Help with overheating issue (Temp Drop with abrupt Overheat)

[dearly]

Hello and thanks for reading my post. I've spent years reading but have never posted before. Hopefully in the future I can help others as this site has helped me in the past. However, I really need some help here.

I cannot seem to find anyone with this issue. I'm in Clearwater, Florida (Salt) and run my boat all year long. It's in a slip (not on a lift) but I flush it out after ever use. Gear is 5.0 Mercruiser (Non-EFI) with Alpha One Gen 2 Drive, Raw water cooled. A few months ago it started to overheat under load at high RPM. I used a IR Thermometer and found the manifolds and elbows were getting warm. I replaced the manifolds, risers, elbows, water pump housing and impeller and thermostat (160 degree).

Now here's the problem: When under load and higher RPM's (2,500 +) out of nowhere the temp gauge will suddenly drop down to 100 degrees and then climb back to 170 degress (Normal temp reading for this sender). If I don't back down on the throttle to 2,000 RPM's or less the temp will climb until the alarm sounds. If it sounds I back off and idle for a few minutes and it cools down again. Sometime I can run for an hour at high RPM and Load without the problem showing up. But it's happening more and more lately.

Any ideas? I sure would appreciate any input you might have. Thanks in advance!

 

[outlaw32]

I would check to make sure the thermostat is not sticking. From there I would do compression check to make sure you don't have blown head gasket

 

[Espos4]

You may have a restriction in the output from the pump inside the outdrive (Alpha 1 Gen 2) up into the engine. The hose can become restricted, and the reduced water volume could cause your issues.

You flush it out after every use, but have you cleared the block drains recently? Sediment can build up inside the block, causing poor circulation. They may be very difficult to access, depending on how your engine is situated in the bilge.

 

[CliffA]

i would take a look at the water circulating pump on the engine.....even if there are no signs of water leaking from the weep hole the pump could not be moving enough water through the engine to keep it cool at higher rpm's.....this happened to me on a previous boat.....i did all the things you did which did not help....the circulating pump visually looked fine but i replaced it any way....problem solved...

good luck....

cliff

 

[dearly]

Thanks Outlaw32,

I would check to make sure the thermostat is not sticking. From there I would do compression check to make sure you don't have blown head gasket

I removed the thermostat yesterday and went out to test. Temp never went too high but still had the abrupt drop in temp for a few seconds while at high RPM's and then it climbed back to 170. Did not want to push it so I would back down. Thanks for the compression check idea. I thought I might get that recommendation so it will be my next step. I'll let you know. Thanks again....

 

[dearly]

You may have a restriction in the output from the pump inside the outdrive (Alpha 1 Gen 2) up into the engine. The hose can become restricted, and the reduced water volume could cause your issues.

You flush it out after every use, but have you cleared the block drains recently? Sediment can build up inside the block, causing poor circulation. They may be very difficult to access, depending on how your engine is situated in the bilge.

Thanks Espos4: Sea Ray dealer mentioned a problem with a possible blockage in the hose through the transom. Said I would need to have the engine pulled to check it, Yikes!!! Not ready for that yet. Considering creating a closed loop and circulating some phosphoric acid to flush the block. Might be able to start the flush circle at the outdrive pump hose?

 

[dearly]

Thanks CliffA: Good point, I wondered about the engine circ pump. Did not want to remove it for nothing and could not figure out how to test that it was moving enough water. Just still can't seem to account for the abrupt temp drop just before overheating. Wondering if a water circ pump hose is collapsing and the sender gets a quick shot of cold water just before over heating? Maybe a head gasket issue as Outlaw32 suggested if air is getting in the system and going past the sender unit? Wow, this is great stuff! I never thought I would get so much feedback so soon. I'm going to try and help other here as you have helped me in return. I have a lot of knowledge in marine equipment but this issue is driving me crazy. Thanks to all of you for your help....

 

[outlaw32]

Thanks Outlaw32,

I removed the thermostat yesterday and went out to test. Temp never went too high but still had the abrupt drop in temp for a few seconds while at high RPM's and then it climbed back to 170. Did not want to push it so I would back down. Thanks for the compression check idea. I thought I might get that recommendation so it will be my next step. I'll let you know. Thanks again....

I see below here where it check hose have to pull motor that is not right.

 

[wakeup 19]

I am having similar issue. feedback appreciated and I am watching closeley

 

[Korkie]

I just went through similar. Although on Bravo III drives. My problem was poor inflow of raw water to pump through OD due to blockage (Bravoitis) On the bravo drives, it can be checked by removing elbow at stern and if its the case, it will be obvious. Also, if you have SmartCfraft, what is you water PSI as you are idling, running, etc. It should go up from 4ish at idle to 19+ at WOT. Mine never went above 9.5 at WOT. Solution was adding through hulls and Ting in the ODs to the intake side of the raw water pump. Engine temps now at 165 no matter what speed. Water PSI at 25.8 at WOT. Don't know how Alpha drives set up, but consider the restriction of water to the engine for whatever reason.

 

[Cyrus]

Thanks Outlaw32,

I removed the thermostat yesterday and went out to test. Temp never went too high but still had the abrupt drop in temp for a few seconds while at high RPM's and then it climbed back to 170. Did not want to push it so I would back down. Thanks for the compression check idea. I thought I might get that recommendation so it will be my next step. I'll let you know. Thanks again....

Risers and manifolds?