Mercruiser overheating- How to check raw water pump flow rate

I have an overheating issue on a 1977 merc 140hp. I replaced the raw water pump but still am having problems. I want to confirm that the water pump is delivering proper flow after my replacement.

How much flow rate should i see at the t-stat intake hose? I definitely see flow when I run at 2000 rpm but I am not sure if it is sufficient. Trying to rule this out before replacing the manifold/inspecting the exhaust shutter.

One other piece of info. The engine runs cool if I hook a hose directly to mufflers on the lower unit, hot if I run from tub of water.

Greg

I seem to remember 20 seconds to fill a gallon jug...not sure. It runs cool on a cold garden hose because the heated cooling water and hot exhaust enter the tub of water. Insert a 1-2' section of clear water hose between the T'stat and the water inlet hose and look for air bubbles. If you see them then exhaust is entering the water stream at the impeller. Use a laser thermal temp. gun to check temps on the water inlet, T'stat hsg., exhaust manifold and riser. How old is the manifold? Any rustcicles on it?

it sounds like the impeller is damaged, the water pressure on muffs is forcing water in keeping it cool, the tub method, water level 6 inches above the cavitation plate is the bare minimum for testing in a tub.

Pull the riser from the manifold.
Look at the sea water transfer ports to see if these are closed down from rust scale.
You may have good flow from the sea water pump, but if these ports are restricting flow, it will cause issues no matter how well the sea water pump is working.

Often these ports can be cleaned up with a round file or small tools, and made to be functional again.

If the mating surfaces are questionable, a good machinist can dress these up via a surface sanding machine.
New gaskets, and you're on your way.

Here's a good example of restricted transfer portsm, and a mating surface that has caused the gasket to breach..... or a gasket that has breached the mating surface.
Chicken or the Egg???? You choose!





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I just went through all this on my engine. It turned out to be a melted and scored housing on raw water pump. I replaced impellar without looking carefully at the housing. You will get water flow even with a bad pump and or housing (depending how damaged it is) but it might not be supplying enough to cool engine properly. My engine would run to almost 200 degrees at idle with a brand new thermostat within say 10-15 minutes. If I throttled up to say 1500-2000 rpm on trailer with muffs it would drop down to like 175 degrees. However, you should check the condition of your exaust shutter to see if it is melted maybe blocking exaust gases which could make engine run hot. See what it looks like and check your raw water pump real good. See if any blades are missing from impellar. If so, check water pickup tube on upper unit for debris or blockage. Maybe run a hose from heat exchanger supply hose backwards toward the drive in hopes of pushing any blockage towards the drive. (of course with the lower unit removed already) I had a few chunks from my impellar at the entrace side of heat exchanger that I had to remove. If you ran the engine without enough water height in the tub, it is possible that you may have cooked impellar and exaust water shutters. Maybe check power steering cooler for pieces of impellar if indeed you have power steering. Just a thought. I am not too familiar with the 140 engine so you might have a problem with your water pump and or thermostat.(could also be your risers or exaust manifolds but try the easier things first) (go that route last after checking all the other stuff) I am assuming you have a closed cooling system with antifreeze. It sounds like the impellar is weak if the engine runs cool on the hose. But you have to understand that the hose pressure is forcing water up through the drive and into your raw water cooling system so it does not give you any information. Did you check supply line from raw water pump to heat exchanger? On my old damaged pump, I had a soft flow of water at idle. With the new oem water pump kit with the lower housing soup to nuts the flow rate at idle was significantly greater. It was basically obvious at that point that my raw water pump was weak and not able to produce enough water flow to cool system. At first,
I tried the easy route by replacing just the impellar on the raw water pump. That attempt failed miserably and I ended up back at square one with little or no information as to solve the problem. Had to tear down lower unit again, install new oem raw water pump kit, put everything back together again, fill with drive back up with oil, etc.,etc. My point: Once is bad enough so be be diligent,thorough,know what to look for (manual helps), and sytematically take one particular part of the cooling system at a time making sure all is well and good before reinstalling anything! Just saves for alot of bother and frustration. You'll figure it out one way or another. best of luck, Tom