Common Onan generator issue

[joedaddy805]

I noticed a lot of people have issues with the fuel pump on these. I was wondering if anyone has found out the real issue with the fuel pump failing. I was having issues awhile back when i would go camping in hot weather my gen would die. I noticed that every time it died the fuel filter I put in was empty. So I replaced the fuel pump like everyone else said and did. But went camping this weekend and now I'm back to the same thing. Hot weather gen turns off. Ive noticed others have had the same issue and was wondering if anyone has found a way to fix this bad design. Im gonna try a few different things to see if I can fix this issue, but was wondering if there was already something out there to fix it right away.

[eddieelk]

JoeDaddy805, my similar problem was caused by a collapsed rubber hose from the gas tank. Also, make sure you have your gas tank at least 1/2 full. Eddie Elk.

[joedaddy805]

Mine is shutting down cause my fuel pump overheats. Its a brand new fuel pump and acts just like my old one.

But when its midnight and i run my gen with the a/c till morning it runs great. But once the heat comes on it doesn't last long.

[pchuckie]

How do you know the fuel pump is overheating and not producing fuel pressure?

I don't disbelieve you that the problem revolves around a increase in ambient temp. I'm not saying don't trust your gut or follow a hunch, but follow through and prove your hunch right before you start swapping parts

You didn't mention what vintage or model generator you have. 5.5 Marquis, 4.0KY when it shuts down is it displaying a fault code? Likely 36 if it's fuel related.

I've seen more than one person remove an Onan fuel pump and take it to Napa and Mr helpful parts man sell someone a pump that looks identical but isn't close in operating pressure.

When it shuts down I'd be real curious if you loosen the fuel line connection to the carb and press and hold stop and see if the pump is pumping?

[joedaddy805]

I know the fuel pump is overheating cause i relocated from the original location from the bottom of the gen. Its mounted a few feet away from the gen and its actually mounted so its a lil below the frame so it gets some wind blowing on it so it would keep the heat down. So it was running yesterday when it was pretty hot and it died so i looked at my fuel filter and it was empty and felt the fuel pump that is far from heat and it was so hot i couldn't keep my hand on it. So after the pump has cooled down to the touch it runs great till the fuel pump heats up again. Left it running all night when temp was down and it ran for 8 hrs straight with a/c on with no problems. Oh and its a Marquis 5500. If you have any extra tips that might be it let me know.

[pchuckie]

Electric motors get hot when they run, especially when they don't have full fuel flow through them to keep them cool.

[twinboat]

The fuel flow helps cool the pump. A slight air leak in the suction side could be a problem.

An empty gas filter is a clue. Carbs don't suck fuel. If the pump stopped, the filter would have gas in it.

The air is comming from somewhere.

[joedaddy805]

If it was a issue with the hose wouldn't it do it all the time?

[Waiter21]

Another thought: A hot fuel pump could be vapor locking. i.e.

• pump gets hot, both from the engine running and the pump running.
• pump creates a slight negative pressure (it sucks).
• negative pressure lowers the evaporation point of fuel.
• fuel hits hot pump and evaporates inside pump. Fuel isn't flowing through the pump very fast, not enough to provide any substantial cooling of the pump.
• Fuel pumps don't pump air very good, vapor lock.

QUESTIONs

1) Do you open the genny compartment door when running the generator? I put vents in my door to help it ventilate.

Generator Compartment vents | 2001 Coachmen Mirada 300QB

2) When you replaced the pump, did you use the Onan pump or the Airtek pump? You may have replaced a bad pump with another bad pump. (Airtek pump is the same pump, at half the price)

3) Thoughts on sucking air. Remove the hose from the carb, put the end in a small container and turn the pump on (hold the STOP button), watch for air bubbles.

4) bad hose - Fuel line hose to pump could be bad. i.e. hose is multi layers, when the layers begin to laminate, the inner layer can get sucked down and basically plug the line. This is a problem with older brake hoses causing brakes to lock up.

[joedaddy805]

When i run the gen i have the door open. I did use an Airtek pump. I'm gonna go buy some new fuel line and see if it helps any. If that doesn't work I'm gonna go back to thinking its the fuel pump.

Im thinking if i should take a new fuel line and wait for it to act up then install the new fuel line and see if it makes a difference. Cause I can go run my get right now and it would run forever here at home cause the temps are in the 60's and 70's most of the time. It needs hot heat for it to act up.

[twinboat]

Originally Posted by joedaddy805

If it was a issue with the hose wouldn't it do it all the time?

Could be borderline. During daytime use a few more things may be on creating a bit more load. Even the AC will draw more power in the higher daytime heat.

[pchuckie]

The door is a intergal part of the cooling system, running the generator with the door removed would reduce the effectiveness of the generators cooling system. The cooling system on his 5.5 Marquis is done out the bottom of the geneartor.

Unless you have the cut sheets on the two fuel pumps there is no way you could possible say they are the same pumps. I have changed several dozen so called same pumps.

[joedaddy805]

I have ran it with door open and door closed same thing on both. I never run it with the cover off either.

[pchuckie]

While running have you ever heard the generator make a nasty hammering sound? Almost like a worn piston pin and the piston is slapping the cylinder head?