auxiliary fuel pump for onan 4000 microquiet

I'm way past due to do an oil change/service on my generator.  While I'm in there I thought I might consider adding one of those in line low pressure pumps that I've read some folks install to reduce the vapor locking potential.

Do folks tap into a power source in the generator's controls, or find a 12volt source elsewhere?

Is the idea to install the pump nearer the tank, or do you think it would be just the same to install anywhere in line?

Also, could it be that there is no oil filter in this model?  nothing is coming up in the manual or in the cumins online parts store under the model KY Spec Code P

Can't say that I've heard of folks adding an aux pump to their generator feed. Or of a vapor lock problem with gensets.

from heat....
I had several occurrences during the hotter months where the generator would shut down.  In my research I came across several folks that theorized it was heat, and the fuel in the line vaporizing.  Maybe I misunderstood, but I thought that some folks had installed an inline low pressure fuel pump to help keep the line charged.  Not sure if it was here or using a google search finding posts on other forums....

Regardless, I'm pretty much convinced it was heat related.  Sometimes occurred when driving on the road, others when sitting parked in a hot parking lot/sun/pavement...and moving to shade seemed to help a bit.  The error codes were not consistent...although I can't recall now for sure which ones I was seeing.

I'd be more suspicious of a heat related control board issue than vapor lock, although the existing electric fuel pump could also be failing under heat. The only time I've heard of adding a secondary inline fuel pump was related to the in the tank fuel pump failures on early 90's era F53's with the 460 engine.

If you do add one keep in mind that Onan's require a very low pressure fuel pump, 5 psi max

  If you really think it is heat related and the fuel pump getting is hot then you might try moving the existing pump out of the generator bay and closer to the tank. I found the best way to eliminate vapor lock is to add a fuel filter that has a return line to the tank and placing it near the carb  . The problem may be in finding a place to tap in the return line, once I added one to the fuel filler hose.

catblaster said:

  If you really think it is heat related and the fuel pump getting is hot then you might try moving the existing pump out of the generator bay and closer to the tank. I found the best way to eliminate vapor lock is to add a fuel filter that has a return line to the tank and placing it near the carb  . The problem may be in finding a place to tap in the return line, once I added one to the fuel filler hose.

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now that's an interesting idea...but too many unknowns for be to know where to start in rigging up something like that.  I assume the return line would constantly bleed off some of the pressure and flow.....wouldn't want to bleed off too much....might result in starving the engine + overworking the pump.
Moving the pump though seems like maybe even a better idea than adding a booster.  I think I'll look at things with that option in mind next time I'm under the hood and looking to consider options.

I did cut a vent in the door a while back in hopes that it would pull cooler air from outside rather than hotter air from under the coach.  I still need to go in and duct it better because it's still bleeding in a lot of air from under.  Have some weather stripping to do it, just need to do the work.

Hoping to get to an oli change this weekend and plan to nose around in there a bit more and "brainstorm"

I had an '86 Dodge Caravan that would vapor lock.  Mechanical fuel pump & carb.  Original gas pump quit, aftermarket pump did not have enough thermal insulation from the block.  Starting cold, it ran great.  On a longer trip, stop for gas, restarted great, drive 30 ft and quit.  After it cools down, you were good to go.  I learned a few ice cubes placed on the top of the fuel pump could get it going in 5 min instead of 45 min.  Replaced with factory pump and the problem was resolved.

Vapor lock is possible.  In my case, it was the pump.  When running, fresh, cool fuel kept things running.  Stop, then the carb was starved.

Can you identify where the vapor lock may be occurring?  Where is fuel getting hot enough to vaporize before it reaches the carb?

I really haven't verified anything really..... and it wasn't even my original idea.  I will say that I think the idea is very plausible.  The fuel line feeds into the filter at the side of the generator (front in my case) and crosses the air plenum.  This is the intake air so in theory it should be cool.... but it's rather close to the pavement and very restricted down in there.  The way the coach came from the factory, the air gets sucked in from below up between the MH's access door and the front panel of the genny to the intake which is on the generator's front panel.  Heat from the pavement, heat from the generator's engine, and heat from the big V-10 sitting not so far away....

So it seems that some folks have installed a fuel pump (I assume) closer to the tank to help keep this line pressurized and flowing.  I've seen a lot of chatter on this come up online in the hot months... the generator runs happy as can be in the morning and evening, but will die during the day, hard or impossible to restart until it cools off a long time, then it dies again in short order....

"..wouldn't want to bleed off too much....might result in starving the engine + overworking the pump."

Not very much danger in that, I have used those type filters on 500 cu/in Caddys and first learned on a 427 corvette that kept vapor locking so bad you could hear the fuel boiling out of the carb.

You have probably heard already that fuel pumps are designed to push rather than pull and work best when the inlet is as low to the tank as possible, meaning they won't suck very well uphill. Just thinking that one thing you might try  is wrapping as much of the fuel line and pump with insulation, it could help in determining if this is really your problem.

Dont know if you have basement air but those unit dump their hot air under the coach so the generator would pick it up, I reversed the airflow on my AC so it dumps out the side instead of underneath.

blw2 said:

I'm way past due to do an oil change/service on my generator.  While I'm in there I thought I might consider adding one of those in line low pressure pumps that I've read some folks install to reduce the vapor locking potential.

Do folks tap into a power source in the generator's controls, or find a 12volt source elsewhere?

Is the idea to install the pump nearer the tank, or do you think it would be just the same to install anywhere in line?

Also, could it be that there is no oil filter in this model?  nothing is coming up in the manual or in the cumins online parts store under the model KY Spec Code P

Click to expand...

I have a 2011 Fleetwood Storm and my first summer with my RV in the south, our Cummins Onan 4000 RV GQ had the problems you describe- vapor lock leading to autoshutdown.  It apparently is a common problem with these Fleetwood Storms and the Thor Aces.

Fleetwood has a modification kit to fix the problem.  Thor moves the generator to a rear compartment.  When I talked to the Fleetwood engineer about this known issue, he told me to do the first two most effective things first:  put a fuel recirculation line in the generator and cut an air intake in the side of the compartment door.  The next step is to move the exhaust pipe out from underneath the generator where radiating heat from the pipe can contribute to the fuel vaporizing.

I cut a hole in the compartment door and installed ducting and a louvered air intake so that the generator could suck in cooler, outside air, instead of heated air that was directly adjacent to the generator.  I spliced the fuel line between the fuel pump and the carburetor and put a return line back to the fuel tank.  Apparently, the little 12V DC fuel pump runs continuously anyway with generator operation, so putting the splice in supposedly doesn't shorten its life.  We'll see.

There were some guys that either moved the fuel pump away from the generator and/or added a second "switched" fuel pump in line with the old one.  There has been success with that too.

But having said all of this, I'm not sure I'd go through the trouble if your generator isn't shutting down under load in hot/humid conditions.  Is it a known issue for your RV make/model? 

There are some very simple methods of alleviating vapor lock, if that is indeed what is happening. One of the easiest is simply wrapping the fuel lines with insulation, preferably one with a reflective skin. Another is to place a shield of some kind between the heat source and the line, basically a diverter and heat sink. A metal plate is often sufficient.  Yet another is moving the fuel line or pump enough to get further away from the heat source.

I believe the fuel pump in the Onan is already an electric-powered, constant flow type.  They are much less susceptible to vapor issues than the old mechanical pumps. I'm not familiar with your 4000, but isn't the pump mounted on the genset itself? That would suggest that any vapor lock is probably in the suction line rather than between the pump and the carb, but you have to figure out where it is to know where & how to fix it.

Gary, I'm not 100% sure yet where the pump is in the genny.  I'm fairly sure it's a desperate electric pump mounted someplace in there, so there is likely a short line form the pump to the carb.... but I do assume the problem is most likely on the suction side of the pump.
I have read of folks trying to insulate their fuel lines without any luck. 

@ualdriver, yes it's a known problem.  It gave me fits last summer.  We're planning a grand canyon trip this summer, so I want to do everything reasonable that I can to try to make it better in preparation fro that trip to the desert.  I think I might have mentioned before, I have already cut the vent hole in the door.  i did that before I started seeing a bunch of folks doing it last summer.  The thing I haven't done yet though is to duct that opening to the generator.  Plan to do that soon, maybe this weekend if I can get to it.
I'm curious, so you just put an inline tee in the pressure side of the pump feeding the carb?  what size line and how did you get it feeding back into the tank?  I imagine these pumps don't give a huge flow rate so I wouldn't want to bleed too much away from the carb....

Anyway, I hope I can get in there to look things over this weekend.  Thanks everyone for some great things to consider!

I have read of folks trying to insulate their fuel lines without any luck.

Click to expand...

OK, suit yourself. Looks like you have your heart set on adding a pump, so go for it.

blw2 said:

Gary, I'm not 100% sure yet where the pump is in the genny.  I'm fairly sure it's a desperate electric pump mounted someplace in there, so there is likely a short line form the pump to the carb.... but I do assume the problem is most likely on the suction side of the pump.
I have read of folks trying to insulate their fuel lines without any luck. 

@ualdriver, yes it's a known problem.  It gave me fits last summer.  We're planning a grand canyon trip this summer, so I want to do everything reasonable that I can to try to make it better in preparation fro that trip to the desert.  I think I might have mentioned before, I have already cut the vent hole in the door.  i did that before I started seeing a bunch of folks doing it last summer.  The thing I haven't done yet though is to duct that opening to the generator.  Plan to do that soon, maybe this weekend if I can get to it.
I'm curious, so you just put an inline tee in the pressure side of the pump feeding the carb?  what size line and how did you get it feeding back into the tank?  I imagine these pumps don't give a huge flow rate so I wouldn't want to bleed too much away from the carb....

Anyway, I hope I can get in there to look things over this weekend.  Thanks everyone for some great things to consider!

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Yup, you definitely want a duct going to the intake.  As a side note, when one puts their hand over the duct to "feel the suction" created by the operation of the generator, it was pretty amazing to me how much air that little generator sucks in!

Gary is probably right (as he normally is!) about wrapping the fuel line.  However, it would be a huge PITA because the area on my generator where the fuel line runs within the generator, and also where the fuel line is the hottest, would be difficult to reach.  Further, the fuel pump on my generator is in that same, extremely difficult area to access.  However, when that pump does fail someday, I'm definitely moving it OUTSIDE the generator enclosure.  That's a no brainer.  I don't have near the technical knowledge that Gary possesses or maybe even you have, but I don't understand how the fuel vaporization problem would be on the "suction" side of the pump.  My generator pump sits in the lower corner of the generator and its green enclosure, so the fuel line that feeds into the pump is entirely OUTSIDE of the hot generator enclosure.  I'm guessing that the fuel pump gets really hot, heats the fuel as the fuel passes through, and the fuel vaporizes between the fuel pump and the carburetor.  As a layperson, that's my guess.  If my logic is incorrect, let me know!

I think the return fuel line is the same size as the fuel line between the generator fuel pump and the carburetor.  On my Ford F53 chassis, the fuel tank has a small "nub" with a plastic cap on it, sorta on top of the tank.  The nub is a port into the fuel tank, and it is accessible from underneath the chassis.  Per Fleetwood's instructions, I put a "T between the fuel pump and the carburetor, then ran the same sized fuel line all the way back to the fuel tank, removing the cap and attaching the return fuel line to the nub.  My understanding that with the Chevy chassis, one has to drop the fuel tank to access a similar nub.

As far as bleeding too much fuel away from the carb, it doesn't appear to be a problem although admittedly I haven't ran the generator with this new configuration for more than an hour.  I did run the generator under load with the air conditioner on high, and there was no indication that the generator was being starved of fuel.  I also have not read of anyone else who did the mod running into that particular problem, and there is a lengthy thread on this topic either on this forum or the iRV2 forum. 

Finally (sorry for the long post), if you throw an "at comcast.net" to the end of my username, I'll send you the Fleetwood docs that were sent to me to fix the issue.  Off the top of my head, it had a parts list that had the fittings and fuel line sizes (I think!) if you're interested.  I realize we have different RVs, but maybe it will help you?

thanks ualdriver, that surely does help.  I had not read about this return line mod before, but it makes total sense..... and your description makes total sense.  Mine probably has different line sizes and such..... but i'll definitely have a look.

what you are saying about  the location of the fuel pump and the suction/discharge side question makes sense.  The theoretical says that fluids will boil if pressure is reduced enough, so pumps often cavitate on the suction side....lower pressure gets you closer to the vaporization point, now add some heat.... and "houston, we have a problem".
    also you would think that a primed pump would push through any vapor locking happening on the discharge.....but since you say the suction side is completely outside of the hot box, then it's possible I suppose that the vapor locking is happening on the discharge side..  I'm really not a vapor locking expert.  Around 17 or the early years of my career were working with industrial pumping systems, but haven't really dealt with fuel systems all that much..... so it's guessing for me.

I never had time to work on it over the weekend, but finally got over just to have a look at it yesterday
I'll try to attach photos

so the first shows the generator.... A typical unit.  The plenum on the left is the air intake.  The fan pulls air in here for cooling and combustion.
behind and under those blue electrical connectors is a circuit board, and behind that is the filter and pump (second picture)
It basically sits on the bottom of this hot box.  A huge draw of air through it when it's running, but it's also getting heat soaked form the genny's engine, but also from the nearby pavement etc....

As it came from thor, the air is drawn in through a space approx 1-1/2 to 2 inches I believe behind the door that hides the generator.... so it's pulling air mostly from below the generator...which happens to also be were the cooling air is exhausted after it cools the engine.....

I can completely understand how this fuel system gets heat soaked.

It looks like it would be very easy to install a tee in the rubber fuel line right before the carb....but I didn't have a chance to investigate where to return it to.

I think my 1st order of business when I do the oil change & work, is to finish the ducting project to route more air from the louver vent I put in the door (3rd photo).

Then I'm going to focus on looking for ways to return a small tubing back to the tank.  I figure I'll restrict it by using a much smaller tubing, that way it'll increase flow through the pump some but not so much hopefully that it robs the carb too much

& if I can't find an easy way for that return, I'll focus on moving the entire fuel pump away from the hotbox.  Maybe find a place to mount it on the chassis frame ....

thoughts?

blw2 said:

I never had time to work on it over the weekend, but finally got over just to have a look at it yesterday
I'll try to attach photos


thoughts?

Click to expand...

Sounds like you're going to do what I did.  Sounds good to me!  I like the idea of a small diameter line going back to the fuel tank so that the carburetor gets its fuel "first," as long as there is enough flow to keep the fuel circulating and bowling.  Let me know how it goes.  Come summer, I'll post back here if I have any problems as well.

Hi All,

I am new here to RvForum.net, but have been a long time member at two other well known RV forums. I just wanted to throw my two cents into this discussion by backing up UALDriver's comments and confirming what Brad (the OP) has suspected.

We have 2010 Fleetwood Encounter 32BH (same model as UALDriver, we have actually conversed about this before) that has the Onan 4K mounted just behind the engine on the driver side of the MH. Fleetwood never installed baffles/insulation between the engine compartment/exhaust system and the generator compartment, allowing heat from all three sources (plus ground) to build up and vapor lock the fuel system on the generator. I have heard that the same thing applies to other manufacturers, including Thor.

On ours, the generator will only overheat on days when temps are above 90, we have been driving with the generator running for at least an hour, and (here's the key thing) we are driving uphill. Any other time, even temps up to 100, the generator will run fine all day long, but as soon as we hit a hill large enough to force the MH to downshift and speed the to drop below ~50 for more than a couple of minutes, the generator will die. You might say I have over analyzed this, but I can accurately predict when it will shut down every time. My theory says that above those speeds there is enough air flow from under the MH to temper the air that does come from the engine compartment, but below those speeds the lack of baffles to divert the air allows it enter the generator compartment and overheat and/or vapor lock the generator.

Adding the auxiliary fuel pump will help keep the fuel from vaporizing, but it won't keep the generator from ingesting all that hot air. I would think that might hasten the generator's demise in the long run.  Fleetwood's fix (as UALDriver mentioned) was to cut a hole in the generator access door, install a duct with gaskets to allow the generator to pull air directly from outside. Well, when my DW asked what I was intending on doing to solve the generator problem, both Fleetwood and I were quickly overruled, she would not allow a hole to be cut in the side of the MH, no matter how nicely I dress it up!

So, onto plan B. I researched 12v fans and came up with the idea to use a bilge blower to pull air from between the grille and radiator. The blower is about 5"x 5"x 8" long uses about 4.5 amps and the manufacturer claims about 220CFM. From what I can tell, it blows about 1.5 times the amount that is needed by the generator. BTW, this was a highly scientific test using my hand anemometer to feel the amount air being sucked in by the generator compared to that being blown by the bilge blower. I would show you the calculations I used, but don't have enough space to do so.  ;D

I installed it last weekend using 4" HVAC duct from Lowe's with a switch located just below the generator switch near the instrument console. The blower is loud when sitting still and only the generator running, but I suspect once engine and road noise are factored in, I won't be able to hear it all.  The intake is just in front of the wheel well under driver. The first vertical piece of duct connects to an elbow connected to the bilge blower that is mounted in the forward most corner of the chassis. A longer piece of duct work runs over the wheel and through the mud flap terminating just above the  air intake for the generator. I may need to install another elbow to aim the airflow more directly at the intake. It appears that the suspension travel and steering movements shouldn't contact the duct work, hopefully.

The first test will come in about a month, if the weather is hot enough at the time of our trip. A more thorough test will be in July and then also in August. Hopefully, it will keep everybody (including the generator) cool and happy. I'll report back once I find out more. Good luck. Tom

Welcome to the site and thank you for the good info. I'm not sure how widespread this problem is, but if it works for you after testing, maybe write the modifications up on Word or other similar program, include some pics, and maybe get it posted in the library section for easy reference to others in the future.