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Overheat at low rpm
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Author:  Heathryan [ Tue Jun 15, 2010 8:19 pm ]
Post subject:  Overheat at low rpm
Anyone have a similar issue? I have a 2002 348 and one of the Volvo Penta's (5.7 GXi) is overheating but only at idle (below 1100 rpm). When I throttle above 1100 rpm the temp comes back down to a normal operating range. We have checked the impeller and confirmed a good water flow to heat exchanger. Any help on troubleshooting would be greatly appreciated. Thanks, Heath
Author:  txvista [ Wed Jun 16, 2010 1:59 pm ]
Post subject:  Re: Overheat at low rpm
Is it happening all the time? I had something similar on my starboard engine, and turned out to be a bag stuck to my intake. Once removed, all was good.
Author:  billcruise [ Wed Jun 16, 2010 2:40 pm ]
Post subject:  Re: Overheat at low rpm
I am having the same problem with both engines 5.7gxi but I my engines are raw water cooled (no heat exchanger). I've changed thermostats, impellers, inspected the thermostat housings, looked for 'old impeller debris' in the hoses and I'm a bit baffled. What about the manifold risers? Could they be corroded and reducing flow? Anyone else experiencing this problem? It seems to be slowly and progressively getting worse.
Author:  LazyCAT [ Wed Jun 16, 2010 9:11 pm ]
Post subject:  Re: Overheat at low rpm
If there are no blockages in the hoses and the impellor is good, I would start looking at the water circulation pump . The vanes inside could be worn down or a small leak can reduce flow.
Author:  billcruise [ Wed Jun 16, 2010 9:45 pm ]
Post subject:  Re: Overheat at low rpm
My impellers are brand new and there are no visible water leaks... hmmm
Author:  LazyCAT [ Thu Jun 17, 2010 9:23 pm ]
Post subject:  Re: Overheat at low rpm
Just thinking out loud to make sure we are on the same page.

From what I understand the impellor brings water from the lake/ocean to the engine and what is the waterpump on a car engine (circulation pump ) circulates the water around the engine. If either system is insufficient the engine overheats. The circ pump is driven by the same belt as the impellor, so if the belt is not tight and is slipping the water doesn't flow fast enough at low rpms but will flow fast enough at higher rpms. Same applies to the circ pump itself, if the vanes inside the pump are worn down or broken not enough water flows through the engine at low rpm but flow will be sufficient at the higher rpms.
Author:  billcruise [ Fri Jun 18, 2010 8:55 pm ]
Post subject:  Re: Overheat at low rpm
Lazy, all reasonable ideas but at least with my 5.7GXI the impellers are new and are direct driven off of the crankshaft (i.e. no belt). The circulation pump is driven by the serpentine belt but a) I've never seen one slip and b) any belt would tend to slip at high RPM, not idle. I think there's something else to this.... other ideas anyone? Pls help! 8)
Author:  LazyCAT [ Sat Jun 19, 2010 10:54 am ]
Post subject:  Re: Overheat at low rpm
Since you have a problem on both engines, do they share any part of the cooling system such as the intake? or are they both completely independent from one another.

You mentioned the manifold risers. Here in south Florida the life expectency is 5 years due to the agonizing brutality of the salt water, how long is it for you and have they ever been checked?
Author:  Off-Piste [ Sat Jun 19, 2010 12:53 pm ]
Post subject:  Re: Overheat at low rpm
Our 2005 V 258 did the same exact thing when new. It has dealer installed closed cooling and they replaced the impellor. My marina manager said he'd seen this a lot, to rev it up on start as the manifolds take a lot to fill. That worked, even though the dealer got it to "draw" I still rev it up to about 2000 on start. In replacing my impellor myself this year (volvo Penta 5.8) I have a smaller impellor then is called out in the parts list. I'm guessing they did a mid year upgrade to the water pump.
Author:  kinkster [ Sun Jun 20, 2010 1:38 am ]
Post subject:  Re: Overheat at low rpm
I had the same issue with my Mercruiser 5 liters, it turned out to be the sender unit(water pressure) for both engines were faulty.
The dealer replaced one and one engine still going off so I went to the dealers and got another one and replaced it, the engine management reset itself and all was good.
I couldn't believe the coincidence, that both would go bad at the same time!
Author:  Heathryan [ Sun Jun 20, 2010 8:30 pm ]
Post subject:  Re: Overheat at low rpm
No progress for us this weekend, we took an IR thermometer and directly under the thermostat housing the temperature difference between engines was about 14 degrees. At the gauge it was reading about 25-30 degree difference. Starting to look into the potential of a ground issue.

Still same results though, under 1000 rpms the engine will heat up to about 200 and immediately after a move the throttle up above 1100 rpms the temp guage goes right down. It goes down so quickly that it makes me wonder if it could really cool down that quickly. I appreciate any suggestions or feedback to continue troubleshooting this issue?

One additional note, on the same engine we also have a high oil pressure (higher than the starboard engine that is running at normal oil pressure and normal engine temp). Could this be connected to the overheat at low rpm?
Author:  billcruise [ Tue Jun 22, 2010 7:44 pm ]
Post subject:  Re: Overheat at low rpm
Talked to a Volvo service tech today. He suggested close inspection of all water hoses, especially the hose running from the transom to the suction side of the pump. Apparently he has seen some kinked and restricting flow. I'm going to do that and carefully look for impeller debris. Still open to other suggestions.
Author:  ScottA [ Fri Jun 25, 2010 5:29 am ]
Post subject:  Re: Overheat at low rpm
I had this problem and found that the water intake hose had built up with salt on the outside and crushed the hose reducing flow.
I would imagine this would be only in boats operated in salt water
Author:  wkearney99 [ Mon Jun 28, 2010 9:40 am ]
Post subject:  Re: Overheat at low rpm
Be aware that hoses can get soft over time and collapse. But that tends to be more of a problem at higher RPMs, not lower. That's where the incoming suction pulls hard enough to collapse the hose, greatly reducing the amount of cooling water. For a problem just at lower RPMs I'd definitely look toward blockages as the cause.

And don't assume that the new impeller hasn't also failed. Sh1t happens. Check it.

Also consider that your manifolds may need service. It's possible you've got corrosion or other material blocking the passages inside them or their risers. They tend to last longer in fresh vs salt water, but they'll eventually fail in either.

When debugging gauge issues it helps to have a spare gauge and wire handy. That way you can connect right to the sensor and eliminate the in-boat wiring and gauge from the equation.
Author:  LouC [ Mon Jun 28, 2010 9:49 am ]
Post subject:  Re: Overheat at low rpm
when the impellers were done, was the wear plate also replaced? if this is worn and not replaced new impeller may not make a good seal up against it and may not pull as much water as it should. I'd especially check the belts, and take out the thermostat and look in housing for any debris (any past impeller failure?) in a Volvo I"d guess the broken off bits of impeller would wind up in the thermo housing. Finally test the thermostat...if raw water cooled it should start to open at 160*...sometimes with closed cooling they run a higher temp stat...
How about the heat exchanger...does it need to be cleaned?

Manifolds/risers can corrode and block exhaust flow, one way you could tell is to see how hot they get when running on plane...if they are warm at idle but really hot after running on plane that's a possiblilty...
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