89Johnson 150 Overheat

[Hekoa]

Had boat out and overheat alarm sounded. Replaced both thermostats and impeller, flushed out system. And reassembled. Took it out for a trial and after about 30 minutes running at 5000 rpm the alarm sounded again. Checked head temps and port side was 152 and the starboard side was 223. Pulled head covers expecting to find restrictions but there were none. Compression test revealed all cylinders equal in compression (within 5 lbs). Any ideas what may be causing the overheat? Thanks for any help.

 

[Bosunsmate]

Re: 89Johnson 150 Overheat

It might be a failed or swelled water deflector.
3 per head

 

[Will Bark]

Re: 89Johnson 150 Overheat

Did you also replace the poppet relief valves; that one may be sticking and not opening up at WOT

 

[Hekoa]

Re: 89Johnson 150 Overheat

It might be a failed or swelled water deflector.
3 per head

How can I determine if they're failed or swelled? There are three and it looks to me like they completely block the flow of water.

 

[Hekoa]

Re: 89Johnson 150 Overheat

Did you also replace the poppet relief valves; that one may be sticking and not opening up at WOT

I did replace the poppet valve. Thanks for the reply.

 

[Bosunsmate]

Re: 89Johnson 150 Overheat

How can I determine if they're failed or swelled? There are three and it looks to me like they completely block the flow of water.

Post a photo. They a meant to block the flow in one direction but not others

 

[emdsapmgr]

Re: 89Johnson 150 Overheat

Here is a pic of how a properly-installed deflector should be positioned.

 

[Hekoa]

Re: 89Johnson 150 Overheat

Thanks to all who posted. It was the deflectors...two of them had completely closed off circulation to one side of the bottom two cylinders.