1974 15hp Johnson temperature problem

[Rad2101]

Hi there,

I recently acquired a little 1974 15hp Johnson. After checking it all out and adding a few missing parts. I was ready to test in my barrel. Nothing worked first go. After a carb kit and points clean/adjust I had it running smoothly. It was however getting a little hot. Id already checked the water pump and it seemed to pump water ok. I pulled the thermo and tested it. It opened in hot water and closed back down when colder. It was opening and seemed ok. To be safe I ordered a new one anyway. The new one is a 143 degree F unit. As an experiment from something Id read here I tested both side by side with a piece of string suspended in a pot of water over a hotplate. I brought the water up to temperature slowly and found the new thermo opened much earlier than the old. So I believed this to be my problem. I installed the new thermo with a new water jacket gasket and run it again in my water barrel. No difference. This motor is getting too hot. With the thermo removed it runs cool.

Could I have a blocked outlet passage somewhere? The new thermo seems to operate ok. The water pump seems to be ok. I can probably get away with running it without the thermo where I live but its there for a reason and there must be another issue if its getting hot with the thermo in place. Im going to run it next with the water cover off and thermo out to see how well the water pump is pumping. Im pretty sure that will be ok and Im wondering what to look at next.

I have another of these motors and I notice at the back of the midsection a lot of water comes out of the holes there. This motor doesn't seem to output much water from these holes.

any ideas?

cheers

 

[Grub54891]

Not sure if it's possible on that motor, but is the thermo installed upside down? The spring must be on the hot water side of the system.

 

[flyingscott]

That motor should have one hole on the leg for the idle relief. If the motor was ever overheated the grommet at the top of the water pipe could be melted and restricting flow. Or at one time the water pump disintegrated and parts.of the pump are in the motor.

 

[Rad2101]

Im pretty sure the thermo will only go in one way. I had thought about that and will have another look. I'll remove the thermo and run with the cover off. If its pumping plenty of water here then the pump must be ok. I'll move onto the water exit side.

If I had the thermo out and lower unit off could I put a hose onto the water pipe and expect to see water exiting without the motor running. Without the thermo in place wouldn't the water just go around the motor and then exit? Would it come out the relief hole?

I'll play around a bit more and see what I find.

cheers

 

[OptsyEagle]

Water should be shooting out of that exhaust relief hole just under the powerhead on the back of the motor. The water spray will vary as the T-stat opens and closes but it should still shoot water. If you are running the motor in a barrel make sure the leg of the motor is submerged to a level above where the water pump impeller sits. Those pumps are not self priming and if the motor is not submerged in enough water the pump will not work.

When you say it is too hot. The test is to put your fingers on the water jacket of the powerhead and if you can hold your fingers on it for 3 seconds or more it is not running too hot. If it requires you to pull your fingers away immediately it is running too hot.

From what you have said, your t-stat is fine so put it back in. I suspect it is the upper water grommet restricting water flow. It is a relatively common failure on that motor but unfortuneately it requires the removal of the powerhead to replace it. Not an overly difficult job but a little more work then changing an impeller, etc.

 

[oldboat1]

water cover gasket is 0318917 — don’t think there’s an extra spring on that one, but still hard to manage in the space. Make sure the gasket slot on the lower left is properly in place or flow will be blocked.

Might remove the head and work on the t.stat and cover on the workbench. The gasket is a little tricky, and might use a little gasket sealer to keep it in place. The slotted rubber gasket for the t.stat fits over the flange.

To remove the head, free up the bottom bolts and back them partly out. Lift the head out vertically.

 

[boobie]

I go along with the grommet at the top of the water pipe being closed up.

 

[oldboat1]

Thing is, the motor runs cool without a t.stat installed.

 

[OptsyEagle]

I did not notice that the original poster indicated that the overheat condition went away when he removed the t-stat. If it did then definitely ignore my comment that the t-stat is fine. That would obviously be where to focus his attention.

In any case, he does not want to run that motor too long without a t-stat. The 1974-1976 9.9/15Hp are notorious for fouling spark plugs. Three things he should do if he is not a fan of rowing (although usually they only foul one plug at a time and still run slowly with the other cylinder). He should discard any champion spark plugs and use NGK B6HS or B7HS spark plugs. For some reason they last a little longer in those outboard models. 2ndly, set the rich/lean properly and lastly, always use a good working t-stat. Colder temperatures will foul the plugs much quicker. He should also avoid trolling for long periods of time and he should change his spark plugs after a couple of seasons, at the most.

None of that has anything to do with his overheating problems, but it will be his next problem if he doesn't use that motor properly. There is a reason why OMC overhauled their magneto design after only 3 short years of use and fouling spark plugs was almost definitely on their list of reasons.

 

[Rad2101]

Thanks again guys for your attention. Much appreciated.

As I mentioned before. If I had the thermo out and lower unit off could I put a hose onto the water pipe and expect to see water exiting without the motor running. Without the thermo in place wouldn't the water just go around the motor unrestricted and then exit? Would it come out the relief hole?

Not seeing anything out the relief hole makes me think a blockage somewhere still.

cheers

 

[flyingscott]

Maybe you should replace the waterpump on ypur motor. Seems ok is not really a good indication of if it is good or not. Did you assemble the lower unit correctly with the watertube alinged correctly? You should have water out of the relief hole within 20 seconds That is with or with out a thermostat.

 

[Rad2101]

It runs cool with the thermo out so I'm assuming its getting water in. With the thermo in its red hot after about 5 mins. Way too hot. Burning fingers and removing finger prints. I've had the water cover off a few times now and have run it with and without the thermo. checked the gasket and installed a new one every time. Lucky I bought a bunch of gaskets in case I had to repeat this process. Down to my last one. I just want to know if I can push water up the water pipe and check if its coming out the right places.

I might be looking at two different problems here. There might be a thermo issue either with the seating of the thermo seal or being restricted somehow when its in place. The water out the relief hole may be something different. If the rubber seal at the bottom of the exhaust was damaged or missing (see part 38 in Picture) would this stop water out the relief holes? Does this require exhaust back pressure to get water out the relief holes? Would a missing or damaged gasket stop water exiting these relief holes


View attachment johnson exhaust.bmp

 

[Rad2101]

Sorry did the second picture work better?

 

[racerone]

# 38 is not the problem.

 

[oldboat1]

#39 would be the grommet in question here: http://ww2.iboats.com/Exhaust-Housin...iew_id.1532500

I don’t think that is your issue, though — think the problem is (or is in the vicinity of) the t.stat. I would go ahead and do the test you have mentioned — take off the l.u., and pump some water up the tube. I don’t think you will see water out of the relief holes just under the powerhead as I think the leg has to be flooded for that (as would happen in the lake). My 9.9s spit some water from those ports shortly after start up, when in the lake, and I get a steady stream when the t.stat opens (or at least that’s my assumption, and might be wrong).

But with or without the t.stat installed, you should see hose water exhausted down into the leg. That is, water would circulate around the head and exit out the inner exhaust tube up in the leg.

It’s clear you are getting water past the t.stat when a stat isn’t installed, but you might not with the stat in place. My guess is that there is a blockage in the housing, so no bypass. I’ve gone through a few of those t.stat cover gaskets too, but you may have to pull the cover off and insure the holes in the housing are open, and that the gasket is not blocking the small opening (from recall) on the lower left side. Additionally, the flange fits into the slotted gasket that goes around the stat. If you put it underneath the flange, the clearance might not be correct — not sure.

So pull off the l.u., and run some water up the tube. If you have an IR gun, btw, you can use it to see where there is a blockage. The water from your garden hose is probably around 40F, so anything much warmer than that would indicate the hose water isn’t passing through.

 

[OptsyEagle]

Yeah, I doubt you would have a problem if you tried to run water up the copper tube with a hose. Just make sure you start off with reduced water pressure from your hose. I am not sure what water pressure your house runs with but many are in the 60psi range and I doubt anything in that motor was designed for that type of water pressure, so increase the pressure on your hose slowly. It shouldn't take much pressure to get water from the bottom of the motor to the top so a little more then that for you test is all you need.

By the way, that is a very weird problem. Where the water cooling system only works when you remove a "fully functional" thermostat. I will be interested in what fixes it. Keep us posted and good luck.

 

[Rad2101]

HI Guys,

Sorry about the delay in getting back to you. I have had to order parts and wait. Things take a while to get here in Oz. Im hoping to get back onto this in a week or so. Interesting comments however and the idea that the bypass might be blocked sound possible. Stat out and runs cool. Stat in and way to hot.

I'll start by dropping the lower unit with the stat out and pump some water up the tube. If that seems ok I'll put the stat back in and do the same. Should water still exit? Will it exit via the bypass?

I'll double check gaskets and make sure I haven't blocked the bypass. I was pretty careful putting the stat in around the gasket and thought how easy it would be to have that sitting wrong.

So I have a new impeller, a few more water cover gaskets and a IR thermometer. Hopefully that will give me an idea of exact temps.

thanks again

 

[OptsyEagle]

The only problem with "the bypass being blocked" theory is why does that passage all of a sudden clear up when you remove a functional thermostat.

 

[Tim Frank]

My money would be on a marginal water pump. The thermostat does add a restriction....even when open.

 

[thatone123]

I have seen a number of these 15's blocked with salt residue. One pf these motors was so blocked that I dug a half box of Morton salt residue out of the passageways. Surprised this was not mentioned here unless you said it was used in fresh water only.