Generator Operating Temperature?

[Serene]

I have a Cummins Onan 6kVA Generator (MDKBJ Spec C) on my vessel, which has been layed-up for around six months while I was out of the water for my refit. I started the generator for the first time this weekend and saw what I thought was an alarmingly fast rise in coolant temperature, so shutdown the generator.

In previous usage I have checked oil pressure, the pumping of raw water out the exhaust and the coolant temperature, before adding AC loads to the generator. Typically I would add the AC loads when the temp reached around 67 degrees celsius and then I would let the generator run for the required duration. Most typically two hours to recharge batteries, pull the freezer temp down and to heat water for evening showers.

The generator started no problem, but I couldn't see raw water cooling being discharged overboard, so I monitored the temp for a period of time (3mins) to see if the muffler needed to be re-filled with raw water before discharge overboard started. I then shutdown the generator as I was concerned about an overheat situation.

At no time did I receive any warning or errors on the control panel.

I then checked the seacock and strainer to ensure no blockages (none found) so removed the raw water pump and replaced the impeller. I was expecting to see all of the vanes stripped from the impeller, but it was surprisingly intact. Although it did have some deformity in shape. I replaced with a new impeller anyway as I already had the pump removed.

On the next start, I monitored some raw water output from the exhaust, but the temperature kept climbing up to 95 degrees celsius before I again shutdown the generator as I was worried about overheating. Again no warning or alarms.

But I cannot remember at what operating temperature the generator should stabilise, therefore I don't know if I actually have a problem or am being overly cautious.

The next step I was thinking was to replace the thermostat, in case that is frozen closed, so no coolant is being allowed to flow.

So questions;

1. What operating temp should I be seeing?

2. Am I being overly cautious given no alarms or warnings?

3. Any tips on what to do next?

 

[SeaDogAK]

If you’re getting less than normal raw water output from the exhaust, I’d check the heat exchanger for blockage, and look for impeller bits clogging the lines. You could pull a hose connection upstream from where the raw water discharge goes into the exhaust to see if there is normal flow coming out (being careful not to scald yourself with hot water).

95 C seems a bit high, but I don’t know how high that engine is supposed to run. My Westerbeke runs around 80C.

 

[Serene]

Yeah I felt it was high too, but I cannot find anything in any of the operating or installation guides which suggest the normal temp range.

 

[BruceK]

I think my later Onan runs at 90C.
You have a significant change so it`s right to investigate. Anything obstructing the exhaust?

 

[tiltrider1]

Your generator should have a high temperature shut off circuit. Since it is not shutting down it’s possible that your guage is reading incorrectly. The best way to determine this is with an IR Temperature Gun.

 

[Lepke]

Diesels run more efficiently at about 205° to 210°, 96°-99°C. Fuel burns better with less soot in the exhaust and oil.
I haven't seen a diesel generator w/o a high coolant temp shutoff since before 1970.

 

[Bmarler]

There should be significant output of water from the exhaust. Reduced flow and high temp indicates that there are issues in the raw water system. As mentioned, the heat exchanger may be partially plugged, or the injection point at the exhaust elbow might be crudded up with rust. Maybe pull the hose and poke around with a screwdriver.

 

[Serene]

OK so a few ideas to continue troubleshooting. Thank you. I do feel there is less raw water flow out of the exhaust, so next step is to do what BruceK and Bmarler have suggested and check for flow obstructions. Easy enough to take the raw water hose off the exhaust elbow and look for both flow restrictions in the hose and potential soot buildup in the elbow.

Next step after that will be to check the heat exchanger.

Thanks for the suggestions.

 

[SeaDogAK]

OK so a few ideas to continue troubleshooting. Thank you. I do feel there is less raw water flow out of the exhaust, so next step is to do what BruceK and Bmarler have suggested and check for flow obstructions. Easy enough to take the raw water hose off the exhaust elbow and look for both flow restrictions in the hose and potential soot buildup in the elbow.

Next step after that will be to check the heat exchanger.

Thanks for the suggestions.

If you start disconnecting the hose at the exhaust elbow and work your way back to the water pump disconnecting each joint in turn, you’ll isolate where there is a blockage. When this happened to me I had a massive plug of old impeller bits in the outlet from the water pump. Impeller bits can also collect in the end of the heat exchanger, or you can have scale blocking the impeller tubes.

 

[Serene]

Spent a few hours today troubleshooting. Removed the sound shield to get access and then removed the hose from the exhaust elbow. The elbow was clear with no obstructions in the inlet. But I have very little raw water cooling flow.

Have already replaced the impeller and given low raw water flow have ruled out the thermostat.

Am thinking I need to clean the heat exchanger.

Question is, can I do some sort of chemical flush, or do I need to remove the heat exchanger to clean it properly?

 

[SeaDogAK]

You need to figure out what is blocking it. On mine it was old rubber impeller bits in the outlet from the water pump - just downstream from the impeller - that I could physically remove. They can also gather in the end of the heat exchanger. I would take both ends off the heat exchanger and inspect it to see if you can find a blockage. If there is gunk in the heat exchanger tubes you may be able to clear them with a metal rod. If it is scale you might be able to use a chemical cleaner.

 

[Insequent]

I'd first pull the intake end off the HX, as well as any other 'choke points', to look for physical blockages. But then get this stuff, dilute it and set up a recirculating loop (driven by an external pump) through the raw water cooling system including the HX.. No need for engine to be running. Best to stop the pump for a few minutes at a time to allow the acid to work better.
Barnacle Buster 5 litre
For doing my main engines the 5litre size was good. You may be able to use their 1 litre size bottle of it for you Onan.

 

[Serene]

Yeah understood on looking for physical blockages, but the reason I am thinking this is unlikely is because the generator has previously been running fine and when I changed the impeller there were no vanes missing. So don't think there are loose vanes anywhere in the system.

But the boat was out of the water for three months and has been sitting for another three months while finishing a refit. So six months of the same raw water sitting in the system with no circulation. Made me assume this may have caused calcium/growth build-up.

Focus is now on the input side of the heat exchanger. Will pull hoses and have a look. If nothing found, will need to try the chemical cleaning process.

I have the manual and have read up on the process and schematics so I know the steps required.

 

[Serene]

I'd first pull the intake end off the HX, as well as any other 'choke points', to look for physical blockages. But then get this stuff, dilute it and set up a recirculating loop (driven by an external pump) through the raw water cooling system including the HX.. No need for engine to be running. Best to stop the pump for a few minutes at a time to allow the acid to work better.
Barnacle Buster 5 litre
For doing my main engines the 5litre size was good. You may be able to use their 1 litre size bottle of it for you Onan.

I have ordered the Barnacle Buster and am getting all of the bits and pieces together to pump it through the heat exchanger.

Am thinking I will dump the BB into a bucket and use a cheap bilge pump connected directly to the generator start battery with alligator clips and a switch wired inline so I can turn it on and off.

Will get some 3/4" clear hose to connect to the bilge pump and input side of the heat exchanger and the output side back into the bucket. The generator raw water hoses are 3/4".

Question; will using a cheap bilge pump to circulate the BB from a bucket be suitable?

 

[Insequent]

As a temporary system it should be fine to use a bilge pump

 

[Serene]

As a temporary system it should be fine to use a bilge pump

Thanks. I won't leave it unattended while it circulates, so will be keeping an eye on things anyway.

 

[Greg QS]

mine runs solid 185 (85 c)

 

[Serene]

Problem solved!

Spent the morning on the boat. Removed the inlet and outlet hoses from the heat exchanger and connected up a hose from a bilge pump sitting in a bucket of Barnacle Buster to the inlet side, and then on the outlet side connected a hose back into the bucket.

Wired up the bilge pump with a switch and alligator clips to connect directly to the generator start battery.

First run and no fluid was pumping. Initially thought the cheap bilge pump wasn't powerful enough, but after blowing into the inlet side hose I realised it was completely blocked.

Removed the end cap off the heat exchanger and while there were no impeller vanes, there were two small shells in there, but not enough to completely block flow. Cleaned those out and also noticed some fouling in the heat exchanger.

Removed the outlet side and blew down that hose towards the exhaust elbow and no blockage.

But.....I did notice the outlet barb cast onto the side of the heat exchanger was completely blocked. Used a small screwdriver to remove the blockage. Re-connected the bilge pump hoses and was able to circulate the Barnacle Buster.

Let it run on and off for nearly an hour, then reconnected everything and ran the generator.

There is now a LOT more raw water flow out of the exhaust and the coolant temperature stabilised at 70c.

Ran the generator for 30mins under load with no issues.

Very happy with the outcome and thankful for all of the help offered here.

 

[Insequent]

Good outcome!

 

[Serene]

In hindsight I probably should have flushed it with fresh water before laying up. Lesson learned!