Vapor Lock

Vapor lock is caused by gas boiling in the gas line.
If the pump is at the engine then the line back to the tank is sucking fuel. The suction lowers the atmospheric PSI and lowers the boiling point. Higher altitude lowers boiling point.

In tank pumps help so much because now the fuel is pressurized the entire length of the line which RAISES the boiling point.

Winter gas has a lower vapor/boiling point so if you use it in the summer you are asking for trouble.

It could be Vapor lock or a cracked fuel line.

wa8yxm wrote:
...Vapor lock happens with the fuel pump is pumping vapor only, no gas, this requires several things. One is heat and another is no fresh (cool) gasoline entering chamber to cool it.

THIS can only happen if the pump is engine mounted (Electric pumps mounted at a distance tend not to vapor lock,, in tank pumps (not applicable here) DO NOT Vapor lock, ever) and the motor off so not drawing gas.. once it is running vapor lock is out of the question...

I'm not sure exactly what you are saying here, but this could definitely be vapor lock. Unfortunately, I have too much experience with vapor lock on my old coach and the symptoms he describes could be a result of vapor lock. I'm not saying it is, however.

We haven't heard from the original poster since last week. I hope he updates the post.

A dirty fuel filter will cause it to act out of gas after a short run, then allow more gas to enter, but again after a short run kill it..

Vapor lock happens with the fuel pump is pumping vapor only, no gas, this requires several things. One is heat and another is no fresh (cool) gasoline entering chamber to cool it.

THIS can only happen if the pump is engine mounted (Electric pumps mounted at a distance tend not to vapor lock,, in tank pumps (not applicable here) DO NOT Vapor lock, ever) and the motor off so not drawing gas.. once it is running vapor lock is out of the question.

But dirty filters.. Those cause just the issue you cite, along with fuel line issues discussed up-thread.

My genset aggravation was similar to yours. I had replaced the fuel pump, only to have the same problem occur. I wished I had ran a fuel hose and gas can to old pump 1st...that would of saved me some $$. Come to find the rubber fuel hose had a crack in a bend, causing it to suck air when it got hot enough. I replaced hose and it runs great.

A small air leak in the main fuel line to the generator will cause this exact issue. Had the same issue on my 2000 Onan 5500. Would quit after a few hours on the road in 100 degree temps. Cool off a little and it would run again.

A good test is to disconnect the fuel line at the generator, attach a shorter piece of fuel line and drop it in a gas can and see if it will run a couple of hours in the same temps. If it quits it's most likely the fuel pump. If not, look at replacing the line all the way back to the main tank.

Thought my issue was vapor lock as well, but it ran for several hours from the gas can. Replaced the line and had no other issues.

Do you hear any clicking sounds coming from the fuel pump?

esyers wrote:
My 5500 started acting the same way. Turned out to be the fuel pump. New pump and no problems since.

yea, I was wondering that myself. Just went out and started fine. Everything else has been fixed or rebuilt.

My 5500 started acting the same way. Turned out to be the fuel pump. New pump and no problems since.

The hose could be collapsing inside, that causes fuel starvation. When suction is reduced when genset shuts down the hose opens up and allows the fuel to get through.
I would still be looking at the hoses!

Deano56 wrote:

enblethen wrote:
Doubtful on vapor lock. I would start by checking all the rubber hoses for cracks especially around connectors. It could be sucking air causing it to basically run out of fuel.

I will check hoses in morning, i will tell you this I just started up and it was racing pretty high which I didn't think it could with the governor set for 1800, or am I missing something thinking that?

If it's running out of gas how does it start up again and run fine for a while