Telltale dribbles after replacing impelller and ensuring hose is clear

[IanCanda]

I recently purchased a pontoon boat with a Mercurcy F25ELPT 4-stroke. During the 30 minute pre-purchase water test I noticed the tell tale was only dribbling. The vendor stated that it had been like that the 8 years he owned the boat. My 1974 Mercury 1150 (tower of power) that I have been maintaining over the years, peed like a racehorse when I had it out last month so this piddle from a modern engine doesn't sit well with me. The water flow diagram in the Mercury service manual for this engine shows the flow from the lower unit pump going to the exhaust adapter plate from where it fills the powerhead and, until the thermostat opens, can exit only through the fuel cooler to the telltale. After the thermostat opens, it also circulates the waterjacket and exits throughthe exhaust and idle relief. I pulled the hose off the inlet side of the fuel cooler - no flow at idle speed. I also confirmed the tell hose was clear. I then replaced the impeller on the water pump. The old impeller was still intact but cracking so replacement was needed. New impeller but still nothing from the telltale. I then put a garden hose directly onto the lower unit water tube without starting the engine. I got a dribble from the telltale and all the water coming down the exhaust.
My thinking is that the thermostat is stuck open. That could explain how the previous owner had 8 years without overheating but zero flow at idle and only dribbles at higher RPM from the telltale.
Does this make sense for a modern engine? My 1974 1150 pees like a racehorse at all RPM (it doesn't have a thermostat). I wouldn't have thought the thermostat opening would affect the telltale water flow so much.

 

[Tech Tony]

Try blowing air backward down the fuel cooler inlet line and see if anything pops loose., also check to see if the fuel cooler itself is blocked. If it is the fuel pump will eventually vapor lock and won't start till the temp cools off.

 

[IanCanda]

There was no flow on the water hose to the inlet side of the fuel cooler. Good idea. I'll try that to see if there is a blockage between the fuel cooler and the exhaust adapter plate. I agree also good to ensure fuel cooler itself isn't blocked. I'll do that but I see it as a separate matter.

However, if there is a blockage between the exhaust adapter plate and the fuel cooler, with the thermostat closed (as it should be with cold engine) shouldn't I just have had no flow (or spray back) from the garden hose on the water tube and not all the water coming down the exhaust?

 

[Tech Tony]

You càn check the thermostat in a pot of boiling water to see if its working right. But to me logic dictates there is something blocking the line to the fuel cooler.

 

[IanCanda]

You càn check the thermostat in a pot of boiling water to see if its working right. But to me logic dictates there is something blocking the line to the fuel cooler.

Today I blew 50psi air into the water tube feeding the fuel cooler with the lower unit off. It blew clear but most of the air was coming out the exhaust. There was some pressure on the bottom end of the water tube but I could easily block it with my finger.

I decided to hand it over to the local boat mechanic whose been maintining motors for over 30 years. He agreed the thermostat should be checked. He'll also run it with muffs and see if water is getting to the thermostat hole.

I'll post more when he reports.

 

[IanCanda]

Today I blew 50psi air into the water tube feeding the fuel cooler with the lower unit off. It blew clear but most of the air was coming out the exhaust. There was some pressure on the bottom end of the water tube but I could easily block it with my finger.

I decided to hand it over to the local boat mechanic whose been maintining motors for over 30 years. He agreed the thermostat should be checked. He'll also run it with muffs and see if water is getting to the thermostat hole.

I'll post more when he reports.

Mechnic found the thermostat was full of debris and stuck way open. New thermostat and now its is peeing like a horse.