Exhaust alarm sounded. Troubleshooting thoughts??

[ScottC]

I would be grateful for anyone that may have some thoughts on this…

Background
I have a wet exhaust system with a Vetus muffler and outlet under water at the extreme aft end of the boat. Exhaust elbow is “new”. Replaced at the beginning of 2022.
About 8 years ago, I had my yard install a Vetus exhaust temperature alarm. Simple (75 C) sensor in the hose about 2 feet after the exhaust elbow. Connected to a siren.
Until last week, this alarm has never sounded.

Fast-forward to last week
I was idling around in choppy seas near the entrance of a large VTS controlled bridge in the Gulf of Patras. Of course, just as the bridge told me I could proceed, my exhaust alarm sounded. First thing I checked were my two engine water temperature gauges. Both expressed happiness.
I shut off the diesel engine and proceeded using my electric motor. I placed a friend at the helm while I flipped up the engine hatch for a quick inspection. I could hold my hand on the exhaust elbow. All looked fine. All smelled fine.
Started the engine again, checked that I could see water moving in the raw-water strainer and proceeded at 7 knots (1850 rpm) for the next few hours until we reached our destination. I concluded that I must have picked up a piece of plastic in the rather polluted Patras harbor that was blocking my water intake until I turned off the diesel engine.

Fast-forward to yesterday
I was idling around in VERY choppy seas at the entrance to the Corinth Canal awaiting permission to enter. Alarm sounded again (!). I switched to electric propulsion to maintain position and performed checks once again. All fine. Engine water temp fine too. Started the diesel engine when we were called to enter the convoy going through the canal, checked the raw water strainer and all was fine. Ran 7-8 knots through the canal and then 7 knots for another 3 hours after that to our destination. All fine.

Today
It occurred to me that if my water pump impeller is beginning to fail, it might produce insufficient water to keep the exhaust cool at low rpms, but adequate amounts at higher rpms. So today, while at the city quay in the Epidavros, I ran the engine at idle speed (850 RPM) for 75 minutes trying to replicate the phenomenon. No alarm. I then revved it up a bit to 1200 RPMs for an additional 15 mins. Still no alarm. In both prior cases, where the alarm sounded at idle-ish speeds, it happened within 30-45 minutes from starting the engine.

My conclusions thus far

• Could it have been due to back-pressure caused by some rather heavy waves hitting me from astern while I was trying to maintain position/not move forward? I’m sure in some cases, the exhaust outlet was briefly exposed due to the somewhat unusual wave action. On both occasions where the alarm sounded, I was experiencing heavy waves from astern while trying to hold position / not move forward.
• I feel it’s unlikely a faulty sensor, as it’s a very simple sensor and it does not sound immediately upon starting.
• I guess it could have been plastic blocking the intake. But TWO times in one week??? Hmmm….
• I feel that if the impeller is failing, I should have been able to reproduce the problem with my tests at the dock this morning. I couldn’t. Furthermore, the engine water temp remains always normal.

Any thoughts or ideas??

 

[CharlieO.]

I would check\replace the impeller. Check sea strainer and water flow to it. Check sensor to be sure it hasn't moved and is still in optimum position on the exhaust hose. The sensors are fairly cheap so I wouldn't have a problem replacing it.

 

[sunchaser]

A Murphy gauge telling you temps and a locked in highest reading would answer your questions. And yes, change out the impeller, maybe annually if it shows any signs of stress.

 

[OldDan1943]

If you are missing pieces of the impeller, blow backwards to dislodge the pieces to remove.

 

[CharlieO.]

This is where one of those infered temperature reading guns would come in handy for a quick check.

 

[rgano]

It might have been an interesting experiment to briefly increase engine RPM in neutral to see if the alarm would silence.

 

[lwarden]

I've seen that if I run my engine off the hose I get a hot spot at the top of the exhaust hose after the mixing elbow. I'm sure if I had a sensor there and let it run for long enough it would probably get hot enough to trigger it. I presume this is due to reduced water flow from the hose and the shower spray in the mixer is not full enough to provide flow to the upper holes. There still seems to be quite a bit of flow out the exhaust so it could be misleading to use that as a gauge. Reduced flow from either a worn impeller, blockage on the pickup side etc. could all likely lead to triggering an alarm.

 

[SteveK]

The impeller and strainer need to be eliminated as cause. Your thought of intake blockage is possible, cleared after engine stopped. Reduced flow from a blockage at idle and alarm sounds. Impeller should be good with reduced flow, but change it as cheap insurance.
As for the engine temp gauges reading normal, they will, they did for me as well. Neither one of us let it run long enough after the alarm to affect the coolant temp.
As per post 7, I was wondering why the alarm sensor was 2 feet from riser.

 

[ksanders]

I did not get from the original post...

When the alarm sounds have you right away touched right next to the sensor?

75C is pretty hot to the touch, it should be immediately clear if it is a sensor problem or an actual hot temperature.

Since it is intermittent in nature and your boat is 4 years old I would also just change the impeller, since 4 years is a pretty fair time to proactively replace this maintenance item.

 

[ScottC]

Thank you for all the comments and suggestions thus far!

· I agree checking the impeller is a good idea. Checking/changing the impeller is, unfortunately, a major piece of work on my boat.
· The impeller was changed at the beginning of the season and I have not been in sandy/silty waters since. Of course, the boat yard in Malta that did my winter work used my spare impeller and did not replace it. I discovered this the night before we were to set sail. I should not have travelled without this spare. I don’t want to think of opening the water pump without a new one (along with a new cover seal) on hand. Am checking around. I have a Jabsco part number. I don’t think it will be too difficult to obtain. Of course, I have three more impellers at home in Sweden on the shelf.
· Raw water strainer basket looks clean, but I guess I will be going for a swim to check the intake strainer.
· The sensor is mounted about 2 feet after the exhaust elbow because that’s where my yard decided it was best to install it. I’m not sure why.
· The sensor is still in its original position. It has not moved. It is not loose.
· As the alarm was sounding, I touched the un-insulated exhaust hose near the sensor and it did not feel especially hot. Just warm. No problem holding my hand on it. Unfortunately, I don’t know what this should feel like to the hand, having never put my hand on it while running before.
· Regarding the Murphy gauge suggestion: Interesting idea, but I think more than I can do right now. Makes me think that maybe long-term, I should consider changing the alarm sensor to a sensor that could give me an actual readout at a new gauge on at the helm.
· Infrared temperature gun is also an interesting idea. I have one. Sitting on my desk at home (I bought it for a different reason than boat use). I may try to find one here in Greece to keep on the boat full time.
· If the alarm sounds again on my 21 NM run tomorrow, I will do as Rich suggested and increase the RPM briefly before shutting down. If that stops the alarm, I will have learned something.

 

[Comodave]

Keep in mind that these exhaust alarms usually go off way before the engine temperature gauge shows the engine being hot. At least that is what mine do.

 

[High Wire]

The impeller blades could be fine but the rubber part is slipping on the metal hub. That happened to me last year. Pull the impeller, inspect and/or replace with a new spare. Its fairly easy and a job you (and just about any other cruiser) need to know how to do.

 

[twistedtree]

Definitely a curious one.

One key question is whether the exhaust is really overheating, or if it’s an alarm malfunction of some sort.

When it happens, if it’s possible to leave the engine running and quickly get a hand carefully on the exhaust, that will tell you if it’s really overheating vs a false alarm. Feeling the exhaust under normal conditions will give you a reference point for normal operation.

An exhaust overheat without an engine overheat suggests an abrupt interruption of water flow, to whatever extent that might provide a clue. Perhaps a zinc or something large that intermittently blocks flow?

 

[Bmarler]

When I put in my exhaust alarm I decided to use the one made by blue seas. It has four channels, comes with four sensors. Each channel has user defined set points for high and low temperature alarms and a few different ways to display the information. You get actual temp numbers and can set the alarms after you see what normal looks like. I have my exhaust alarm at 105 Fahrenheit as it usually runs around 92-94 degrees.
I read the generator exhaust, block and tranny cooler with the other channels.

 

[OldDan1943]

Something else I learned in the Navy.

Use the back of your hand to check for high temp. If you use any part the ‘flat of your hand’, you hand may contract, you may not be able to remove your hand from the surface. If you use the back of your hand and your hand contracts it will contract away from the surface in question.

Of course an infrared gun works better.

Definitely a curious one.

One key question is whether the exhaust is really overheating, or if it’s an alarm malfunction of some sort.

When it happens, if it’s possible to leave the engine running and quickly get a hand carefully on the exhaust, that will tell you if it’s really overheating vs a false alarm. Feeling the exhaust under normal conditions will give you a reference point for normal operation.

An exhaust overheat without an engine overheat suggests an abrupt interruption of water flow, to whatever extent that might provide a clue. Perhaps a zinc or something large that intermittently blocks flow?

 

[PNW]

wow great tip with the contracting hand thing!

 

[Greg QS]

Same advice when using yourself as a electrical tester. Us the back of hand or finger so you pull away from the eclectic. LOL

 

[ScottC]

Update:
The impeller was changed and the cooling system flushed over the winter. Since I was not present when this work was done, I cannot comment about the condition of the impeller. I now have a temperature gun with me for the next time, should this occur again. Thus far, it has not. And thus far, I have not been in sea conditions similar to what I was in the two times this occurred last year (heavy waves from aft, while sitting stationary... hitting my under-water exhaust).

 

[OldDan1943]

Get 2 spare impellers.
When you change the impeller, back flush the cooler looking for bits and pieces of the previous impeller

 

[Comodave]

You said they used your spare impeller, how old was the spare? Rubber ages even sitting on the shelf. If the impeller was 3 or 4 years old it could go bad quickly when put into use. I don’t stock a lot of spare impellers for that reason. If I were to stock say 4 impellers and I change them every 2 years the last impeller could be 6 to 8 years old when put into use. I stock 1 spare for each engine and replace it when I use the spare.

 

[SteveK]

What Dave said. Changed impeller with stock one and it only lasted a month. The one replaced would have lasted a month too.

 

[ScottC]

You said they used your spare impeller, how old was the spare? Rubber ages even sitting on the shelf. If the impeller was 3 or 4 years old it could go bad quickly when put into use. I don’t stock a lot of spare impellers for that reason. If I were to stock say 4 impellers and I change them every 2 years the last impeller could be 6 to 8 years old when put into use. I stock 1 spare for each engine and replace it when I use the spare.

Interesting and valid point! I think my spare was probably more than 4 years old!
Make sense, as I've experienced the same "degradation" using windshield wiper blades that are too old!

 

[Maerin]

An impeller change also needs to address the cover plate. They wear, causing the clearance to the face of the impeller to get sloppy. That can be enough to erode performance, particularly when at minimal flow/rpm... read: idling. Either flip the cover, face it, but carry a spare. It won't degrade over time. Should change it every other impeller swap or as it shows wear. Usually overlooked.