Hollow exhaust sound and exhaust alarm

[tdsull]

I pulled the sea water pump and heat exchanger last winter. Cleaned and replaced all moving parts. Replaced engine coolant.
The previous year I replaced the mixing elbow with a stainless steel unit and replaced the 4'-0" section of hose before getting to the fiberglass elbow.

Boat has been running great this year. Except on Saturday when I started it up the exhaust alarm went off.
I was getting plenty of water discharge. I ran it for a bit in the marina and then the alarm went off.
Engine never overheated I ran at 3000 RPM for 2 hrs. The engine exhaust sounded hollow, eerily similar to when the old carbon steel mixing elbow started to go.

Played the whole day and came back. While pulling into the marina and throttling down the exhaust alarm went off again.
Water discharge and engine temp was fine.

The boat is being stored high and dry for the first time and I haven't been opening and closing the engine seacock.
Reading through other post's I notice comments about the line emptying when the boat is lifted and losing prime, makes sense and I will close the
seacock from now on.

Boat is 18 years old an I may have a bad exhaust sensor; however, I don't like the hollow exhaust sound. Any thoughts?
This week I will pull the pump and see if loosing prime caused any damage to the impeller.

[TorreyWP]

As soon as you started up your exhaust alarm went off?

Seems unlikely to me that right away the probe got so hot it set the alarm off. This condition coupled with not having any issues at cruise then getting an alarm again after cruise makes me think you might have either a flow issue or an issue with the probe itself. Likely not a bad probe but maybe it has something stuck to it causing water/exhaust gas to flow around it differently.

I know you said flow out of the exhaust overboard looks good and engine temp is good, thats good. As we know, the exhaust alarm is the first line of defense against overheat. That said, flow may "appear" good...strainers may "appear" clean...

I think you are on the right path to get a look at your impeller.

Let us know what you find and good luck!

Also, do you have any way in your Yanmar gauges to see actual exhaust temp or just a light and an alarm? If no temp, I wonder if anywhere in the literature if the threshold of that alarm is listed. Just curious.

[tdsull]

Yes as soon as I started it up the alarm went off. The day before I had cleaned the strainer, there was nothing really inside it.

Looking back I had a similar situation in May of 2017. Happened for 2 weeks then went away for the season.
Sea water pump was reconditioned in April of 2018, had no broken vanes etc.

2 weeks ago there was a rattling in the stern of the boat when we left the marina. It sounded like something was rattling around in the sea strainer.
I couldn't see anything then the noise went away with a little throttle.

I think your right, something (or someone) got sucked up and is slowly making its (their) way thru the system.

[RobS]

There is no probe. This alarm is not a temperature alarm. The 6LP OEM setup is a raw water flow switch and it is located on the raw water intake of the intercooler. Your statement that the day before you cleaned the strainer leads me to question if there is an air leak at the strainer that is limiting low rpm flow. Re-seat the strainer cover and be sure the cover gasket is cleaned and lubed.

[carolmarie]

Rob is right on.
I had the EXACT issue on my 2001, 6LP-STE.
It WAS the raw water flow alarm sender. It was eroded; after the replacement no more high exhaust alarm at low rpms!
Don.

[RobS]

Rob is right on.
I had the EXACT issue on my 2001, 6LP-STE.
It WAS the raw water flow alarm sender. It was eroded; after the replacement no more high exhaust alarm at low rpms!
Don.

Yup. I believe Yanmar did away with this sensor on later model engines. Flow sensors can be problematic - as with pretty much everything that sits in sea water at the dock, it is not "forever"...

[tdsull]

I had no issues this past weekend. No alarm on start up nor throttling back.
Checked the strainer before leaving the marina and again when we got back. Maybe I had been sloppy putting it back on the last time.

Given that I had this issue last spring for two weeks and then it disappeared and came back for one weekend this year leads me to believe it is a flow sensor issue.

The rattling I heard a few weeks back must have been the rudder zinc shaking loose, as of 9am this morning its no longer on the rudder.

Thanks for the responses.

[Yearwood]

The alarm has a plastic shaft and a float rides on the shaft. Over time, especially in muddy or sea water the shaft will erode and the float hang up (actually hang down i.e. fail to rise). Result high temperature alarm sounds. It is actually a low water flow alarm.