1988 70h johonson chronic overheating problem

[horiconducker]

Ive got a 1988 70hp johnson that I have had a overheating problem that has me stumped. I have owned the boat for 5 or 6 years with no cooling issues and have done regular (3 years) water pump services. Motor can idle all day and stay around 140-155 deg. As soon as i increase rpms to planing speed temps start to rise 160-180 i try to shut it down then. (verified temps with IR temp gun) otherwise overheat alarm will sound shortly if i don't stopI. have installed a new complete brp water pump kit and thermostat. I did all the cooling system updates with the water pump kit, including new thermostat spring, and relocated tell tale to the top of the block also removing tell tale restrictor. I have removed the head to check for any blockages and build up, found none( freshwater motor). I installed a water pressure gauge and get 6-7 psi at idle and around 19 psi at WOT. Today i took the boat out and pulled the thermostat completely out and took it for a ride at 4k rpm temp stayed nice and cool around 110-115 deg with no thermostat, which leads me to believe poppet valve isn't opening properly, but its all new parts. Im not really sure what to do next, bad base gasket or upper water tube grommet? thanks ducker

 

[oldboat1]

Sounds like a t.stat assembly issue, or a bad-from-the-box unit. Should be a 140F thermostat. (Is the vernatherm sensor end pointing toward the engine?)

 

[horiconducker]

The new thermostat I bought today and the 5 year old one both seem to work properly controlling temps at idle, they just don't seem to push off the seat far enough at higher speeds. The cone shaped part of thermostat or vernatherm seats on the rubber valve seat in the head I believe that is the correct way

 

[oldboat1]

sensor would be the flat part sort of like the eraser end of a pencil.

 

[horiconducker]

In this picture should the top pointed part of the thermostat be toward engine or t-stat cover. I have it installed with the top pointed part to engine ,and the bottom threaded end to the thermostat cover, do i have it installed wrong?

 

[oldboat1]

That would be correct -- bullet shaped part toward the engine. If I understand the assembly correctly, the vernatherm would actually be inside that thermostat assembly, with a spring.

 

[horiconducker]

yep its inside

 

[oldboat1]

parts diagram you have probably already seen: http://www.iboats.com/Cylinder-Crank...iew_id.1552892

[ed. might dunk it in hot water and check for movement]

 

[horiconducker]

Oh yeah, have studied the diagram much, the original thermostat assembly (that i threw out 5 years ago) could be taken apart. The new superceded part number seems to not screw apart, just one piece, i had to replace the entire assembly.

 

[oldboat1]

Ouch! Not exactly progress. Hope you get it working. (Might want to try some flushing while the thermostat is out. Freshwater, but still could be some sand or silt in one of the passages. Flush down from the t.stat opening.)

[i.e., not technological progress in my view....]

 

[horiconducker]

I had the head off and it was all clean. I ran water and compressed air bottom to top and top to bottom while head was off. Today I started the motor with the t-stat cover off and at 2k water was gushing out in a 1 ft long stream out of the housing.

 

[oldboat1]

dunno what to tell you that you haven't thought of. Does seem to be thermostat-related.

 

[James R]

It may be dirty carbs and it's running lean. When were the carbs last rebuilt.

 

[flyingscott]

What spring did you put it in. Was it the cone shaped one or the straight one. I don't remember which one but one of them gets a different thermostat housing cover should have came with the kit. Something you might want to double check. Another thing how many vanes were on the waterpump was it a 3 vane.

 

[horiconducker]

I put straight red spring in. The cone shaped spring gets the new cover. At the lake yesterday I was trying everything cone shaped spring won't fit. No it's a 5 or 6 vane impeller.

 

[flyingscott]

Did you pull the impeller out to look at the key some of those impellers had problems with stripping out the key

 

[horiconducker]

Yes i have had problems stripping out the key with aftermarket kits, but not with the BRP kit, key on brp kit seems to be a little thicker in the brp kit

 

[horiconducker]

Something I noticed when I installed the thermostat with the head off was that the thermostat can only push back around 1/8 in before the threaded end of the thermostat hit the thermostat cover. Knowing that today I took my old thermostat and ground 1/8 of an inch off of the threaded end of the thermostat so it can push back 1/4 in but it still runs hot

 

[flyingscott]

The thermostat should only move about an 1/8" you do not want it pushed all the way because it still needs to regulate temp. By letting it move that far you are trying to bypass it completely. It still sounds like you have a flow problem try replacing the impeller with the old one because even though it's rare I have had the spin the hubs.

 

[horiconducker]

Wouldn't I see that in my water pressure gauge then? I get 6 psi at idle and 19 psi at wot. It also runs nice and cool 110-120 with Tstat assembly completely removed.