!! Thanks for sharing the fix. Could help the next guy. Enjoy the engine....

Johnson 6hp 1974 cooling problem

Cadet

Commander

Petty Officer 2nd Class

Cadet

Chief Petty Officer

Petty Officer 2nd Class

Fleet Admiral

Rear Admiral

Supreme Mariner

Cadet

Cadet

Chief Petty Officer

Lieutenant Junior Grade

Petty Officer 2nd Class

Supreme Mariner

Cadet

Cadet

Chief Petty Officer

Cadet

Supreme Mariner