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Old 08-19-2020, 09:26 PM   #1
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Generator overheating blues

Okay so Iíve read a million threads here on what to fix on my Onan QD 8000 generator and canít figure out what to do next. Here are the symptoms:

Generator overheats and shuts off (giving the 33 overheating error code) consistently after 30-40 minutes under moderate load (running one AC) if the outside temps are above 85 degrees. If cooler than 85 outside it will run one AC just fine indefinitely, but will overheat if I run both ACís regardless of outside temps.

Hereís what Iíve done (following advise on this forum in other threads)

Changed oil and new oil filter
Flushed and changed coolant
New thermostat
New radiator camp
New air filter
New fuel filter
Generator is not on a slide so I just had Cummins diagnose and do new water pump and belt

They told me it was fixed and they load tested it (not good enough apparently) because nothing is any different. Exact same issues. Not even a little better (Cummins charged me $1,400!).

Iím at my wits end. I see people running their generators and ACs all day long in the campsites Iím visiting without issues so I know mine should be working better than this. Even the Cummins onan website says this model should run two ACís and more without issue.

I donít think itís the temp sensor (although I have one on hand just in case) because normally it wonít run more than a minute if the sensor is bad, and since it runs longer, then shuts off when overheating that tells me the sensor is doing its job. Temp gun verifies 220 degrees at the water pump and thermostat housing, so it is definitely overheating.

Anything else I can do????
Thanks so much for your wisdom!
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Old 08-19-2020, 09:47 PM   #2
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Have you measured radiator temp with your temp gun when it overheats?


(ETA: should be close to your measured 220 near inlet, much cooler towards outlet, with a gradient in between indicating flow pattern, with no discernible cold spots. Cold spots indicate sector blockage with reduced cooling capacity. And I assume your tech guy knows how to burp the air out of the system.)
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Old 08-19-2020, 10:13 PM   #3
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Have you measured radiator temp with your temp gun when it overheats?


(ETA: should be close to your measured 220 near inlet, much cooler towards outlet, with a gradient in between indicating flow pattern, with no discernible cold spots. Cold spots indicate sector blockage with reduced cooling capacity. And I assume your tech guy knows how to burp the air out of the system.)
Haven’t tried the temp gun on the radiator, if Cummins doesn’t figure it out tomorrow I’ll perform that test myself. And since normally I’m the “tech guy”, what’s the proper way to burp the system? All I read online is to fill with coolant, fill overflow with coolant, run the generator for a few minutes. Then top off coolant (adding coolant with hose fully extended upward). Is there anything I’ve missed?
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Old 08-19-2020, 10:17 PM   #4
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Originally Posted by Dan4Governor View Post
Okay so Iíve read a million threads here on what to fix on my Onan QD 8000 generator and canít figure out what to do next. Here are the symptoms:

Generator overheats and shuts off (giving the 33 overheating error code) consistently after 30-40 minutes under moderate load (running one AC) if the outside temps are above 85 degrees. If cooler than 85 outside it will run one AC just fine indefinitely, but will overheat if I run both ACís regardless of outside temps.

Hereís what Iíve done (following advise on this forum in other threads)

Changed oil and new oil filter
Flushed and changed coolant
New thermostat
New radiator camp
New air filter
New fuel filter
Generator is not on a slide so I just had Cummins diagnose and do new water pump and belt

They told me it was fixed and they load tested it (not good enough apparently) because nothing is any different. Exact same issues. Not even a little better (Cummins charged me $1,400!).

Iím at my wits end. I see people running their generators and ACs all day long in the campsites Iím visiting without issues so I know mine should be working better than this. Even the Cummins onan website says this model should run two ACís and more without issue.

I donít think itís the temp sensor (although I have one on hand just in case) because normally it wonít run more than a minute if the sensor is bad, and since it runs longer, then shuts off when overheating that tells me the sensor is doing its job. Temp gun verifies 220 degrees at the water pump and thermostat housing, so it is definitely overheating.

Anything else I can do????
Thanks so much for your wisdom!
Everything you list there can be done without taking the metal housing off the generator, except for maybe the water pump and belt. Do you know if Cummins took the housing off and did any sort of inspection?

It's very time consuming to take all the bolts off and remove the housing but there is a chance that a bird or animal has nested up in there. That would restrict the air flow and cause overheating.

I know from experience that you can't run the generator with a moderate to heavy load, without the housing on it or it will overheat. It just doesn't get the air flow needed.
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Old 08-19-2020, 10:39 PM   #5
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Originally Posted by windsorbill View Post
Everything you list there can be done without taking the metal housing off the generator, except for maybe the water pump and belt. Do you know if Cummins took the housing off and did any sort of inspection?

It's very time consuming to take all the bolts off and remove the housing but there is a chance that a bird or animal has nested up in there. That would restrict the air flow and cause overheating.

I know from experience that you can't run the generator with a moderate to heavy load, without the housing on it or it will overheat. It just doesn't get the air flow needed.
You are correct about airflow (from what Iíve read).
I took all the covers off when I did the thermostat and verified temps with the temp gun (kept covers on with a couple bolts until it overheated, then pulled covers off real quick to check temps). I also had previously inspected the belt with covers off and I never saw any internal blockages like a bird nest or anything. It was surprisingly clean in there. Because itís not on a slide, Cummins had to pull the generator out of the motorhome entirely to replace water pump and belt and yes you need to take all the covers off for that job as well, so I assume they wouldíve inspected everything. I told them to diagnose the overheating and they said water pump was leaking (I never saw the leak in my inspections, maybe it was internal?). But nonetheless the problem persists.
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Old 08-19-2020, 11:25 PM   #6
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But nonetheless the problem persists.
If you're running fine on no-load but overheating under load, you're shedding some heat but not enough. The waterpump would be a natural guess to start. (Not a cheap guess, sadly.)

I don't know if you can spot check the radiator temps across the whole surface - without a slide, I don't know if you can get in there with the genny in place - but an internal radiator blockage would cause this problem (as would low air flow, per the panel suggestion.) If you cut out a portion of the radiator surface area available for heat exchange, what you describe is exactly what you get.

Burping varies for each engine configuration, and an air bubble seems less likely given you had the problem both before and after a coolant replacement, but make sure you run it for 5-10 minutes under load and at high rpm and then top it off. Just idling it sometimes leaves air in the loop. If you have the coolant fill that pulls out, hold it up high as you top it off.

Has it ever run correctly? And, how old is it?
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Old 08-20-2020, 02:27 AM   #7
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Dies it act the same sitting as while driving down the road ?

Could the fan be on backwards ?
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Old 08-20-2020, 03:33 AM   #8
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What year?


My two bits, sounds like coolant restriction. Block coolant passages or radiator. Flushing may help but be temporary like 12 months. I would look at the radiator, especially if the unit is older. Besides, sounds like you have replaced everything else in the cooling system.
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Old 08-20-2020, 04:42 AM   #9
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My 8000 was doing exactly as yours. It would run abt 30 minutes and shutdown. I was running in 108 temps at the time and it was no fun with AC

I replaced the following:
Thermostat
Radiator cap
Flushed system
Temp sensor

Afterward mine ran fine coming across the desert in 120 temps.
I think the temp sensor is your issue. I did a lot of research on this forum and from what I found the sensors is almost always the issue. Someone actually rigged up a second sensor with an A-B switch as a backup

Good luck and let us know what you find
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Old 08-20-2020, 05:29 PM   #10
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Another item to check other than the cooling system would be the exhaust. A partially plugged exhaust can cause overheating.
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Old 08-20-2020, 05:51 PM   #11
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Oprators Manual

Sounds like you have done everything the Manual suggests except check for overheated inverter heat sink.

Years ago, I had an automobile engine overheat due to a baffle inside the block that corroded away. Have not heard that one in modern engines.

Operators Manual
HIGH TEMPERATUREóFAULT CODE NO. 1
(Engine coolant or inverter heat sink temperature exceeded design limit)
HIGH ENGINE COOLANT TEMPERATURE FAULTóCODE NO. 33

(Engine coolant temperature exceeded design limit)

Corrective Action:
1. Check the engine coolant level and add coolant and repair leaks as necessary (Page 19).
2. Check for and remove any objects blocking the air inlet or outlet openings in the bottom of the genset.

3. Reduce the number of appliances connected at the same time. (Note that high altitude and high am-
bient temperature decrease engine cooling capacity.)

4. Clean and flush the cooling system to remove coolant passage fouling.
5. If the genset overheats only underway, see the coach manufacture regarding air baffles or other
means to direct cooling air into the genset.
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Old 08-20-2020, 10:47 PM   #12
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Thanks so much for the great replies! You’ve given me a few more things to check out. The motorhome is still currently with Cummins since the water pump didn’t fix the issue they are comping labor to pull it out again and fix it. This time they think it might be the radiator, which they sent to a local radiator shop to be inspected/cleaned, or replaced if necessary. I’ll keep you guys posted and I’ll mention these extra things to them for inspection tomorrow.
To add the information some of you requested:
I bought the motorhome in May of this year and the generator has always done that since day one (didn’t run it long enough or hard enough during the test drive apparently). It’s an 05 model, only 800 hrs, so it hasn’t been run a whole lot and was likely poorly maintained (which is why I didn’t mind doing all easy fixes, it probably needed it anyway). The generator does run better if the coach is moving, but will still overheat if running both AC’s sitting or moving.
An interesting note I learned today from Cummins, yesterday when they “load tested it” after they replaced the water pump, they load tested it to 90% load without issue OUTSIDE THE CHASIS, so they put it back in and called it good. This morning after I told them it was still overheating, they load tested it INSIDE THE CHASIS and it overheated at 50% load. So if radiator cleaning/replacing doesn’t fix it, I’m guessing it’s an airflow issue of some kind.
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Old 08-21-2020, 05:23 PM   #13
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Well guys, itís worst case scenario. Old radiator did have blockages, put in new radiator, still doesnít work right. Air flow is fine, exhaust flow is fine, temp sensor is fine, inverter heat sink is fine. Cummins now believes at this point there is a blocked coolant passage somewhere in the engine block. The cost to strip down and soak the block then put back together will be more than the cost of a new generator ($10k). So basically I just paid $2,500 to have all new parts put on a generator that has the same issue as when I started and canít be fixed.
Thanks for all the suggestions guys. Please let me know if there is anything else I can try. For now I just told them to put it back in and Iíll just deal with it working at about 30% capacity because I donít want to spend another $10k right now.
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Old 08-21-2020, 06:56 PM   #14
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Cummins now believes at this point there is a blocked coolant passage somewhere in the engine block.
Couple of options (that sort of depend on what the blockage consists of):

1). If you suspect the blockage is something other than rust: Go to a pool supply store and buy a couple gallons of muriatic acid. Disconnect the drained engine block from the hoses, plug the lower openings, and pour the acid in and let it sit for an hour, and then drain it and flush it several times. This will work if the blockage is sediment or precipitate or just basic chemical sludge. (If you have some sort of part that has slipped inside and is blocking things, this won't help. But it won't hurt, either.) Nasty stuff to work with - it's a strong acid - gloves, glasses, flush everything clean.

I wouldn't do this to a new engine, but I've rescued dying engines with it.

2). If you think rust is the culprit, Evapo-rust makes an engine block de-ruster they call Thermocure. Like the acid, you pour it in and let it sit for a specified time. Does a nice job of cleaning out rust blockages. Again, flush thoroughly. Multiple applications work best.

If you can find out from the tech what the radiator blockage seemed to be made of - rust or gunk - you can choose one of the above options. If no one knows, I'd start with the Thermocure. https://evapo-rust.com/thermocure/


(Rust blockage is what happens when people run engines with water instead of antifreeze.)
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