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  1. #1

    Default 1989 70HP johnson vro overheating

    I relocated the telltale to top of the block according to the service bulletin. Recently installed a new water pump, new thermostat. Ran the engine on muffs, after 2 min my temp sender shows 190F so I turn the engine off. I don't have a temp gun but the block really felt hot to the touch, a three count was impossible. Pee stream strong. I took the power head off and everything looked normal so I decided to take the exhaust manifold plate of. Same thing here nothing that made me react, I expected to at least find some kind of blockage.

    What is the next step?

  2. #2
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    Default Re: 1989 70HP johnson vro overheating

    Compression test and/or leakdown test

  3. #3
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    Default Re: 1989 70HP johnson vro overheating

    Run it in a barrel and do away with muffs....
    Pappys Sales and Service
    Owner and Factory Certified Technician
    Former racer KDBA (Kentucky Drag Boat Association)

  4. #4
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    Default Re: 1989 70HP johnson vro overheating

    Recently had the same experience with my 90 after similar work. Mine has two thermostats, one bank was cool but the other hot enough to activate the overheat horn. I removed the return hose on the suspect side and tried it again. Plenty of water flow?? I could see the thermostat in the housing and used a screwdriver to depress the spring thinking I may have got some gasket sealer on it when I installed them and it was stuck closed. (hopefully it wasn't bad) It seemed to operate as it should so I replaced the return hose and fired it up. Haven't had an issue with it since. Just an experience that might help.

  5. #5
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    Default Re: 1989 70HP johnson vro overheating

    Quote Originally Posted by faztbullet View Post
    Run it in a barrel and do away with muffs....
    This. Muffs are fine for fogging to winterize but testing like this requires a barrel.

  6. #6

    Default Re: 1989 70HP johnson vro overheating

    My mechanic performed a test, not sure if it was a leakdown. It stated: "Leakage upper crankshaft seal as well as (certain) leakage crankcase halves". I don't have the proper tools for performing a leakdown test and my mechanic lives far away from me.

  7. #7
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    Jun 2013
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    Default Re: 1989 70HP johnson vro overheating

    Were there any symptoms before the water pump and t-stat replacement? If so, what were the symptoms? Was the old impeller missing any parts? I ALWAYS check for blockages or leaks in the water pump to powerhead supply tube with LOW pressure air. There are more effective ways to check for blockages than powerhead removal, etc. Perhaps you expected to find something? That leads me to believe that the impeller was breaking apart.
    Last edited by timguy; 07-20-2018 at 09:16 PM.

  8. #8

    Default Re: 1989 70HP johnson vro overheating

    Quote Originally Posted by timguy View Post
    Were there any symptoms before the water pump and t-stat replacement? If so, what were the symptoms? Was the old impeller missing any parts? I ALWAYS check for blockages or leaks in the water pump to powerhead supply tube with LOW pressure air. There are more effective ways to check for blockages than powerhead removal, etc. Perhaps you expected to find something? That leads me to believe that the impeller was breaking apart.
    Before the replacements I noticed higher than normal operating temperatures, still nothing that triggered the SLOW mode. My problems started happening after I was fishing really shallow and muddy waters (2-3 feet). I pulled the head (not the powerhead as I stated earlier). Does anybody know where the cold water enters the powerhead. According to the water flow diagram it's on the exhaust side but I can't find the entrance hole on the engine. I guess I could find it if i take the lower unit off.

  9. #9
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    Default Re: 1989 70HP johnson vro overheating

    The powerhead is fed water via the waterpump through a water tube,all this can be observed with lowerunit removed

  10. #10

    Default Re: 1989 70HP johnson vro overheating

    Installed the new head gasket with a torque wrench according to the manual. I ran the engine in a barrel with water level high above the water pump. Operating temperature at idle (neutral) was 176 degrees fahrenheit.

    Compression test top 121 psi, middle 119 psi, down 120.

    When I took the head gasket off I recall the bottom right screw being wet (water).

    Still I think the operating temperature is on the high side?

    The old impeller was in one piece and in good shape.

    I am going to do another testrun in a barrel and check the engine temp with a temp gun.
    Last edited by gabbu; 07-28-2018 at 05:16 PM.

  11. #11
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    Default Re: 1989 70HP johnson vro overheating

    Okay, before you were at 190, now 176, it's better now. The sediment is slowly working itself out of the motor. Try force feeding it on the muffs and that's all you might need to clean it up, keep it at about 2000 rpm and monitor temp, of course. I have a good feeling that it should correct itself.

  12. #12

    Default Re: 1989 70HP johnson vro overheating

    Quote Originally Posted by timguy View Post
    Okay, before you were at 190, now 176, it's better now. The sediment is slowly working itself out of the motor. Try force feeding it on the muffs and that's all you might need to clean it up, keep it at about 2000 rpm and monitor temp, of course. I have a good feeling that it should correct itself.
    It's definetely going in the right direction I test ran the engine again. Discovered a leaking head gasket (lowest part). I did resurface the head by myself, but not well enough, will leave the head (next week) for resurfacing in a local marine shop. You can tell by the picture cylinder number 3 (lowest cylinder) is almost 5 degrees celsius warmer than the others.
    Click image for larger version. 

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    I am hoping this will be the final piece of the puzzle. People are telling me all the time to buy a new engine, but I am not letting this one down.
    Last edited by gabbu; 07-29-2018 at 10:52 AM.

  13. #13
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    Default Re: 1989 70HP johnson vro overheating

    What method did you use to recondition head surface

  14. #14

    Default Re: 1989 70HP johnson vro overheating

    Quote Originally Posted by johnnygjr View Post
    What method did you use to recondition head surface
    360 grit sandpaper, figure 8 motion. Didn't do it long enough apparently, so I left the head at a machine shop for resurfacing. If I am lucky I'll receive it tomorrow. Should the gasket be installed completely dry? I've read it a hundred times (dry head gasket) but the mechanic said it could be installed with a sealer, just to be on the safe side. However, I'll do another test run in barrel and hoping for a big improvement.

  15. #15
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    Default Re: 1989 70HP johnson vro overheating

    Iv'e used sealer on all my head gaskets,even though some of the newer gaskets come with sealer built in

  16. #16

    Default Re: 1989 70HP johnson vro overheating

    Testrun made in barrel after head was resurfaced by machine shop. Put the old head gasket with hylomar, since I couldn't wait for the new gasket to arrive. Temp
    stayed almost the same, maybe one degree less on each cylinder. Idled in neutral, Top 167F, middle 167 F, bottom 180F.

    Took it out on the lake, temperature exceeded 190F so I shut it off. 4000 rpm was the hottest (190F), full throttle 6000 rpm (180F), idle in neutral 1000rpm 176F.

    Do you guys have any idea what to do next, except installing the new head gasket.

    The telltale was not warm at all.
    During the test run in barrel I noticed water coming out in only one of the two exhaust relief valves. After a couple of minutes water came out of both holes. Don't know what that means, though.
    Last edited by gabbu; 07-31-2018 at 02:44 PM.

  17. #17
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    Default Re: 1989 70HP johnson vro overheating

    The problem started after running in muddy, shallow water, right? That leads the average person to think that there is a circulation restriction. Could be where the input water tube enters the powerhead. Did you check that as you were replacing the water pump impeller? Make sure the inlet to pump is clear too.

  18. #18
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    Default Re: 1989 70HP johnson vro overheating

    Pull off thermostat housings place l/u in a barrel,fire it up breifly,water should gush out of the housings

  19. #19

    Default Re: 1989 70HP johnson vro overheating

    Quote Originally Posted by johnnygjr View Post
    Pull off thermostat housings place l/u in a barrel,fire it up breifly,water should gush out of the housings
    Ran the engine without thermostat housing and water was gushing out. I measured the temperature (with my neighbours heatgun) on top of the thermostat and it showed 140F. Managed to hold the hand for three seconds on the head. On sunday I'll take the boat for a longer run. Hope it will stay cool.

    I have only been idling in neutral. Not possible to run the engine in forward/reverse gear or all the water would splash out of the barrel.

  20. #20

    Default Re: 1989 70HP johnson vro overheating

    Click image for larger version. 

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    According to the manual, the heat check is performed on top of the thermostat housing. 163F (73C) is considered overheating. I think the numbers measured on top of the thermostat housing are below that, even though I seem to have a hot spot near that upper limit.
    My temp gauge was showing 176F (80C) when these pictures were taken. I can perform a three count on the head and a five count on the thermostat housing.
    These tests were performed when idling in neutral, I am not really sure how the cooling system will perform under other circumstances. Testing it on sunday.

  21. #21
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    Default Re: 1989 70HP johnson vro overheating

    Nice photos, brother. Remember, the water in the tub heats up, It will cause higher readings. The lake or ocean temp rarely exceed 80 degrees. Your garden hose, of course....way less. In a closed cooling system like a car, for instance, it once heats up, then remains stabil. Not so in a tub or barrel, besides that, the exhaust passes through the water, further heating it. Try your heat tests at the dock for more accurate results. Good job.

  22. #22

    Default Re: 1989 70HP johnson vro overheating

    I have performed 4 testruns on water. The purpose with my latest one was to find out my WOT rpm, which was 5790 rpm with a 14x17 aluminium prop. I took out my 24V battery bank (64kg) and my terrova 80lbs (35kg).

    During high rpms the cooling system is performing really well and this is when the temp is at it's lowest: 75C. Idling is 82C and cruising is approximately 88C. This is with one passenger and no extra weight (batteries+electrical motor). Bare in mind that the readings are approximately 20C lower on my thermostat housing.

    Thanks for all the help guys! After 4 testruns with no further issues I feel much more confident. The engine is firing up at first crank, and idling nicely at 800rpms. In the close future I will replace all the fuel lines, remove the inline filter (since I already have a fuel filter with water seperator) and remove the carburetors for a rebuild. I noticed one on the carbs are leaking, when the engine is fully tilted and fuel line removed, which makes me suspect a bad gasket or float not closing the valve completely.

  23. #23

    Default Re: 1989 70HP johnson vro overheating

    The purpose with my latest one was to find out my WOT rpm, which was 5790 rpm with a 14x17 aluminium prop.

    During high rpms the cooling system is performing really well and this is when the temp is at it's lowest: 75C. Idling is 82C and cruising is approximately 88C. This is with one passenger and no extra weight (batteries+electrical motor). Bare in mind that the readings are approximately 20C lower on my thermostat housing.

    Thanks for all the help guys! After 4 testruns with no further issues I feel much more confident. The engine is firing up at first crank, and idling nicely at 800rpms. In the close future I will replace all the fuel lines, remove the inline filter (since I already have a fuel filter with water seperator) and remove the carburetors for a rebuild. I noticed one on the carbs is leaking, when the engine is fully tilted and the fuel line removed, which makes me suspect a bad gasket or float not closing the valve completely.

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