Mercruiser 3.7l 470 Overheating - no water now in sea water line at heat exchanger

• Hi All - 3.7l ran well for a couple of hours at 3200rpm, went to idle and then noted the engine got right up on the gauge, it was boiling out overflow tank. Turn it straight off and then used one engine to get back to shore.
• Have checked header tank and the gasket was okay.
• Pulled the sea water hose off at the heat exchanger and there is no water in there at all. I just cant see why it would have emptied all the water out of the line, thought there would be some water left in there.
• There are no leaks from the engine/hoses.
• The engine turns over fine, have not tried to re start the engine as yet but trying to identify what the issue could be.

Given the sea water line is empty does that mean it is a impeller that has failed?

If it is the impeller, does anyone have experience putting in a separate Jabsco impeller pump running on 12 volt and not using the one in the sterndrive?

If the impeller has failed what else do i need to check?

Rebuilding your seawater pump is part of the zen of marine maintenance

o2batsea said:

Rebuilding your seawater pump is part of the zen of marine maintenance

Click to expand...

Use an Complete OEM pump.... All aftermarket pumps are junk.

The engine driven pump delivers water based on the engine's RPM/needs.... an electric will run at one speed. Just rebulld the original... but inspect the impeller. I suspect you will find missing vanes.... you have to find them. they will restrict water flow if not found and removed.

sandkicker said:

The engine driven pump delivers water based on the engine's RPM/needs.... an electric will run at one speed. Just rebulld the original... but inspect the impeller. I suspect you will find missing vanes.... you have to find them. they will restrict water flow if not found and removed.

Click to expand...

Sage advice - the OP will likely find a blade missing off the impeller and blocking the water flow out of the pump or in the PS cooler if there is one. No need to reinvent the wheel jusy replace with an OEM impeller

Thanks All - The impeller was checked/replaced less than 20 hours back so was a little mystified.

Understand importance of replacing the impeller with original part and the need to check it etc but i was trying to first understand:

1. Is the fact that seawater line is empty/dry mean that it is definitely a sea water pump failure/blockage? I cannot understand how the water could drain out or even boil out dry from this point and hence the question.

2. If point 1 answer is yes that means we need to get the boat slipped to remove the bottom of leg to gain access to the pump (unless you know some other way); that's the reason I was asking if a a Jabsco could be fitted to save us having the cost to slip again...just to get us through for now. As for flow, we can size it on the max flow at max rpm so no issue there, if too much flow the thermostat will close on the coolant line anyway i believe.

Appreciate your advice.

If you are experiencing absolutely no water flow from outdrive and Impeller was done so recently.
1. Leave water hose disconnected from where you removed to confirm no water.
2. Start engine and confirm NO watef flow.
3. If confirmed then you have to pull boat to find out why.

If you install an auxillary water impeller pump, where are you going to draw the raw water from?

Have you looked to see if there is something blocking the lower unit intake holes?
If you possibly sucked up a bag and blocked water for a minute or two while running, you very easily could have destroyed the impeller in the drive.

Thanks for the point to point feedback - will go through the process but looks like it maybe coming out.

Thought we maybe able to run a line out the transom - there is a spare hole through there, put a stainless dropper on...just an idea to possibly save the slip fee.

The volume of water from outdrive should be high.
I forget the exact amount but the water at 1000 rpm should fill a 5 gallon bucket in a minute or two possibly less time.

Re: "Thanks All - The impeller was checked/replaced less than 20 hours back so was a little mystified...."

Running dry for 20 seconds even once will fry the impeller... so 20 hours ????

Yes understand the running dry bit for 20 seconds but it has not run dry unless something covered both inlets which is possible but were in blue water 20 miles off shore...yes checked the inlets and all seemed clear.

I am still trying to understand why the hose would have been empty but maybe it was as the coolant was all hot and I turned it straight off so may have boiled out.

Will try to get to it and check if in fact there is water flow in the coming days as suggested by Jack Mass - there is a slight tight bend in the hose where it goes over the back of the motor also and maybe that also didn't help but that's been there for months.

OUTCOME cracked impeller hub: pulled out the impeller and all fins looked like new!
Further investigation found the impeller plastic hub was split and had been spinning on the shaft.
The key had taken most of wear and were able to get new impeller, key and shim to fit well and all running now.
Had me mystified on the running dry advise as it just didn't make sense but at least we have now found out what it was.

Thanks all for your input and hope this outcome may help someone else.