Onan 4000 generator cranks but won't start

Hi all,

I have a 2015 Cummins Onan RV QG 4000 generator. It has somewhere between 300-1000hrs (not sure, I've only owned it for about 2 years and it was a previous rental...might not be the original generator on this RV and I don't trust the hour meter coming from the rental company). About 1 year ago, I replaced the oil, air filter, and spark plug. Since then there has been 50-100 hours. Up until recently it has been working fine.

Recently, it won't start. It cranks and cranks very strong, but doesn't start.

To troubleshoot so far here is what I have done to no avail...
- I have a full tank of gas in the motorhome.
- The motorhome is level.
- Altitude adjustment set to 0 (I am near sea level).
- I checked the oil to make sure there wasn't too much (it is to the low end on the dipstick as most seem to recommend).
- I swapped in a good spark plug to make sure that wasn't the problem, and it didn't fix it.
- I checked the air filter and it seems fine (...even tried starting it without the air filter and it didn't start).
- I changed the fuel filter with a new one, primed it and tried starting it several times and that didn't fix the problem.
- I disconnected all the batteries (and my solar) per someone's suggestion to "reset" the generator. Hooked everything back up and it still didn't fix the problem.

It still cranks very strong...just doesn't start.

The only error code I could get from the lights was '4' (which is over-cranking). I don't think in all the testing that I did I ever let it crank for more than 10-20 seconds at a time.

I have reached the current extent of my generator knowledge! lol.... I'm hoping some of you can help me expand that knowledge with some more suggestions/recommendations.

One thing I've learned about owning an RV (...I actually have 3 now) is that it makes sense to try to learn to fix as much yourself as possible because RV mechanics are a)busy, b)expensive, and c)a hassle to drop off/pick up/etc. Plus, it's fun/satisfying when we can fix our own rigs and not be as dependent on others.

Thanks in advance!
Chris

smarty
Explorer II
Jul 23, 2021
there are several great videos on utube that show exactly how to troubleshoot what you are noting
Dusty_R
Explorer
Jul 23, 2021
When you checked the spark plug, did you also check for compression?
It takes 3 things to make a gasoline engine engine work, compression-fuel-spark.
Grit_dog
Navigator
Jul 23, 2021
It appears you have spark, although I’d expect yellow/blue spark.
Also has to if it backfired and lit the ether on fire.
Disconnect fuel line from carb. Prime the pump. Do you hear the fuel pump and are you getting fuel?
Mostly likely cause of well maintained runs fine suddenly not running is the fuel pump. You can see the fuel pump in the generator. It’s in line in the fuel line coming from the tank.
Quick fix, go buy a little electric fuel pump for carburetor from the parts store. They’re relatively universal and about $30-40. If your fuel pump isn’t pumping.
Something else could be blocking fuel from the tank. Not super likely but try sucking fuel through either by mouth with an extension of clear hose so you see before swallowing
Lol. Or a vacuum pump like for changing oil.
jdc1
Explorer II
Jul 23, 2021
OP says he's getting spark. It's carbureted, so we know it's getting air. He tried starting fluid. All that's left is compression and timing. Check the compression ($15.00 tool). If you have 80+ psi then you are okay. Move on to timing problems. If you have less than 80psi, you have a huge problem.

Of course, he could have a faulty oil pressure sensor that won't allow it to start.
docsouce
Explorer II
Jul 23, 2021
Take the front cover off prime it for a minute or so then push the choke closed with your finger and hit the start while holding the choke closed. If your not sure which way is closed try one way and if it doesn't fire try the other, or take off the air cleaner and look down the throat of the carb. This is the only way my Onan will start when cold.
Snowman9000
Explorer
Jul 23, 2021
Agree with the previous post. The two I’ve owned were bad cold starters. The choke seems to hang up. You can turn the choke closed using the linkage connector on top of the carb while cranking.
SJ-Chris
Explorer II
Jul 23, 2021
enblethen wrote:
When you removed the spark plug, was it wet, dry oily?

The spark plug wasn't wet. It did have some black carbon on it. I have another RV with identical Onan generator that is working fine. I pulled the spark plug from that one and tried it in this generator that isn't working. Still not working. I did the spark test and it is generating a rapid spark as it is cranking.
enblethen
Nomad
Jul 23, 2021
Did you try disconnecting the remote start?
Have you tried stating from remote start?
SJ-Chris
Explorer II
Jul 23, 2021
Grit dog wrote:

Disconnect fuel line from carb. Prime the pump. Do you hear the fuel pump and are you getting fuel?

The fuel line going to the carb is connected with a hose clamp I'm not familiar with. Here is a photo:

Can someone tell me the proper way to remove this clamp? I don't want to just man-handle it to get it off.

Is it okay to replace that clamp with a standard hose clamp from home depot (...the kind with the screw to tighten)?

When I do remove this hose, if I push the primer button should I see gas spurt out of this hose?

When cranking, and if the fuel pump is working, I guess I should see fuel squirting out of this hose when disconnected, right? If not, then it seems like a bad fuel pump (since I know my fuel filter is brand new) right?

Let me know.
Thanks!
Chris
SJ-Chris
Explorer II
Jul 23, 2021
enblethen wrote:
Did you try disconnecting the remote start?
Have you tried stating from remote start?

Is the remote start switch simply the start button inside the RV? I have the same result (...just lots of cranking) from either start button.

Now does one disconnect the remote start? Could this actually be causing the issue (a bad remote start switch)? I'm no expert, but that doesn't seem likely to me (but if it's easy to disconnect I'm willing to try).

-Chris