Need help accessing Onan generator fuel pump

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csshafer

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Feb 8, 2016
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Working on a 2004 Holiday Rambler Admiral with an Onan Marquis Gold 5500 generator.  The generator will run smoothly for about 20 minutes, then starts sputtering, and eventually dies.  It will only restart after a cooldown period and I'm thinking it needs a new fuel pump.  The fuel pump should be readily accessible from underneath based on several YouTube videos, except for the fact that HR put a metal plate under the corners of the genset which blocks access to the fuel pump. 

I'm thinking I should take out the mounting bolts from underneath the generator and try to jack up the front of the generator enough to remove and replace the pump.  This will also involve unbolting the exhaust pipe but I don't see any other way to get to the fuel pump. 

Any other ideas?
 
csshafer, I have kind of developed an interest in all things generator. Can you please tell me if you were able to resolve the issue? It seems from all the reading I have been doing, that most of the time, the culprit is the fuel pump, and one of the causes is most likely because they are either old or suffer from heat issues and vapor lock. I would be interested to hear how your generator is running now days.  Thx.
 
Sounds more like vapor lock than the pump itself. I'm having a similar problem on my 2015 C class thor 31L. I'm not familiar with the 5500 (mine is a 4000).  I Have to remove the electronic control and even then it is fun (scarcasm intended) to work on the pump. My plan is to mount the pump outside the gen housing and make a better air intake to get cooler air. I also think the location of the exhaust pipe is a factor heating up the air going in the intake since it's right under it. It acts up in warmer weather but not cooler.
 
PS; if you do decide it's  heat causing vapor lock, i started a tread on Sept 20 and got a lot of good input. A couple guys posted pics of what they did to help solve their heat problems. It's about 2/3 down page 3 here in motorhomes section.
 
The symptoms on my 5500 were the same as OP. It seemed to be fine when I got the coach last March. Within a couple months it started dying as described. I guess it could be related to the warmer temperatures. Replacing the fuel pump solved the problem.

Seems like a common symptom and fix.
Tom
 
Heli_av8tor, glad to hear replacing the fuel pump seemed to have solved the problem. It confirms my theory that in some cases, for many reasons, the fuel pump is to blame, and most likely, as in your case, it may not involve vapor lock, but may solely be isolated to the fuel pump. I still don't know much about the inner workings of the fuel pump, but I tend to believe that in the other cases in which vapor lock is suspected, it does not occur within the fuel pump, but in the fuel line, either before, or after the fuel pump. I also think it is a unanimous belief that vapor lock is created by excessive heat, causing fuel to turn into a gassy mixture. Vapor lock comes and goes, meaning that it dissipates and the evidence evaporates, depending on the degree of the heat that caused it, disappearing along with the problem, so it would be hard to simulate it in a bench test. This problem, in modern engines, especially with fuel injected engines, was solved a few years ago, but with generators, it is a different story.  I have thus far not heard of vapor lock in diesel generators, so it may be restricted to gasoline operated engines, and therefore, caused by gas and not diesel fuel.  Well, suffice it to say that I am still working on my theories.
 
csshafer said:
.................The fuel pump should be readily accessible from underneath based on several YouTube videos, except for the fact that HR put a metal plate under the corners of the genset which blocks access to the fuel pump. 

I'm thinking I should take out the mounting bolts from underneath the generator and try to jack up the front of the generator enough to remove and replace the pump.  This will also involve unbolting the exhaust pipe but I don't see any other way to get to the fuel pump. 

Any other ideas?
Click to expand...

You may have considered this already, but if not:  How about just doing an "abandon in place" mod for the existing (sick?) fuel pump?  cgmartin (IIRC) relocated his pump to the frame of the MH where it is installed in a metal box.  I believe he posted pics?  It was a good looking install and just has to be better than the original fuel pump location inside the hot generator box.  A new fuel pump (and a few knuckle scrapes, etc in that tight location) fixed my "run for 20 minutes and quit" problem; but the relocation approach cg used is on my bucket list.  If I ever install a new fuel pump, I will do it like he did.  If you can re-route the outlet hose from a new/relocated fuel pump to the carburetor on your genny in a safe manner (no rubbing or chafing) you would likely end up with a fix ................ and a better design than new.  Re-routing the 12v power lead would be req'd also, but probably be less of a challenge that re-routing the fuel line?  Best of luck on a fix............these gennys can drive ya nuts, for sure.

Safe travels.............
 
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