Mercury 20 no water flow

Author Message   Greg H. Visitor Posted on Saturday, May 10, 2008 - 07:58 pm:    Hi all, I recently picked up a 97 20HP Mercury O/B. It appears to be in very good shape but it is not pumping water through the port up by the power head. I replaced the water pump and had no improvement. Now I don't know where else to look. The exhaust is coming out through the water inlet. I have it completely submerged in a large bucket. I have also blown air through the water port and the air comes out the lower unit, so there does not seem to be a blockage above the pump.   Graham Lamb Senior Member Username: galamb Post Number: 3159 Registered: 05-2007 Posted on Sunday, May 11, 2008 - 08:29 am:    Greg, you are going to have to pull the lower unit again and give it a good once over. Recheck the gasket/seal on the water pump cover. Make sure the impeller key didn't slip during re-assembly. See if you can jury rig a garden hose to the water tube and check for water flow. If the motor has been run in salt water the water tube maybe rotted/have a hole or the water jacket in the powerhead could be full of build-up. Even running is silty water can clog the powerhead. Remove the thermostat and after inspecting/testing it, shine a flashlight in the hole - you can see a fair bit of the water jacket from there....   Greg H. Visitor Posted on Sunday, May 11, 2008 - 08:40 am:    Thanks for the tips. I'll work on this today. Greg   Greg H. Visitor Posted on Sunday, May 11, 2008 - 10:49 am:    Well I pulled the lower unit and there is definitely a blockage. If I run the hose up into the pump tube I get nothing out. If I run the hose into the sight tube I get flow through everywhere except the pump tube. I ran a 1/4" dowel up the tube and it goes about 13" and hits a hard stop. I don't know what to do next and don't have a manual for this motor. Any ideas? I'm not sure where the thermostat is if you can describe to me that would be helpful. Thanks, Greg   Greg H. Visitor Posted on Sunday, May 11, 2008 - 04:05 pm:    I went a little overboard and had the power head nearly off. There did not seem to be a blockage below the engine block. I found the t-stat and it may have been frozen up. I got it working and water is now flowing. I'm not real sure where the issue was, and still think the flow is weak. So, if anyone can explain how the thermostat manages temperature that would be helpful. I know it opens when the engine is up to temperature, but how much flow should I be getting when cold? More when warm? Thanks, Greg   Graham Lamb Senior Member Username: galamb Post Number: 3164 Registered: 05-2007 Posted on Sunday, May 11, 2008 - 04:10 pm:    Works just like the one in your car. When the motor heats up to operating temp, the thermostat opens to let water pass through. If you had to play with it at all to work, take 20 bucks down to your local Merc dealer and get another one. That's a whole lot cheaper than an new powerhead....   Greg H. Visitor Posted on Monday, May 12, 2008 - 07:47 am:    Thanks for the help. I will replace the t-stat (they were closed Sunday). But please help me with the amount of flow I should expect. Usually I see a steady jet stream of water from an engine. This one seems a little weak and I was getting nothing with a frozen t-stat. Should water still get through the sight port with a bad/closed t-stat? Is it possibly still plugged from another direction? Thanks, Greg

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