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Old 07-12-2012, 11:29 AM   #1
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Onan Generator shutsdown

I'm having trouble with Onan 7.5 quiet diesel generator. Generator shuts down with coolant overheat code. Runs for about 20 min under load until the return temp to the pump reaches 165 F. So far I have, replaced the temp sensor, had the radiator cleaned by a radiator shop, and tested the thermostat. Water pump belt in good shape nothing slipping. I dont have any idea where to go from here. Amy help before espensive trip to Onan dealer would be appreciated. Pfeiffrw
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Old 07-12-2012, 11:34 AM   #2
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Not familiar with that specific gen but is there a cooling fan either pushing or pulling air across the radiator? If not I would add one. Also, where the temp sensor connects I'm assuming it screws into a coolant passage. If so did you make sure there wasn't any crud clogging it up keeping actual coolant from coming in contact with the sensor?
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Old 07-12-2012, 12:20 PM   #3
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Temp sensor passage was thoroughly cleaned when new one was installed. Yes there is a direct drive cooling fan that pushes air across radiator. No blockages
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Old 07-12-2012, 02:18 PM   #4
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The most common cause of genset overheating is an airlock in the cooling system from improper bleeding following coolant replacement or service. I don't know anything specific to that model but check for some arcane procedures that may be required to get it properly bled.
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Old 07-13-2012, 11:25 AM   #5
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Good point. This is very common in cars and trucks also. The easiest way I've found to make sure all air is out of system is to install a flushing tee in one of the heater hoses and keep the cap off while filling. This allows air to escape since it's a high point. You could try installing a tee or disconnect a coolant hose that is at the highest point in the cooling system and fill through that hose if possible. Even if you cant fill through that hose, filling while the high point is vented should help "burp" the system.
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Old 07-13-2012, 12:37 PM   #6
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The heater hose is good advice for a vehicle but not particularly relevant for a genset. Some gennies are inherently hard to get all the air out and the guys that know a particular set will be the ones to give the best advice. In general you want a bleed point at the highest level in the system. Some sets have such convoluted plumbing that one bleed point isn't sufficient. Air lock is the most likely cause of the OP's troubles but remedying the airlock may not be dead simple. It may take a specific sequence for the bleeding in order to avoid simply moving the airlock from one high point to another.
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Old 08-06-2012, 04:40 PM   #7
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Onan overheat

I have the same problem if the outside temp is over 85.
Onan coach service in Harrisburg, PA has replaced the temp sensor, thermostat, water pump and cleaned the radiator to no avail. So far they have charged me over $1,300.00 and I have not been able to get off their parking lot without the system stopping after 15-20 mins.

Is the next step a new generator or a new service facility. Onan will not stand behind their service. Their answer is give us more time to troubleshoot at your expense.
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Old 08-06-2012, 06:23 PM   #8
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I just changed the coolant on my QD8000. In the process I found a few things that I didn't expect. I thought it was a simple matter of opening the bottom drain and replacing the fluid. Opening that drain only drains fluid in the lower part of the engine below/after the thermostat. To get the rest out, you need to run the generator to get the thermostat open to drain. I tried that however it shut off - overheated - as the fluid drained to the lower part of the engine but did not siphon more fluid in from the coolant reservoir. I noticed a panel beside the breaker that could be removed to access the radiator fill hose. I found that the rad hose was collapsed and an air lock existed between the coolant reservoir and the engine. I manually filled that part of the hose and left the hose extended out of the opening. I ran the generator again and it was fine and siphoned some fluid from the reserve tank. If you are having shutdown problems, I suggest you check the rad fill hose. When extended, it sticks about 3 inches beyond the panel and must be compressed significantly to get the panel cover back on. That could cause an airlock and shutdown. I'm not really sure that I have properly described what happened as I am not overly mechanically inclined however it might provide a possible clue to the shutdown.
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Old 08-11-2012, 05:12 PM   #9
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I have heard of Bobmac"s problem causing this and also another one: bad electric Fuel Pump. Evidently, several Onan 7.5 Quiet Diesels have this problem. After the gen. warms up, or if it is hot outside, the electric fuel pump flow decreases enough to stop the gen. Replacing the fuel pump fixed this on other threads I have seen. Don't know if this applies to your problem.
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