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I googled volvo 5.7 gxi overheats at low rpm and was stunned by the number of people with the same problem... my digging is a "solution search" but sharing my experience may (repeat may) help suggest some things that this problem is related to.
Last week I was about 20 miles away from home on USCG Aux patrol, patrolling a swim across the Great South Bay on the south shore of LI, NY - the reason I mention that is there were very long periods of idle or low RPM and also that the environment was quiet so I could hear the engines.
That said, I suddenly heard a metal to metal, high pitch sound from my starboard engine. I immediately shut it down and figured we could finish the patrol and then either lubricate something or make a run home with the noise.
Later when I started the engine the noise was gone and the engine ran fine. In fact, both engines ran fine and no overheating of any kind at high or low speed. We cruised home at about 35 to 40 MPH without incident. However, when I got home the port engine showed signs of leaking which I hoped was a loose hose. There was a fast drip of water out of the weep hole under the pump assembly - so no such luck on the loose hose. I took off the pump (these are easy to get to and easy to remove).
The ball bearing in the housing was rusted and the metal around the bearings was disintegrated and parts of it were all over the inside of the housing (photos available). The next day I went to my local mechanic and he ordered a new pump but I told him to hold the order because I thought it would be wise to check the starborad also since that is where I first heard the noise. I took one second to determine that it too had to be replaced because there was the same metal filings coming out of the weep hole as on the port pump. I ordered two (about $360 each) and painted (black where the hoses fit on) and the rest red (I already had the paint which I used for touch up). I installed both in less than two hours and started the engines and let them run for about 15 minutes until the temp came up to around 150 or so and then shut them down.
Today, I went on a family trip to a local cove (the first run with the new pumps) and when I got to the first 5MPH spot, after idling down the port engine sounder started beeping. 200-210-220 so I immediately put it in nutral and ran the revs' up to 1200 or so... it came right down (after 3 or 4 minutes) and at the end of the 5MPH area I went back up on plane with both engines and they sat at a perfect 175. There are three such 5MPH zones, each way, and the exact same thing happened all six times.
So now, if I haven't bored you to tears with my story (laying out the facts) here are my observatiosn...
1 - there was no overheating at any speed (RPM) at any time prior to replacing the pumps due to bad berrings.
2 - both engines had the same problem at about the same time - I mention this because it is important to note that my boat only has about 150 hours in 5 years and I leave it in the water all year - I pull it once per year for bottom paint and outdrive maintenance then back in she goes. Long idle periods could account for rusty bearings.
3 - both new pumps were from the same dealer, had the same p/n and the same look/feel.
This immediately points to the new pump being bad and the only post that seems to relate to this problem is the one that says that an impeller can spin on its shaft at low speeds but work properly at high speeds.
My dealer suggested that I swap the pump and see if it follows the new part of the engine which I will do and re-post the results.
The only thing I will add is that you may wonder how I knew to do the "neutral - 1200 RPM thing" to cool it down.. that is because when I first got the boat this problem was driving me nuts but turned out to be air sucking into the fresh water wash fittings. I tightened everything and the problem was solved. I never got near this area when I changed the pumps and did "glance" at the fittings today and did not see anything unusual...
So my actions this week will be to swap the pumps - which will clearly lead to some conclusion and also re-inspect the fresh water fittings just in case, coincidently, something failed in that area. Then I will repost (unless someone has an answer based on the above).
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