300HP Johnson

[IFLY]

I have a cooling problem with my 1989 300hp Johnson. I have traced it down to exhaust or air getting into the water on the starboard cylinder bank. Any help or info about this problem would be appreciated.

 

[Dhadley]

Re: 300HP Johnson

Double neat motor! If you are sure you are getting exhaust in the water, then remove the head (I would do both heads) and have them surfaced. Do not cut them. Check the block surface for erosion and straightness with a straight edge. Replace the heads with new gaskets. <br /><br />What thermoststs and spark plugs? There are a couple of other things but lets do the basics first.<br /><br />When you get the heads off let me know what you find before you put them back on.<br /><br />Good luck!

 

[IFLY]

Re: 300HP Johnson

Is the head gasket the only way for exhaust to enter the cooling water? Do you know if these are aluminum heads and prone to warping?

 

[CAPTBLACKSMARINE]

Re: 300HP Johnson

yes they are aluminum heads , but i do not know of any head warpage problems with them , if you pull them be sure and torque them properly and in the right order when you go back on with them . if it were me i would be looking into the exhaust covers and gaskets if your motor has the removable ones .

 

[almost retired again]

Re: 300HP Johnson

Hi Jonp,<br />One question; How do you know that you are getting exaust in the stbd. cooling water? What are the symptoms that make you think exaust gasses are entering?

 

[novacane]

Re: 300HP Johnson

yeah how you know this???

 

[IFLY]

Re: 300HP Johnson

I removed the thermostats and observed the water flow comming out of the block to the stat. The port side has a steady stream but the starboard side has aerated water, either exhaust or air. I removed the lower unit, put a hose on the tube going to the engine and ran it again with the same result. I held a clear hose to the starboard thermostat seat and observed aerated water comming out. Somewhere i am getting air or exhaust into the cooling water going into the powerhead. ??

 

[IFLY]

Re: 300HP Johnson

I also checked the compression on all 8 cylinders and found them all to be 100psi. Does this sound correct? I did this just to see if there was a leak in the head gaskets.

 

[almost retired again]

Re: 300HP Johnson

Hi Jonp,<br />I never heard of anyone doing this that's why I asked,it does sound funny that you are getting air in one side and not the other. I don't think that there is only one place that air can enter considering the test that you have done,I would pull off the exaust manifold and check for cracks. I suppose that you could have a warped head not letting the head gasket seat, but that's all that I can think of. Let us know what you find; interesting problem.

 

[Dhadley]

Re: 300HP Johnson

Go ahead and pull the heads and let us know what you find. I've seen things similar before so it may not be that much of a mystery. We do know that all E/J loopers trap air on the starboard side to some degree. <br /><br />Let us know!

 

[IFLY]

Re: 300HP Johnson

Thanks for all the input. What can I expect to see if I pull the heads? Would the compression check not find a leak in the head gasket?

 

[IFLY]

Re: 300HP Johnson

Well I finally found the problem. I pulled the starboard cylinder head and found the head corroded from the cooling water side just through the gasket. The port side was in good condition. I am now looking for a new head. Does anyone know about the new head conversion kits that are out there? Thanks again for the info I have received.