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Hello from a new user, so please cut me some slack if I inadvertently break a rule or two. My 1998 Sundowner 245 VP GSI (5.7) DuoProp is running hot, and the repair shop is running out of ideas to address this issue. First a little history...I've had the boat for three summers with no heat issues experienced the first two summers. Last summer I was running the boat when an audible alarm sounded, making me notice the engine temp was in the red. I took the boat to my repair shop where the thermostat and drive pinwheel were replaced. Took the boat back out and noticed it continued to run hotter than normal, but no alarms, until I had to anchor and let it cool. Took it back to the shop where the seawater pump was replaced. Took it back out and noticed it still continued to run hot, but no alarms, and no engine "shutdown" mode experienced. Anchored the boat and heard a hissing sound from the engine compartment...took a look but found no water, signs of leaking, or ruptured hoses. The shop has run the boat using a tub of water and swears its pumping enough water. Can anyone give a fellow boater some advice or point me in the right direction?

We need more information. First, how hot is running hot? The alarms usually sound at 200*F. Normal temps is 160-175 or so. Over 180 I'd say is too hot.
Next, fresh or salt water? If salt you may need new exhaust manifolds and risers, they will for sure cause overheating if clogged with rust.
Is the boat slipped or a trailer boat? If slipped you can get marine growth in the water intakes and that reduces the volume of cooling water. I've had this problem on mine (salt water). Post back with more info....

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88 Four Winns 200 Horizon
4.3 OMC Cobra-4bbl
2002 Walker Bay 10/2012 Suzuki 2.5
2008 Walker Bay 8

1998 Jeep Grand Cherokee 4.0/Selectrac
2007 Jeep Grand Cherokee 5.7 Hemi/Quadradrive II

Sounds like the only ideas your shop has, is how much $$$ they charge ya. I mean, they replaced pretty much the most expensive items first. How old is the impeller? Has it been changed? Get an IR temp gun and get readings from manifolds, T-Stat neck, etc. Check those readings against what your temp gauge reads.

Oh, and welcome to the forum! Always like seeing new Sundowners join up. Lets see some pics!

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1996 205 Sundowner DLX
5.0FL Volvo SX Cobra
Tow Veh: 2012 Nissan Frontier Pro-4X

Before I replaced the VDO dash in my 98 225 Sundowner 5.7GSI DP the temp gauge would go from normal to pegged high as soon as I plugged the inflator pump for my tube into the 12v socket, as soon as I unplugged it the needle went back to normal. After replacing the dash the problem has never returned.

I would use an IR gun to check temps as NiagaraChillin suggested before doing anything else.

Yes, it sounds like clogged (rusted) exhaust manifolds and risers. If they are original, they are definitely due for replacement. If the boat is used often in salt water, then the manifolds and risers need to be replaced after 5-7 years. The whole set of manifolds and risers can be purchased online for a little over $600 (http://www.ebasicpower.com http://www.marinepartsexpress.com), and can be done by a shade-tree mechanic.

I don't understand why your mechanic is not able to diagnose...there is only a few things it can be. Maybe it's time for a 2nd opinion.

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"Knot Easy" 2000 Horizon 240 Volvo 5.7GS /SX
tow: 2017 Honda PILOT EXL-AWD
prev. boats:
'87 Chaparral 198CXL 4.3 OMC Cobra
'69 Jetstar 16ft Ski Boat, 115hp Yamaha
'68 Aluminum Jon Boat, 3hp Sears
'64 Water Wings

Thanks for the good questions. It's a trailered boat that only runs in fresh water. Not sure how hot is runs, only that the needle goes into the red and a hissing sound begins from the engine compartment. The impeller was replaced and the old one was actually in pretty good shape. The overheating does not occur at lower rpm, more so it occurs at 3500 or higher, and we can get a high RPM of about 4800 at WOT. We boat at 5000ft with an F3 prop set. We rarely run WOT, and then only for a few seconds, but that kicks the temp up and then we must stop to let it cool. I hadn't thought of the infrared gun, but will get one and take readings next time out. Thanks guys!

Using the IR Temp gun will help you localize problems. You can take readings of the intake manifold under the thermo housing (this will be close to the temp gauge in most cases, usually about 10 degrees lower) and also take temps of the exhaust manifolds and risers. Normal should be 160-175 on the gauge, on the intake it will read 145-155 or so. Manifolds, they will be at about 90-100 idling, and if you check them after running on plane they may top out at 135-140 depending on the engine. If they are hot at idle, like 150-160 they may be clogged or your raw water flow is low.
Google "Volvo Penta Overheat Diagnosis", it is a PDF file that goes through the whole procedure to troubleshoot overheating. It will tell you if your raw water flow is low or if the cooling system is sucking air.
These raw water cooling systems are high volume low pressure systems, they are not highly pressurized like a closed system. So anything that restricts water flow in, or out, or introduces air into the cooling water will cause problems.

_________________
88 Four Winns 200 Horizon
4.3 OMC Cobra-4bbl
2002 Walker Bay 10/2012 Suzuki 2.5
2008 Walker Bay 8

1998 Jeep Grand Cherokee 4.0/Selectrac
2007 Jeep Grand Cherokee 5.7 Hemi/Quadradrive II