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Onan 5500 Backfires and Dies under a load

lakecityemt
Explorer
Explorer
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?
37 REPLIES 37

lakecityemt
Explorer
Explorer
185EZ wrote:
Is it still working?
Did you leave the arrestor plug out?


We are out of town after testing it with out the plug, it will be a few weeks before I get back to the RV.

185EZ
Explorer
Explorer
Is it still working?
Did you leave the arrestor plug out?

joebedford
Nomad II
Nomad II
185EZ wrote:
That sure seems like a lot.
I just measured mine. 13' from outlet on gas tank to inlet on gen on a 34' rv.
Maybe they put the tanks under the toy hauler garage to be used to fill up the toys too?
My toy hauler is 42'6". The generator tank is the rear tank right behind the tank for gassing the toys.

lakecityemt
Explorer
Explorer
185EZ wrote:
That sure seems like a lot.
I just measured mine. 13' from outlet on gas tank to inlet on gen on a 34' rv.
Maybe they put the tanks under the toy hauler garage to be used to fill up the toys too?


Yes mine has 2 tanks for gas, one for gen and one for toys. also 2 grays and a black, therefore i'm sure its a weight distribution issue.

185EZ
Explorer
Explorer
That sure seems like a lot.
I just measured mine. 13' from outlet on gas tank to inlet on gen on a 34' rv.
Maybe they put the tanks under the toy hauler garage to be used to fill up the toys too?

joebedford
Nomad II
Nomad II
185EZ wrote:
I doubt they would put a gas tank 35' away
The gas tank on my toyhauler is "only" 30 feet away from my generator.

lakecityemt
Explorer
Explorer
185EZ wrote:
lakecityemt wrote:
Great thought, but instead of it pulling 35 feet from the tank its now pulling it less than 2 feet. Problem also happened with fresh gas from the tank.

No,
electric fuel pumps are placed inside tanks or below the tank especially in high performance applications
Your pump would do better being level pulling 35' than trying to pull 2' from below.
I doubt they would put a gas tank 35' away
Why are you using a separate tank?


This is how it is set up in my toyhauler. THe Genset is under the front bedroom, the tank is in the very back under the garage. 41 foot toyhauler. Unless there are 2 fuel pumps, we just replaced the one at the genset thinking that could be the problem. It was not. It now appears it was the spark arrestor.

185EZ
Explorer
Explorer
lakecityemt wrote:
Great thought, but instead of it pulling 35 feet from the tank its now pulling it less than 2 feet. Problem also happened with fresh gas from the tank.

No,
electric fuel pumps are placed inside tanks or below the tank especially in high performance applications
Your pump would do better being level pulling 35' than trying to pull 2' from below.
I doubt they would put a gas tank 35' away
Why are you using a separate tank?

time2roll
Explorer II
Explorer II
lakecityemt wrote:
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!!
Backfire through the exhaust is often too rich. Backfire through the carb is too lean.

If no ONAN service I would look for a lawn mower or small engine repair shop.

I would be tempted to remove the carb and apply fuel pressure and see if it leaks. Rich will tend to foul the plugs black and lean will have them normal to extra white.

If an ONAN tech sees your message they will want the exact model number and serial number to avoid playing 20 questions.

lakecityemt
Explorer
Explorer
Great thought, but instead of it pulling 35 feet from the tank its now pulling it less than 2 feet. Problem also happened with fresh gas from the tank.

185EZ
Explorer
Explorer
Yay!
Also, electric fuel pumps aren't good for drawing or sucking fuel.
Having the tank below the pump isn't good

lakecityemt
Explorer
Explorer
185EZ wrote:
lakecityemt wrote:
For the past year I have been searching for answers and replacing parts with no improvement.

What year is this toyhauler?
Did it run right the previous year?
Have you checked that it has proper gas flow from the tank?
Have you tried loosening gas cap while running?


2013
Been trying to fix since it acted up 3 years ago... Dad visits once a year and tries to fix it while here...
New fuel pump and we are not running from the tank, directly from a fresh gas can below the generator.

Spark arrester removed and now it runs under a load.

lakecityemt
Explorer
Explorer
Chum lee wrote:
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


Hey Chum,

The altitude adjustment is correct.

We removed the plug and now it runs properly under a load!!
Now what? LOL Can I leave it out? Probably not if its a spark arrestor

185EZ
Explorer
Explorer
lakecityemt wrote:
For the past year I have been searching for answers and replacing parts with no improvement.

What year is this toyhauler?
Did it run right the previous year?
Have you checked that it has proper gas flow from the tank?
Have you tried loosening gas cap while running?