1996 Mercruiser 3.0 overheating at idle

[Kryptonics]

1996 Mercruiser 3.0 slowly overheating when idling, the temperature increases to over 200F and up, if I rev up the engine to 2000-2500RPM the temperature goes down close to 140F pretty fast.
What I already did:

• Replaced the impeller
• Checked water supply

Here is the video of water supply hose detached from the thermostat and running at 800RPM. Please let me know is the incoming water flow looks good enough

https://www.youtube.com/watch?v=694gg3TcsbY

Next things I'm going to do is to check the thermostat and exhaust. If the issue is clogged exhaust, any suggestions on how to clean it without removing from the engine?


P.S. I'm new here so will appreciate any help

 

[wellcraft-classic210]

Hi

I assume your on the water or using muffs for a steady water supply to the impeller engine -- any loss of water supply will damage the impeller in short order. ( As you may be aware )

If you pull your supply hose coming from the impeller and run with a COLD engine for a short time you can see flow and verify the impeller and items down stream are operating correctly I don't know the gpm but it should be a good flow // the experts may be able to comment on expected fountain height etc.


-- If that test shows good impeller flow -- Yes start looking for a blockage and the thermostat and its housing is on that list.


As you suggested-- If you have overheated checking the shudders is probably a very good idea.


Not highly familiar with your engine -- so hoping someone can list common flow related root causes.


Good luck.

 

[Kryptonics]

The boat is on the water right now. The first thing I would like to do is to confirm that incoming water flow is enough to be sure that the new impeller is working right so it will be helpful if someone experienced can watch my video and tell if the impeller is pumping good or weak. Thanks in advance

 

[Bondo]

The boat is on the water right now. The first thing I would like to do is to confirm that incoming water flow is enough to be sure that the new impeller is working right so it will be helpful if someone experienced can watch my video and tell if the impeller is pumping good or weak. Thanks in advance

Ayuh,..... Welcome Aboard,..... Ya got the hose pointed the wrong way to test the water flow,.....

Do the same test, with the hose pointed straight Up,.....
The water should rise 'bout 5" above the hose, before it falls over itself,.....

 

[dan2744]

My 5.7 doesn't like to cool well at anything under 800 rpms. Even with new impeller so I just put it in neutral for a minute and rev it up to let it cool down.

 

[harringtondav]

Go with Bondo's advice. If you want to get science-specific, idle at 1000 rpm in the water (no muffs) and collect the hose outflow for 15 seconds. Shut down and measure the water volume. Your 3.0 drive should be delivering 4 qts. in 15 seconds, 4 GPM minimum.

If it's below that, and since you installed a new impeller, I'd recheck to make sure your lower gear case water tube connection is correct. You're loosing flow somewhere. Getting the copper supply tube engaged in the O rings in the water pump housing coupler can be tricky.

 

[Kryptonics]

You probably right, I removed the thermostat yesterday, nothing changed, checked water channels for corrosion, they look pretty clear for a boat that used on fresh water. Now I am reading complains about Sierra impellers, people have too many problems with them, so I decided to recheck the water supply and I got about 1 gallon in 15 seconds at 1000-1200RPM, but then I decided to drop the RPM as low as possible (probably around 600) and water flow stopped. I think my Sierra impeller might be a problem so I ordered new OEM impeller kit together with the housing, will need to get the boat out of the water (this is the hardest part, that's why I hoped that the problem is somewhere in the engine) and change it up later this week. Will let you know if it fixes my overheating problem in case someone curious too.

 

[Kryptonics]

My overheating on idle problem was caused by this little guy.

 

[B Rad G]

What is that?

 

[Kryptonics]

Water Coupling Assembly, connects impeller housing with the copper tube in upper unit