Onan 5500 Backfires and Dies under a load

[lakecityemt]

Good Morning.. Im hoping to find the Onan guru's that are here... For the past year I have been searching for answers and replacing parts with no improvement.

I have an Onan 5500 with less than 100 hours in my toyhauler. It began backfiring and then dying when a load was placed on it. I have so far replaced the carb, fuel pump and filter, fresh gas directly from beneath the gen. Once it starts backfiring, if the load is stopped, it continues to "put put backfire" until it dies then gives an overheated exhaust code (I assume from the backfiring?). Plugs were replaced, now currently testing to see if there is spark to both plugs. Please someone help me solve this issue!!! I'm in BFE Alaska in a perm set up, there's no ONAN tech here so I'm on my own... Thanks for ideas and suggestions!!

UPDATE: replaced one of the new spark plugs with another new (wasn't sparking well) and took out the plug in the exhaust, now it works great! Can I leave the plug out? Ive been told its the spark arrestor, sounds like an important part to me, HAHA. Could this mean there is a blockage in the tail pipe?

[K_Charles]

You changed 2 plugs and then you replaced one again. That says that is the cylinder to look at. Carb, muffler and gas are both sides, coil or plug wire or stuck valve most likely affect only one cylinder.

[MEXICOWANDERER]

It points to a fuel starvation issue. Classic symptoms.

[lakecityemt]

Choke is fully open when hot.

Spark plug did have a black end, the end where the gap is but the porcelain looked brand new. The guy at the parts store said it looked like it was running hot...

I question if its the spark plug wire...

My dad is here trying to help fix it, well last summer and this summer and he says he thinks its running rich, there is some dark smoke from the tail pipe when it runs. He questions something in the tail pipe? Have to take it out of RV to disconnect the tail pipe from what he says.

[185EZ]

seafoam doesn't fix problems
Seafoam contains kerosene, which is paraffin, which we all know as wax
Anything more than the recommended dose and it will cause more problems.

I doubt it's tight valve lash IF it ran fine for the first 100 hours

You replaced both spark plugs and then you replaced another because it wasn't firing?
Was the porcelain black?
If so you may have a running rich issue.
Can you see that the choke plate is fully open when hot?
Could be a spark plug wire
Need more info

[lakecityemt]

Thanks for the quick replies!

So I have tried Seafoam per the instructions, no help.

Checked altitude settings, its correct

No cummins dealer here but if I ever put her back on the road system, I will take it to cummins if its not fixed by then...

One spark plug was not firing properly, changed that and now it takes a small load (1 of 2 AC units) so thats an improvement!!! But not back to what it used to do!!

[PaulJ2]

Agree with above. Intermittent spark after warm up, or valve adjustment, (valve adjustment clearance to tight when warmed up).

[Big_Katuna]

Stabbing in the dark but maybe a sticky valve?
Get a spray can of Seafoam and follow the directions.

[2scootn]

Check this site towards the bottom (should be on page 2):

https://www.smokstak.com/forum/showthread.php?t=144848&page=2

One person had a valve adjustment done and it fixed the problem, another had the Cummins dealer say it was a bad coil.

We had similar issues on our Onan but it had to do with vapor lock. Also on other forums they mention to check the altitude setting.

Good luck!