Another overheating thread - Please help - Intermittent/Random!

[rmurray]

2003 Volvo Penta 5.0 GXi tied to SX drive. Haven't had any issues in a while. Replaced impeller in July. Shortly after, had it out and it overheated after driving for a while. Alarm went off, slowed to idle speed and it started to cool down. Another ~30 minutes of boating and overheated again. Slowed and cooled. Then, no overheating for a while, until later, overheated again. Gets to roughly 180-200 tops. Pulled thermostat and boat ran cool all weekend, no problem. Temp never even registered on gauge, so running at <120 with no thermostat. Eventually installed new OEM thermostat and same overheating scenarios again. It will overheat, but randomly and after long stretches. Thinking maybe the new impeller is bad. I ordered off Amazon, OEM VP part, shipped in factory OEM box with o-ring and lube. Looks legit, but is it possible its a knock-off? I know knock-off impellers are no good. I'm thinking I should just buy another factory impeller from a marine store I trust, but before I spend another $60, I was wondering if this scenario points to something else. I just find it odd that it will run great for long stretches, and then randomly overheat. Anytime I've changed an impeller, the previous one has been completely in tact, so I don't believe there are pieces of rubber impeller in any of my hoses. Could it be my risers? boat has 350 hours with probably <15 total hours in salt water. Always thoroughly flushed with salt away after brief salt water use. Thanks!

 

[kenny nunez]

Is this fresh water cooled or raw water cooled. If it is raw water cooled remove the 2 hoses from the thermostat housing that go to the exhaust manifolds. Plug your garden hose in either hose then let the water full flow through. Do this on both, there should not be any restriction. If one looks like the flow is restricted then you know where the problem is.
On the discharge hose from the raw water pump to the thermostat housing add a length of clear hose, if there is a head gasket leak you will see the water disappear as the combustion gasses will overcome the raw water pump’s discharge pressure.

 

[ESGWheel]

I have not experienced a difference between OEM and Sierra branded impellers, other brands I have not tried / trusted.

Adding to the idea of using a garden hose (if raw water cooled) into the hoses that go into the manifolds that kenny nunez is suggesting > the concept is there is a restriction in one or both of the manifolds / risers that is just enough that – with the T-stat installed – is causing the overheat issue. Remove the t-stat and that removes a “built in” restriction on the pump that is allowing it to overcome the restriction in the manifold / riser. In other words, you are on the cusp so to speak. If, when pressing the garden hose against the manifold hose there is a squirt of water spraying out between the two, like holding your thumb over the garden hose, then a restriction.

If there is no squirt and the clear hose does not show any head gasket issue, suspect that a restriction in the manifold / riser may still be the issue (it does not take much restriction to cause the 180 to 200 you are seeing). Thus, suggest get a gasket set and pull off the risers. Its not expensive and its easy to do. Be sensitive to the retorque requirements when putting them back on.

Have the manifolds / risers ever been replaced? Even in FW 20 years is a lot.