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Old 07-28-2021, 03:20 PM   #1
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THOR #21234
Onan QG 4000 stops running, possibly heat related?

I've done a lot of research on this, and I know it is a common problem for these to sometimes stop running in hot weather. This seems to be what mine is doing, as it only started when the temps here got into the 80s, however, once it stops, I can start it up again, and it will run fine for another hour or so before stalling again.

I've changed/checked the oil level, replaced the spark plug, and even opened the door to provide better ventilation. Airflow seems to be fine, as I cannot seem anything blocking the fan/vents. I thought the plug had fixed it, as yesterday, I ran the genny from 11am to 4pm with no issue, however it did stall around 5. I am boondocking and working remotely, hence running it all day.

I assume this is heat related vapor lock, but if that was the case, shouldn't it fail to start right back up?

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Old 07-28-2021, 04:43 PM   #2
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Heat does play a part in it shutting down if there isn't enough air circulating. I found that I need to keep a close eye on the oil level more. If it is a bit low and it is very hot out, I get the shut down more often. If I make sure to keep it topped off, less problem with it shutting down. Mine is a diesel generator.

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Old 07-28-2021, 07:03 PM   #3
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THOR #1469
In most cases temps in the 80s won’t impact the generator. Might be something else wrong.

Have you tried running the gen with the compartment door open and see if it shuts down?
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Old 07-29-2021, 02:17 AM   #4
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THOR #21234
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Originally Posted by EA37TS View Post
In most cases temps in the 80s won’t impact the generator. Might be something else wrong.

Have you tried running the gen with the compartment door open and see if it shuts down?
Yup, and that didn't seem to matter.

It's odd that it will run sometimes 5 hours before it does, other times 2. Today it ran fine from 11 to 2 and then stopped. I restarted it, and it ran a couple more hours before stopping again.
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Old 07-29-2021, 04:14 AM   #5
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Maybe try using a box fan to move some air around it? Prop the door open so there's no direct sun.

I'm not sure changing the spark plug would make a difference if it had been running smoothly for several hours?

That's the generator I have, but never had any issues... yet. I read somewhere that the oil level is finicky... too much OR too low can both cause shut downs. Is there an error code displayed on shutdown?
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Old 07-29-2021, 05:18 AM   #6
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Maybe the fuel pump stops for no good reason... You could remove it and the fuel filter and giving them a once over. For the 'possible' vapour lock, the manual suggests looking for a pinched fuel line.
Also, are you running straight 30 oil (for the higher temps)? Is the carb set for your altitude?

Just little things to consider with a inconsistent problem...
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Old 07-29-2021, 05:48 AM   #7
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We have had ours shut down due to heat but never in the 80s . The last time ours did it was 115 out when we were crossing central Washington but it took almost 5 hours of that heat for that to happen.
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Old 07-29-2021, 01:00 PM   #8
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THOR #21234
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Originally Posted by Chateau_Nomad View Post
Maybe try using a box fan to move some air around it? Prop the door open so there's no direct sun.

I'm not sure changing the spark plug would make a difference if it had been running smoothly for several hours?

That's the generator I have, but never had any issues... yet. I read somewhere that the oil level is finicky... too much OR too low can both cause shut downs. Is there an error code displayed on shutdown?
Spark plug was due to be changed anyhow, but was hoping it might help.

I thought it was oil level as well, as I had filled it up to full. I removed some to where it is down to half, but had no change.
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Old 07-29-2021, 01:03 PM   #9
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Originally Posted by OldWEB View Post
Maybe the fuel pump stops for no good reason... You could remove it and the fuel filter and giving them a once over. For the 'possible' vapour lock, the manual suggests looking for a pinched fuel line.
Also, are you running straight 30 oil (for the higher temps)? Is the carb set for your altitude?

Just little things to consider with a inconsistent problem...
Fuel filter was next on my list of things to check/replace. I am running SAE30, but I have not set the carb altitude, as I am not in a high altitude area.
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Old 07-29-2021, 04:30 PM   #10
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THOR #13932
You may have an intermittant fuel blockage.
Neighbor told me of a problem similar.
A piece of plastic occasionally blocked his fuel pickup.
He'd turn the car off, pump suction would release the plastic sheet, he was good for hours again.
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Old 08-04-2021, 07:40 PM   #11
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THOR #2925
Cummins Service

I played this same game with my 4000 for a few months. Ended up taking it to a Cummins service center they even took a bit of time ended up pulling the generator out to find a faulty ignition coil that would get hot and stop working. On cool days it ran like a champ no problems only was a problem 80 degrees and up. Unit now runs perfectly in 100-degree weather all day and night in the south. The coil was less than $50 labor was a big number. I am not sure if it can be changed without pulling the generator.
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Old 08-04-2021, 08:01 PM   #12
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The carb gums up easily and can also cause this problem. Onan sells a carb cleaner part # 326-5342. Problem is your 80 gallon gas tank feeds the genset, so get a 1/4 ID fuel hose about 3ft long, disconnect the main before the fuel pump , plug off, hook your 3ft hose on, stick in a 1 or 2 gallon gas can with the treatment in it and run for an hour under a load. Before you store the RV you should do the same, also drain the carb.
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Old 08-05-2021, 01:28 AM   #13
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Is the light flashing a code and if so, what is it? That might lead you to a reason and a solution.
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Old 08-14-2021, 09:55 PM   #14
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Im having a similar problem but mine shuts off while driving. Runs fine sitting still. Any suggestions?
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