6HP stops pumping water at high RPM (1967, new water pump)

MattFL

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My 1967 6HP Johnson stops pumping water at full throttle. It was in the garage for about 20 years, it started right up so I had a new water pump professionally installed. Now it pumps water just fine, up to somewhere around 1/2-3/4 throttle. But when you open it up water stops coming out of the little exhaust hole as the RPM's come up. If I let off of the gas to 1/2 or 3/4 throttle then it immediately starts pumping again when the RPM's drop, and the water coming out is extremely hot for a few seconds, so it definitely wasn't pumping. Compression is 89# top cylinder, 90# bottom cylinder. It stops pumping even before the boat is up on plane, and the lower unit extends plenty past the bottom of the boat. Please help with any ideas, I'm at a loss on this one.
 

nwcove

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Re: 6HP stops pumping water at high RPM (1967, new water pump)

from personal experience, id be thinking head gasket.
 

steelespike

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Re: 6HP stops pumping water at high RPM (1967, new water pump)

You might check the thermostat. Be sure the passage is clear and test the thermostat on the stove in hot water.
Check the intake screen on the leading edge of the exhaust snout.
A slim chance a head gasket could be leaking compression into cooling passages.Not likely as the compression seems fairly good. And I think I've heard that a bad exhaust baffle can cause problems. But I'm not sure what it does or exactly where it is.
 

MattFL

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Re: 6HP stops pumping water at high RPM (1967, new water pump)

Thanks guys. The thermostat is brand new, replaced with the water pump. The water intake screen is clear. nwcove did your motor stop pumping due to a head gasket? That thought had crossed my mind, maybe the air leaking by the gasket was forcing the water out, but the compression seems so good that I wrote that off. But if your motor did the same thing then maybe it's worth tearing into this thing...
 

kbait

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Re: 6HP stops pumping water at high RPM (1967, new water pump)

I had an '80ish 15hp that stopped peeing, and began overheating at about 1/3 throttle. Turned out to be a swollen upper water-tube grommet, requiring powerhead removal to replace. Here's a schematic of your 6hp (page 8): http://www.marineengine.com/parts/vintage-evinrude-johnson/278871/27887100001.htm]Vintage Evinrude Johnson Outboard Parts & Diagrams - Catalog 278871[/url]

If one of the #57 grommets (upper or lower) is split, perhaps it works at low pressure, and gushes out at higher speeds? That's where I'd look. Also, was the old impeller intact? If it had veins broken off and unaccounted for, you may have a hunk stuck in a cooling passage somewhere, and w/higher pressure with higher rpms, the blockage becomes complete, causing overheat.

Good luck!
 

nwcove

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Re: 6HP stops pumping water at high RPM (1967, new water pump)

my experience with symptoms you're having was with an early 80's 4.5 hp.( no t-stat) could troll all day, strong tell tale flow. at half throttle the tell tale would start to show signs of an issue. at wot, no tell tale flow. could go back to idle and the motor would spit steam for a few seconds then all was good. motor had great compression cold. not to say you dont have other things going on, but worth the $10 and a bit of work. jmo.
 

MattFL

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Re: 6HP stops pumping water at high RPM (1967, new water pump)

Thanks everyone for the replies. I put it back in the test tank and revved it up in neutral as much as I was comfortable revving it, and it kept pumping. So it pumps with no load, but it doesn't pump with a load, so I'm thinking the head gasket is the most likely culprit. So... I've ordered a new motor to replace this one, and I'm putting this one back in the garage until I have time to pop the head off and check out the head gasket. :)
 

F_R

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Re: 6HP stops pumping water at high RPM (1967, new water pump)

Ordered a new motor??? Yep, that oughta fix it.

As for the old one, I sort of doubt that "new" water pump is a complete new pump. I suspect the housing is worn, or the bleed hole is plugged. Or it is not properly sealed to the gearcase. I won't say it isn't a head gasket, but I doubt it.
 

kbait

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Re: 6HP stops pumping water at high RPM (1967, new water pump)

+1 on doubting the head gasket w/those compression #'s. If water discharge stops completely and it's just the head gasket, you'd either be dumping tons of water out the head gasket area - filling the lower pan, or dumping into cylinder(s)-and it wouldn't fire at all on affected cylinder(s).
 

F_R

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Re: 6HP stops pumping water at high RPM (1967, new water pump)

Just thought of a rare-but-possible thing. Is the inlet screen in place in the leading edge of the exhaust snout? Is the plug in place where they inserted it? If that plug is missing, it will starve for water at speed.
 

MattFL

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Re: 6HP stops pumping water at high RPM (1967, new water pump)

The inlet screen and plug are still in place and clean. My wife is tired of me spending time on the motor and not with her, and my 6yr old son is really getting into fishing and these two things heavily influenced my decision to take the speedy route and buy a new motor. I am keeping the Johnson and if I ever get time to work on it I'll drop the lower unit and check out the water pump before removing the head. I'm a very mechanical guy, but I took it to a pro for the water pump when bolts started breaking off. Now that the problem bolts have all been replaced, it should be easy to get back into it.

That said; the replacement is a new 6HP Tohatsu 4-stroke and I took it out for the first time yesterday. Man, I really miss the 2-cylinder 2-stroke Johnson! 2 hours on the new motor, and it starts reliably, shifts nicely and idles consistently, but it shakes like hell! It's a single cylinder 4-stroke and obviously doesn't have a counter balancer. At idle it's not very bothersome, but at mid-speed my whole aluminum boat is vibrating like one of those brook stone massager toys, and the vibrations make a lot of racket. With only 2 hours on it I haven't allowed it to reach peak RPM yet, but I gave it full throttle for a few seconds and the power seems similar to the old Johnson. After 2-hours of trolling around with my son, I burned probably 1/6 gallon of gas (half of the 1/3 gallon integral tank), so it is great on fuel. It's not smooth like the Johnson, but it will allow me to go fishing with my son so all is good. Thanks a bunch for all the tips!
 

HighTrim

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Re: 6HP stops pumping water at high RPM (1967, new water pump)

Ordered a new motor??? Yep, that oughta fix it.

That gave me a good laugh for this evening, thanks.

I too believe there is likely an issue with the pump itself. What impeller got installed exacty?

If you have updated to the plastic housing use part number 0763735....or simply 277181 superseeded to 0434424

If you have the metal housing, ensure it is not grooved, the vent hole is clear, and dress its mating surface to ensure a tight fit. I also seal the housing plate down with 3M 847 on all my waterpumps. OMC Gasket Sealing Compound on the screw threads for the housing.
 

MattFL

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Re: 6HP stops pumping water at high RPM (1967, new water pump)

I'm not sure which impeller was used, but I will find out when I take it apart. Does the exhaust share the same space as the water pump on these things? I'm wondering if a pump that isn't sealed around the base starts to suck air if the exhaust pressure gets too high. Is this what you're implying might be happening?
 

MattFL

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Re: 6HP stops pumping water at high RPM (1967, new water pump)

I finally had some free time and took the lower unit off of this old Johnson and checked out the pump. Man I was jipped by the mechanic, the water pump had no gasket between the wear plate and the lower unit, or between the wear plate and the pump housing. No gaskets anywhere, AND there were only 3 bolts holing the pump housing down, the 4th had broken off and was not replaced. I'm sure this thing was leaking air. The pump housing was metal and definitely not new with light grooving around the inside and corrosion all over the outside. I would have replaced it if I were the mechanic... The impeller was new and looked fine, I didn't see any name brand on it at all so I'm not sure if it's OEM or not. Since I had basically written this motor off, I smoothed the inside of the pump with 220 sand paper and put a small bead of 3M 5200 (all I had in the garage at the time) in place of the missing gaskets and put it back together. I'll give the 5200 a week to cure and try to test it out next weekend to see if that fixes the problem. I don't really want to spend any more money on this one since I've already got a replacement motor, so if this doesn't fix it then it might become a teaching tool for my son when he's old enough to start working on motors. :)

As for the Tohatsu, I've got just over 26 hours on it now and so far it has been reliable and very good on gas. The excessive vibrations is my only complaint, otherwise it seems to be a solid little motor so far.
 
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