Checking free flow raw water cooling

[russ]

I am about to replace the thermostat on my 05, 5.7 Mercruiser and B3 drive due to an over heating problem last weekend. Once all the hoses are removed is there a way of checking that the hoses aren't blocked and the cause of the overheating? So silly or not but if I momentary turned over the motor with releasing individual hoses in sequence would this show if the pumps are working or blocked a hose?
Oh and the boat is still in the water!

 

[dpb]

If you disconnect the hoses that run from the thermostat to the exhaust , at the exhaust end, each in turn (some (1 liter) water will come out from the riser and the disconnected hose when you do this), hold the hose over a bucket and run the engine briefly. Water from the hose = water pump working. No water after say 10 seconds then pump not working. However, ,unless you have seen one running that you know is right you still wont know if what you see is normal flow. Also any restriction is likely to be in the manifold / riser anyway.
With the engine running and having warmed up, put your hands on the top of the exhaust elbows, You should be able to keep your hands on them easily. If they are too hot to do this then there is a flow restriction, and if the exhaust is more than 3 years old there is a good chance your problem lies there. Spliting the riser from the exhaust is easy enough to do to check on the condition inside, just make sure you drain the exhaust of water first (stop cock underneath) and have a new gasket for reassembly should you find that they are OK.

 

[russ]

Thanks dpb. Managed to change thermostat yesterday and temp is now running normally, a little over 175c. I believe the risers are still the 2005 originals. Should I be concerned?

 

[dpb]

Well it depends on the boats history, I was surprised to find that those on mine when I bought it were 16 years old, but one of them was to hot to touch and there were some signs of joints weeping externally so I replaced the entire exhaust system to be safe. Running temp fell from 175 to about 170.
If they are too hot to touch, joints weeping, or the engine is hard to start from cold after being stood for a few days, then takes a while to run smooth, then you should be investigating at the very least.

 

[russ]

Well it depends on the boats history, I was surprised to find that those on mine when I bought it were 16 years old, but one of them was to hot to touch and there were some signs of joints weeping externally so I replaced the entire exhaust system to be safe. Running temp fell from 175 to about 170.
If they are too hot to touch, joints weeping, or the engine is hard to start from cold after being stood for a few days, then takes a while to run smooth, then you should be investigating at the very least.

Top of risers were cold to touch at peak engine temp but the manifold below either side of the risers became hot on starboard side of the engine and a little cooler on port side manifold. Is this normal?