Overheating

[Jim Roe]

After running my Onan generator for several hours, it simply shut down. I pulled the sound shield cover and discovered that the air intake silencer was seriously damaged ( almost melted ). I checked the strainer, through hull, hose to pump, impeller and all seems to be OK. I left the cover off the pump to see if the impeller turned when engine cranked and it seemed to work fine. I assume the engine overheated.

The engine starts and runs for 3 to 10 seconds and then shuts down. The one thing I did not do is pull and check the hose that leads from the pump to the engine block.

Does anybody have any suggestions as what I should do next or is it time to call a mechanic?

Jim Roe

 

[Bob Bradley]

I'm presuming you've already checked the coolant level to be OK.

First thing I'd check is to see if water is coming out the exhaust port on the side of the hull. That will tell you if the raw water pump is working. If it's not, check to see if it has just lost its prime. To do that I remove the outlet hose from that pump and start the genny. If the water starts pouring out of the pump, put the hose back on while it's running (be careful of the pulley and belt) and see if it now comes out the thru hull exhaust port. The problem will most likely recur when you shut off, but at least you'll know that it is the raw water pump.

If, on the other hand, there is water coming out the exhaust port now, it may be the circulating pump or the thermostat failing to open. With the engine cool, take off the coolant cap and start the engine. After a couple minutes, the engine will warm up and warm water should begin circulating into the heat exchanger. You can stick your finger in there and feel the temp change. If the engine starts to heat up, but the coolant remains cool, it's either your pump or thermostat. Take the outlet hose off the pump like you did on the raw water pump and see if it's pumping. If it's not, then that pump is the culprit. If it is, it's most likely the thermostat.

That's at least a couple things you can check before you call the mechanic.

 

[chris piazza]

I think I would check the coolant side next, thermostat, water pump and coolant level. IS there any coolant in the oil? Is there any steam in the exhaust? Start here first and let us know

Chris
Superior Nights 53C

Sorry I just repeated what Bob beat me too

 

[bigbill]

did you check your oil? if the engine is cold when you start it and it only runs for 10 seconds, it doesn't sound like an over heating problem, the engine doesn't even get to operating temp. in that short of time. bigbill

 

[StratPlan61]

Does your genny have automatic shut-downs for High Water Temperature and for Low Oil Pressure? My new NL generator (12kw) had less than 20 hrs on it and both had to be replaced. I had the same symptoms, I couldn't find any other obvious problems. NL mechanic told me these get replaced 'all the time' because they fail often. ???

 

[Genesis]

Check the raw water pump first. Next place is the exhaust shower head - if the water can't get OUT, you'll overheat too.

Those are the two most likely issues.

 

[Beckytek]

Did you check the belt that drives the water pump? My genny did the same thing last week, ran for about 2 minutes and started smelling like something was burning. I shut it down and couldn't start it again. Checked the belt and it was loose. Tightened it and after it was cool started right up and ran. Go for the easy stuff first. Ron

 

[Jaxfishgyd]

Well I see I"m not alone with genset "issues"....

 

[Jim Roe]

Thanks to everyone for the feedback. Since I can not get the engine to start other than just for a few seconds, I can not determine if water is being discharged. I did check the oil and that is ok. The gen is an older model with 3500 hours on it so I do not know if the unit has auto shut down etc.

I will try disconnecting the hose on the back side of the impeller and see if water is flowing out. I won't get a chance to do this until early August since I am traveling.

I will pass on the information at that time.

Thanks again

Jim

 

[Nonchalant1]

First, can you determine what caused the air intake silencer to get so hot? Maybe your preheat switch was stuck on and the preheat elements were too hot?

On keeping it running, if it is like the Onan MDJF series, did you push the red reset button? Maybe the low oil pressure sensor had tripped, which shuts down power to the fuel solenoid. Then, it'll only run for a few seconds because your "start disconnect" switch is letting it start even though your low oil pressure shutdown switch is stuck closed. The oil pressure switch is a sensor switch in the fuel solenoid electric supply line. The switch is normally open (will not pass current) and when it senses low oil pressure it switches to closed (will pass current) and the current heats a coil around the reset switch to make the reset switch open (won't pass current) shutting down the fuel solenoid which shuts down the engine. When you start the engine, a separate "start disconnect" switch overrides the circuit to let it start even though there is low oil pressure. When it reaches normal RPMs, the start disconnect switch stops working and the low oil pressure switch shuts down the engine again.

This can be tested by finding the low oil pressure switch and disconnecting the wire leading to the reset switch in the control box. Check for oil level first. If it continues to run for 30 seconds, shut it down and replace the low oil pressure switch.

Doug Shuman

 

[REBrueckner]

I am impressed with nonchalants description of the start sequence above...WELL DONE, DUDE!!!!
I've been though it with my owners manual staring back at me, but I sure don't remember all the pieces...

One other thing easy to check: In my cases, the "reset" (also called the crank limiter) red button device burned up several times over several years.... and replacements...when it did that, I believe my unit would not crank over...in any case, if you open your control box cover and look in, you'll see the burned coil on the outside of the of the device along with some melted plastic alongside and the ugly burned plastic odor if you stick your nose close enough....If it has melted, you must find the exterior cause....in my case it was closed points, described in another post nearby by....

 

[bobk]

The preheaters will not get hot enough to distort or melt anything within the sound box. But something did and you need to look to see if there is any tell tale discoloration or burning.

What is the history on the impellers. Have they been changed over the years and the broken blades removed? It's easy to leave them behind up against the HE which impedes water flow. Backflush the HE to see what comes out.

The short run after starting really suggests the unit did shut down for hi temp and was not reset, so it is starting on the over ride switch which allows only a brief run time. Find the reset then try to start. If it runs longer, you can now diagnose the issue. BTW, use an infrared thermometer to check for temperatures not you finger.

Bob

 

[Nonchalant1]

The preheaters will not get hot enough to distort or melt anything within the sound box. But something did and you need to look to see if there is any tell tale discoloration or burning.

What is the history on the impellers. Have they been changed over the years and the broken blades removed? It's easy to leave them behind up against the HE which impedes water flow. Backflush the HE to see what comes out.

The short run after starting really suggests the unit did shut down for hi temp and was not reset, so it is starting on the over ride switch which allows only a brief run time. Find the reset then try to start. If it runs longer, you can now diagnose the issue. BTW, use an infrared thermometer to check for temperatures not you finger.

Bob

My preheater switch (MDJF) got stuck on once and it melted the air intake hose material which dripped onto the heating element, making a smell which alerted me to the problem. They do get that hot.

Doug

 

[bobk]

My preheater switch (MDJF) got stuck on once and it melted the air intake hose material which dripped onto the heating element, making a smell which alerted me to the problem. They do get that hot.

Doug

Interesting, I had a 1986 8KW (3 cyl Kubota) which was wired wrong at the factory and was always on. It took out the batteries, but the area in the box was just warm. Hope I didn't mislead the investigation.

Bob

 

[Jim Roe]

OK, I am back to my boat and did what I should have done in the first place. I pulled the heat exchanger and took it to a radiator shop and had it cleaned the proper way and guess what; everything is working great! I can't believe I didn't do that in the first place. Thanks to everyone who responded.

Jim