My Generator Needs A New Fuel Pump

About a month ago, I was making a video how my PC V10 engine and generator started after sitting idle for two years.  It went uneventful until 20 minutes after I started the generator.  With the a/c cooling down the rig, the generator stopped dead.  I stopped the video and scrapped the idea of posting the event because of the generator dying.

After a while, I was able to restart the generator, but it shut down a second time after 10 minutes.  During the cooler time of day (still hot and humid but less hot), the generator with a/c on ran for 1.5 hours without incident so I shut it off thinking all is well.  When camping the following weekend, it was again hot outside.  The same shut-down happened again.

I researched this to be a very common problem with the Onan generator.  There are so many U-Tube videos on this.  Basically, the fuel pump becomes sensitive to hot operating conditions.  I wonder why the fuel itself is not enough to keep the pump cool.  Anyway, I ordered a new fuel pump and fuel filter (they are removed together) costing me roughly $45 for the pair.  I await their arrival.  The weather is now much cooler so I'll be changing parts without official confirmation that the problem is solved.  According to the videos, changing the fuel pump with the filter seems easy enough.

If you wonder, my 2007 Onan-4000 has only 162 hours of use.

Has any one here had a similar experience?

Here you go. Save or bookmark this pdf.

page  86-88 fuel pump testing procedure 

https://www.rvwithtito.com/wp-content/uploads/2017/08/Onan4000MaintenanceManual.pdf

Ron, it’s a very easy job. I think I did mine around 190-200 hours.  Carb I replaced two season ago with an after market carb. I change the air filter every year and the oil every season regardless of hours run.

Thanks Cal for manual.  I had found it earlier when trouble shooting my Onan.  That pdf file is stored in my computer PC file folder.

I should have mentioned this one detail in my opening post, how I determined the fuel pump was bad.

Before the trouble began, I was always able to hear my fuel pump run when priming from inside the motor home.  When the trouble began, I could barely hear that humm.  I then went outside to attempt to start the generator from there to hear things better, and to bypass the remote control panel inside our PC.  The pump sounded normal during a successful start, but sounded very weak during an unsuccessful start.  Without testing flow rates and using instruments, it was easy to conclude that the fuel pump weakens under extreme high-temp operation.

I then got on-line, did a little research finding You-Tube videos describing my exact situation.

The generator is a very hot-operating piece of equipment.  I surely do not recommend doing this, but if I were in a third world situation without resources, I would relocate the fuel pump outside the generator housing where it is much cooler.

Ron, it’s a very easy job. I think I did mine around 190-200 hours.  Carb I replaced two season ago with an after market carb. I change the air filter every year and the oil every season regardless of hours run.

Thanks Joseph for sharing that.  I wondered if I was the only PC owner who experienced a fuel pump failure.

Regarding my carburetor, mine works great when the fuel pump is working.  My air filter is the original one.  It looks quite clean, but I have new ones coming in the mail so I will replace it when replacing the fuel pump and fuel filter.  You say, replacing the fuel pump and fuel filter is an easy repair, and I believe you.

Another thing I did not mention was about changing the engine oil.  The owners manual states 15W-40 motor oil.  That oil on the shelves is now labelled "For Diesel Engines".  While at the store, I called the 800 number on the label.  The consumer help line told me that oil is normal motor oil, but labelled as such so people don't think about using it in their gasoline vehicles which all require a lower viscosity oil.

Ron, my air filter doesn’t always look like it needs replacing when I change it.  We boon dock on BLM desert and in Joshua tree which is all desert and I guess I figure we’re around more dust than many places so for the few bucks it costs I change it out. The oil change in my opinion is a must being there’s no filter and no oil pressure.

My fuel pump was acting up but more frequently than yours at the time. It fixed my problem till my carb gummed up with crap Ca gas. Cheaper and less frustrating to change it out than attempting to clean/fix it.

. . . . .  Basically, the fuel pump becomes sensitive to hot operating conditions.  I wonder why the fuel itself is not enough to keep the pump cool.  Anyway, I ordered a new fuel pump and fuel filter (they are removed together) costing me roughly $45 for the pair.  I await their arrival.  The weather is now much cooler so I'll be changing parts without official confirmation that the problem is solved.  According to the videos, changing the fuel pump with the filter seems easy enough.  . . . .

Ron,

Based on the description of the fuel pump being sensitive to hot operating conditions, you have me wondering if the generator is experiencing "vapor lock".  As I understand it, gasoline starts turning into a vapor at 140 degrees.  As the generator gets hot, it heats up the gasoline in the gas line and filter to the point it starts to turn the gasoline into a vapor.  The fuel pump cannot pump vapor, just liquid.  I had this happen to a 25HP zero turn mower.  Similar symptoms as your Onan, it would run great for a while but on days above 85 degrees, it would stop after 30-45 minutes of mowing.  After I let it sit for 10 minutes, the vapor in the gas line would cool enough to let the it return to a vapor such that the pump could pump again.  To fix the zero turn, I added an insulating fuel sleeve around the gas line prior to the fuel pump, focusing especially in areas where the gas line runs close to heat generating sources like the engine block or exhaust manifold.  It is really easy to install around the existing gas line:

https://www.amazon.com/Design-Engineering-010669-Sleeve-Length/dp/B079S1XLM7/ref=sr_1_4?dchild=1&hvadid=78615206577519&hvbmt=be&hvdev=c&hvqmt=e&keywords=gas+line+insulation&qid=1600350398&sr=8-4&tag=mh0b-20

https://www.amazon.com/vdp/fbc35aa9b1b94669aa647cd36427d9f4?ref=dp_vse_rvc_0

I have not tried it on the Onan generator, I have not had the need yet.  But it might be worth a try providing even more protection against the heat.

JJ

Hi JJ,

Thanks for that interesting theory.

In the case with the Onan, the fuel line enters from a "forward cool" location and the fuel filter and pump are right at the point of entry.  I don't think the fuel has enough opportunity to heat up and vaporize within.

Given in the past, I have run my generator with a/c running under even hotter conditions without incident, I strongly feel it's my fuel pump.  The parts are cheap enough, surely worth replacing them because of my suspicion.

Hi JJ,

Thanks for that interesting theory.

In the case with the Onan, the fuel line enters from a "forward cool" location and the fuel filter and pump are right at the point of entry.  I don't think the fuel has enough opportunity to heat up and vaporize within.

Given in the past, I have run my generator with a/c running under even hotter conditions without incident, I strongly feel it's my fuel pump.  The parts are cheap enough, surely worth replacing them because of my suspicion.

Hi Ron,

I agree, a new fuel pump will likely overcome the problem.  Since the generator works while cool, I do think vaporization of the gasoline in the lines when hot is contributing to the problem.  A new fuel pump should be able to pump the fuel/vapor combination better when compared to an older/worn pump. Like you said, your generator worked fine in the past.  If this ever happens to my Onan, I will likely remove the generator cabinet door temporarily to see if the increased ventilation has any impact on the issue.  It may prove or disprove the theory.  It would be interesting to know how hot it gets in that Onan cabinet when running at heavy load like with the AC running.  As packed as that cabinet is, it is bound to get quite hot in there.

I did look at my Onan and I see your point about the filter and pump being located just as the fuel line enters the Onan cabinet.   The pump/filter appear they are going to be hard to get to . . . maybe removing the air filter cover will improve the accessibility.  I will be interested to know how difficult a job it turns out to be and if you had to remove anything in the way to get the job complete. 

Good luck!

JJ

I watched a couple of You-Tube videos on replacing the pump & filter.  After disconnecting the batteries, you remove that potted circuit board controller and the solenoid to the left of it and then the pump is easy from there.

As far as running the generator without the Onan cover, I had understood that with the cover installed, cooler air is directed in better and hotter air is exhausted better.

When camping without neighbors, I open the PC access door to help with cooling.  But that did not help when testing at home.

Hi Ron,

I just joined the forum and don’t know if you solved your genset’s possible fuel starvation issue but I thought I’d offer my experience with my genset’s fuel pump issue. Might help someone else.

I believe your generator's symptoms were exactly as mine experienced 18 months ago, and indeed, replacing the pump cured the problem. Once I felt there was a 'pump plus run time heat' relationship, I intentionally tested the hypothesis over two days of cycling the starting of the generator, first from dead cold. It would run 20-30 minutes before quitting. Let it rest for 1/2 hour and it would run again but now only for 5-10 minutes. The shorter the rest periods, the sooner it would quit. Same processes the second day = same results. On two occasions when it quit, I did pull the fuel line into the carburetor and no fuel came out of the carb as the bowl was empty. I felt it had to be the pump.

Along with replacing the air and fuel filters, I replaced the fuel pump. Though not hard to do, the effort was more stressful (to me anyway) than installing the heavy-duty sway bars F&R and/or the SUMO Springs (Solo on front, Maxim on rear). I really didn't want to damage the green colored printed circuit board that had to be moved to gain access to the pump related stuff routed underneath it. And there is twisting and turning the board "just so" in order to get it back in place. Indeed, it is a DIY but it takes one's time and patience. I added extra long electrical leads for future use should it happen again. I would just abandon the faulty one in place and mount a new pump and filter outside the Onan box next time, hence the long leads.

Good fortune to you and thanks for all your sage advice and insight.

Steve

Thank you Steve for your detailed reply.  I had considered mounting the fuel pump outside the Onan to avoid the "Hot Zone", not sure I will actually do so.  I still have my new pump, fuel filter, and air filter sitting on the PC counter top, waiting for better weather.

I am focused on tires right now.  Just tonight, I took a front tire to my church's auto shop and was able to get the tire off and back on the steel wheel, a textbook process, but the balancing machine could not handle it.  Unless something new develops, I think I will rely on a tire shop to take care of everything.

Curious, has anyone replaced the fuel filter BEFORE replacing the pump to see if the pump was just supply starved? Forgive me if I missed it.


  A supply starved pump would not deliver enough fuel and cause the pump to sound different. When replaced with a new pump and filter it would solve the problem. Probably why the filter check precedes the pump check in the manual.


 ""Change the fuel filter at the interval recommended in the Operator’s Manual, or if performance problems occur and bad fuel is suspected.""
(Manual pg  8-19 or PDF pg 86) ( https://www.rvwithtito.com/wp-content/uploads/2017/08/Onan4000MaintenanceManual.pdf    CalCruiser!!)

In Ron's case, we had performance issues, and 2 year old fuel certainly could fall under 'bad' fuel guidelines. So fuel filter replacement would be diagnostic step one.

 The diagnostics for the PUMP say:

A negative fuel supply line pressure
(more than minus 1.3 psi or three feet of lift) will prevent the pump from delivering enough fuel. Measure the pressure at the pump inlet. Inlet pump
pressure must be between a minus 1.3 psi and a
positive 1.5 psi (minus 8.3 kPa and positive 10.3
kPa).

Source - Manual Pg 8-20   or   PDF page 87   https://www.rvwithtito.com/wp-content/uploads/2017/08/Onan4000MaintenanceManual.pdf

So, they want you to add a 'tee' between the filter and the pump. Then measure the pressure to see if there is an inlet restriction. NOT sure how you would end up with a positive supply line pressure, but they do allow for that. Did anyone here actually do this test? In a shop, to avoid unnecessary charges to the customer, I would see where the procedure is a CYA. In the 'shadetree' / 'Throw parts at the problem' mechanic world, i would bet this is never done.

Interesting they do NOT specify a volume outlet check on the pump, which seems to be the indicator people are using here.    An omission, or not a good diagnostic indicator for the pump? Dunno.

Proper diagnostic procedure would say that one checks/replaces the filter before a diagnosis of pump failure is performed. BUT... if the labor cost/effort is easy enough, and parts cheap enough, I guess it makes sense to just replace the pump/filter as a unit. I defer to those who have been in there, I am going by the manual procedures,  but it seems that the fuel filter is a pretty easy job compared to pump replacement. To be fair, i could not find the procedure for fuel pump replacement in the manual. Found 49 occurrences of the phrase 'fuel pump' and not one procedure to replace it. 

Hi Volkemon,

My generator ran for multiple hours with the a/c unit on, but only after the worst heat of the day had passed.

Hi Volkemon,

My generator ran for multiple hours with the a/c unit on, but only after the worst heat of the day had passed.

Gotcha.   With a pump at $30, its certainly not too expensive. Heck, the filter was $15.  We have a gathering of people that had changed their pump here, looked like a good chance to see what other diagnostic efforts might have been tried.  Steve did state that replacing the pump was stressful, be nice to avoid that. I (try to...) avoid unnecessary dirty work with diagnostics. And I freely admit I sorta have OCD's  to 'Find the problem' not just correct the symptoms.    I am the type of guy that cuts apart filters to see what went on.


I was curious if anyone had tried replacing the filter only, then seeing if the condition persisted. I would be a fool to think I wont have similar problems someday. This forum has been a GREAT early warning system.  The filter is a 450-500 hr item (!)  on the maintenance schedules I found, wonder how many have the original. Bet mine is...    I am at 446 hrs,  16 years after being made.