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?

[Chum_lee]

Chris Bryant wrote:
If it started suddenly, I would bet on a broken valve component- spring or rocker arm.

Yes, I've seen that (and/or a tight valve clearance) on 5.5, but IMO, it would happen all the time, even when cold. How does that explain this: "It began backfiring and then dying when a load was placed on it. . . . . . until it dies then gives an overheated exhaust code."

I'm assuming that later after cooling off, it runs (semi) normally when warming up and NOT under load. A compression check and a look at the valve train/clearance is in order. Interesting issue. This is a NEW generator!

Chum lee

[Chris_Bryant]

If it started suddenly, I would bet on a broken valve component- spring or rocker arm.

[Chum_lee]

lakecityemt wrote:

Chum lee wrote:
"I assume from the backfiring?"

I can't see it or hear it run online. That said:

Backfiring implies that unburned mixture IN THE INTAKE tract is being ignited and burned. (not the exhaust) Can you confirm that? Backfiring is a classic condition of a lean mixture which would point to a vacuum leak somewhere in the intake. Are all the breather hoses connected properly? Is the (new) carburetor properly seated on the manifold with all the gaskets? When hot, a lean mixture can lead to pre-ignition/detonation which may sound like a backfire.

What happens if you manually force the choke on with the engine warmed up, loaded, and backfiring? (richen the mixture) Any improvement?

Do you have a compression tester? If so, does the engine meet the compression specs?

Chum lee

Chum you might be a bit over my head, LOL... I will ask him to try and manually adjust the choke when its backfiring. No compression tester.

OK let's try something simple. Your carburetor should have an altitude compensating adjustment screw. It has some control of your fuel mixture adjustment which should be set for your current altitude. It that set correctly? Having it set too high for your current altitude can create the symptoms you have. For example: If you are currently at 1500 feet altitude, adjust (turn) the screw so that the pointer is between 1,000 and 2,000 feet. Look at the screw adjustment on the carburetor, it will be obvious.

Chum lee

[twodownzero]

MEXICOWANDERER wrote:
It points to a fuel starvation issue. Classic symptoms.

It's classic for a lean condition, generally caused by fuel starvation, but he could be sucking air around the throttle somehow as well. All air must pass through the carburetor or the fuel mixture will be wrong.

Verify proper compression, if ignition timing is adjustable check that, and if that is good, go back to focusing on fuel system issues or vacuum leaks.

[lakecityemt]

grizzzman wrote:
When I had that happen to my onan,I found a broken valve spring. Worth a look. ( it had about 150 hours on it.

Thanks, sadly not something we are capable of fixing

[grizzzman]

When I had that happen to my onan,I found a broken valve spring. Worth a look. ( it had about 150 hours on it.)

[lakecityemt]

Chum lee wrote:
"I assume from the backfiring?"

I can't see it or hear it run online. That said:

Backfiring implies that unburned mixture IN THE INTAKE tract is being ignited and burned. (not the exhaust) Can you confirm that? Backfiring is a classic condition of a lean mixture which would point to a vacuum leak somewhere in the intake. Are all the breather hoses connected properly? Is the (new) carburetor properly seated on the manifold with all the gaskets? When hot, a lean mixture can lead to pre-ignition/detonation which may sound like a backfire.

What happens if you manually force the choke on with the engine warmed up, loaded, and backfiring? (richen the mixture) Any improvement?

Do you have a compression tester? If so, does the engine meet the compression specs?

Chum lee

Chum you might be a bit over my head, LOL... I will ask him to try and manually adjust the choke when its backfiring. No compression tester.

[lakecityemt]

I will try that, if it will run under a load, that is the whole problem, LOL...

[Dutch_12078]

lakecityemt wrote:
185 - My dad was not aware of their being a spark arrestor. In the visible portions none were found, its possible its in an area he could not see.

As 185EZ said, the cleanout plug is at the bottom of the muffler. Unscrew the plug and run the generator under load for about 5 minutes.

[lakecityemt]

He did remove a square plug, we will try that again with the new plug

[Chum_lee]

"I assume from the backfiring?"

I can't see it or hear it run online. That said:

Backfiring implies that unburned mixture IN THE INTAKE tract is being ignited and burned. (not the exhaust) Can you confirm that? Backfiring is a classic condition of a lean mixture which would point to a vacuum leak somewhere in the intake. Are all the breather hoses connected properly? Is the (new) carburetor properly seated on the manifold with all the gaskets? When hot, a lean mixture can lead to pre-ignition/detonation which may sound like a backfire.

What happens if you manually force the choke on with the engine warmed up, loaded, and backfiring? (richen the mixture) Any improvement?

Do you have a compression tester? If so, does the engine meet the compression specs?

Chum lee

[185EZ]

Look underneath the muffler
Probably a square 1/4" pipe plug
Remove it and run the gen

Could be a lean condition if the plug porcelain was white but you just changed plugs.
The tip was black so maybe it's more ignition related, wire, coil, circuit board etc.

Check your altitude adjustment

The fact you replaced carb and pump and filters points more to ignition or lack of.

[lakecityemt]

185 - My dad was not aware of their being a spark arrestor. In the visible portions none were found, its possible its in an area he could not see.

[185EZ]

Just throwing stuff out there
Have you pulled the plug on the bottom of the muffler to clean the spark arrestor?

[lakecityemt]

Mexico - New fuel pump and filter, of course I could have faulty parts... again....

I'm thinking we have done as much as we can without a professional and help to remove the generator. I was hoping there could be an answer that was simple, but its not looking that way 😞