70hp Johnson kicking our butts...overheating I think

[turbo4door]

Hi,

Desperately looking for some help with our 89 Johnson 70hp.
It came on a 89 Basstracker 17 that we purchased a few months ago and we have been having nothing but trouble with it. It will run fine at idle, but under any sort of load (say, 3000rpm or above) it will run for about 3 minutes and then enter SLOW mode. Have noticed some steam out the top exhaust ports on the back of the motor when this happens.

Here is what we have done so far:
1. Cleaned thermostat (white chunks around it, see next item)
2. Cleaned water jacket on port side of motor. Removed plate where tell-tale nipple was (before relocating it) and cleaned/replaced gasket. Was completely full of scale and nasty chunky white stuff but is now clean.
3. Replaced water pump
4. Relocated tell-tale nipple to top of block.
5. Has newer thermostat spring (updated part, came with water pump kit)
6. Updated temp sensor

Now, the motor has no alarm on it thanks to previous owner, so I'm not sure what tones it is generating (is there a way to hook up something to tell us, say...an alarm horn of another type...12v maybe?)
I really thought we had trapped air in the tell-tale stream but since relocating it today it has been peeing very strong. It would stop peeing before relocating when going into SLOW mode. Will a lean running condition cause this? It happens at the exact same time every time we test. We can put in, get out of manatee zone and throttle up and it dies in front of the exact same dock every time, so I know we haven't made any progress with it.

Not really sure where to go from here? Are there other areas of the cooling system that we need to look into cleaning? Looking for any ideas at this point. Appreciate any help, we are down-and-out over this...just want to be able to enjoy the boat.

 

[Faztbullet]

Re: 70hp Johnson kicking our butts...overheating I think

Just because its peeing strong doesnt mean its cooling correctly... take the t-stat apart and test the vernatherm or replace it as a t-stat assembly. If this doesnt correct it pull the head and check bottom water port on head as they could be stopped up restricting water flow thru head..

 

[WernerF]

Re: 70hp Johnson kicking our butts...overheating I think

What about the original warning horn in the control box? You're lucky that the power pack is a SLOW mode type (the 1988 70hp model is not). Otherwise your engine would be toast now. So get the horn working again.
The thermostat is most likely 23 years old. Replace it in any case. It's hard to measure out if the t-stat's regulating temperature has drifted away.

Edit:
Is this the same engine as in scsdiver's thread? Exactly the same description except the thermostat/temp sensor replacement. If so, you need to verify the engine's temerature first.

 

[scsdiver]

Re: 70hp Johnson kicking our butts...overheating I think

sorry guys we did not chat with each other first and apparenlty posted about the same time. i replaced the thermostat about a month or two ago so that has already been done. The only reason i dont think either of us thought to check the actual temp is that there is steam coming out of the top exhaust ports which we assumed meant boiling water somewhere.

Faztbullet we were pretty supprised at the results vinegar had on the motor under the water cover would you recommend pulling the head and cleaning it or would you think we could run it in a tank filled with vinegar and be able to get around pulling the head. obviously pulling the head has its complications and im not so opposed to it but there is places we cannot get to even removed the head i would imagine which is why i was asking about doing the vinegar tank.

 

[WernerF]

Re: 70hp Johnson kicking our butts...overheating I think

Some steam at slow speed (not too much) out of the exhaust relief ports can be normal. It depends on the weather (cold & humid) and the exhaust temperature.