Re: Water pressure test.
First of all, did we determine that you engine was overheating at all speeds or just idle or just WOT.
If at idle only then the first thing we want to do is make sure it's not running lean. The same goes for WOT. Your plugs will be your test gauge and after idling for a few minutes the plugs should appear somewhat oily.
At WOT the plugs should appear coffee tan colored, not white and clean or melted. This check is done by running on the water (not hose) at WOT for about a minute then turning the key off and pulling a plug for inspection.
If that's already been tested then install the water pressure gauge so that it tee's in at tube that runs from the water outlet fitting at the top of the block to the poppet valve.
You can do some testing on the hose if you have normal pressure and volume of around 5gpm. I bought a pasture trough that's deep enough to sumberge my lower unit up to the bottom of the cav plate and I do all my land based testing in it. It's a relatively inexpensive investment and works well enough that I can even set in-gear idle speed.
With the engine running at idle speed of approx 800 rpm in neutral you should show some to 2lbs of pressure on the hose and 2-4 lbs in the water.
At 2000 RPM you should show 6-8 lbs on the hose and 10-12 lbs in the water. On the hose the poppet probably won't open at 2000 RPM but in the water it should start.
If your water pressure is not meeting those minimums then the problem is either water pump or poppet valve. It is not thermostats because they do not pass enough water to significantly effect volume and pressure readings.
I go to the poppet first since you can continue your running testing. I remove the poppet and look at the block insert and rubber grommet to see if they are still there. If they are then I remove the poppet from it's carrier and put the covers back on.
I start the engine and I should have zero water pressure at idle and 5-6lbs, at 2000 RPM and temps should not be exceeding 135 degrees. This test is simulating what a motors temp is when the poppet and water pump are working properly. Since we know the poppet is working a overheat during this test is going to either be a bad waterpump or a blockage.
If this test proves satisfactory I reinstall the poppet since these rarely fail and move to the thermostats. I remove both thermostats and test them for opening at the proper temperature. With the thermostats out but the covers re-installed, I run the engine at idle and check the temp. If the temp rises above 145-150 degrees then I either have a blockage or a bad waterpump. If the thermostats tested good I put them back in or I replace them depending upon physical condition.
Now I pull the lower unit and check the waterpump housing for melting, warpage and just general damage. If I am suspecting water pump problems that I don't feel are regular maintenance related I always spend the extra money and buy the complete kit with base.
Once repaired I reinstall and test. If everything is back to normal then the waterpump was the problem. If not, then there is a blockage somewhere.
The first place to look is the heads. Pull each head and look for restrictions in the head, at the top and bottom of each bank and around the cylinders. Note that some merc's have diverters consiting of rubber tubing between the block and cylinder wall. These are supposed to be there and should be replaced if they have gone flat. Not all motors have or require these.
If there is no blockage found then pull the exhaust divider cover and look for restrictions and blockages. If you can't find anything wrong you are going to need to pull the powerhead to continue trouble shooting.
Hope this helps.
Steve
