1988 70h johonson chronic overheating problem

flyingscott

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Then replace the thermostat 110 is to low you should be about 140-150 maybe 160 depending on water temp.
 

horiconducker

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Just another observation I had was the temp at the top of the thermo housing (where the oem manual states to check) was always at 140 deg. The block and head by the spark plugs is hot at high speeds but cool at idle.
 

horiconducker

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The 110 was running with NO stat in at 4k. It runs 140 -150 at idle WITH stat in.
 
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horiconducker

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I know this motor isn't suppse to have the water diverters like my old cross flow, but that made me wonder what forces the water around all sides of the cylinder, can't the water just flow up one side of the cylinders? Sure seemed like it when I had the head off
 

oldboat1

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Assume you are running a 50:1 mix -- or know for certain the vro2 system is working well, if running on vro. (If on vro, you might try running on premix and see if the motor runs cooler.)

A low tech test (but effective) is to pull the l.u. and flush up the tube, rigging something to fit reasonably snug over the tube. The water should exhaust down into the leg and out the exhaust port (also the tell tale indicator). Might be good to do this with the thermostat out (cover on), then with the thermostat in place -- won't test the thermostat, but will insure the the bypass is not blocked. With cover on, does water exit at the normal places -- exhaust port, down and out leg (tell tale indicator as well, although that doesn't show circulation). With an IR gun, you can locate cold spots as well as hot spots, of course. So you might check temps at various locations while flushing to see what you find. Hose temp would be, what, 40 to 50F? In any case, you should be able to confirm circulation.

I get a little nervous when I measure a temp around 160, but your operating temps might be pretty normal as flyingscott suggests. 140 to 160F is usually cited as the normal range. I think 180F is too high, but then again it probably depends on where you get the reading. I'm not sure about temp readings at the t.stat cover without actually taking a temp again on one of mine. I'm pretty sure, though, that the temp at the cover was cooler than the head surface temp when I measured (can't say for sure, offhand -- sorry). Just intellectually, I would think the temp at the t.stat should be a little cooler as cooling water is being released through the stat. Upon first opening, it's probably hotter because the water there was heated up in the engine prior to full circulation.

Easy to overthink. If it smells hot, buzzers going off, burns your hand -- it's too hot. Below that, "normal" probably varies with the motor and the environment.
 

horiconducker

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Well i decided to pull the powerhead this afternoon looking for a bad base gasket or something. What i found when i looked at the bottom of the powerhead was a nut wedged into where to water enters the the block into the exhaust cavity. The funny thing is i think it has been in there since the motor was manufactured in 1988 because the nut is to large to fit up the water tube,( yes i tried it to see if it fit.) must have been dropped in assembly and floating around in the small cavity in the adapter plate until last fall (when i started having overheating issues) when it probably got wedged in passage partially blocking water flow. thanks you all for all the helpful input and advice, this is a great forum. thanks ducker
 

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oldboat1

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clearly would have been our next guess....:rolleyes: Good sleuthing, ducker. Hope you report back after testing -- thumbs up.
 

horiconducker

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Well i finally got to get out on the water for a test, all is well no more overheating. At wot temp on top of thermo housing was around 110 and the block was around 140, much cooler than before. Water pressure is still 6-7psi at idle but now jumped up to 24-25 psi at WOT. Used to be 19-20psi at wot. thanks for all the help ducker
 
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