 Unfortunately this was a very common place for me to be over the course of four years... The Onan/Cummins QG5500 is one of the most common gensets in modern RVs including gas-powered Class A and B motorhomes and 5th wheel trailers. The straightforward design is made to operate within compact basement compartments while maintaining a decent operating temperature, it’s relatively inexpensive and because it’s such a common unit, and parts are readily available from multiple sources. And that last point is a good thing, because to say we have not had good luck with our QG5500 would be putting it lightly. It’s been common to find my head buried in the midship compartment on the driver side of our motorhome diagnosing or repairing our genny over the years. Some days you’d find my legs hanging out the side of the rig from underneath that compartment - It just depends where the problem is that week. At this point we (or a technician) have replaced almost every part of the generator outside of the engine itself. That’s been stressful and expensive, but it has lended us one gleaming bit of good news: I’ve become a bit of a handyman on the ‘ol QG5500, repairing friends’ units and walking them through troubleshooting issues over the phone. I’ve also found a couple of “must do hacks” for anyone with a QG5500 - even a brand new one! The symptoms We’re not alone in our frustrations with the QG. Face it, if you’re reading this article you probably can’t keep your generator running. Or it won’t start. Or it’s puking fuel all over the place when you prime it. Or it’s making a terrible noise. Or it’s running but not making power. I’ve actually experienced all of these things in the past three years! Every one of these issues is incredibly frustrating but through a process of reading every available piece of literature possible then applying them with a heavy amount of trial and error, I’ve managed to overcome each. Okay full disclosure, a couple times I did pay some dudes with forklifts and more tools than me to do the bigger stuff. When it comes to issues with gasoline Onan generators, there’s a lot to cover. Today we’re going to stick to the most common issue that plagues these things even when they’re brand new: stalling. The codes If (when) your generator stalls, you’re going to have a flashing light on your generator button. When you click it once, the light will blink in a different sequence. This is your code. For example, if you click the button and it blinks “1,2,3… 1,2,3,4,5,6…” That's code 36. Here is a list of common Onan fault codes and some very basic checks/fixes: Code 2: Oil pressure fault (check & top off or change oil) Code 12: Overvoltage fault Code 13: Undervoltage fault (disconnect some of your connected appliances) Code 14: Overfrequency fault Code 15: Underfrequency fault (disconnect some of your connected appliances) Code 22: Governor overload fault (disconnect some of your connected appliances) Code 33: Low cranking speed fault (check your battery connections and charge level) Code 36: Engine stopped without command fault (one of the most common faults, this can be a lot of things!) Now, there are all sorts of codes in between and in addition to the ones above but in my experience they are ancillary. In fact, I’ve had failures of parts which should have triggered specific faults but instead I only got Code 36. It’s like the part caused an issue which in turn caused the engine to stall but the system was too dumb or lazy to let me know. It was just like, “Yeah I stopped working but I don’t know why. You figure it out!” If you’re getting a Code 36 you can probably attest: It’s infuriating because there usually isn’t always a “smoking gun” jumping right out at you. In this article I’ll explain all the problems we’ve experienced, how I diagnosed the issues and all the fixes I’ve done! At the end I’ll share the two things YOU should do to your QG5500 even if it’s running fine! Our first issue The most common thing to fail on these units is the fuel pump. Speak to anyone who’s fumbled their way around their Onan and they’ll probably tell you they’ve replaced theirs. In fact, many RVers (including myself) carry a spare fuel pump in their rig! For us, the fuel pump announced its departure with a sudden stall and a “Code 36”. When we pressed the button to prime the fuel system, the pump did nothing. Keep in mind there are other reasons this could happen (which we’ll talk about later) but I got lucky and this was indeed the fix. In my case, I replaced the pump with a Chinese one from Amazon. There are many generic brands on Amazon and I’ve used three. They all worked fine, although one of them (Waterwich brand) did not have the mounting tabs in the right place. I should note that we did not have three pump failures, I simply misdiagnosed the pump and replaced it without proper diagnosis. More on that later! If you need to replace your Onan QG5500 fuel pump and want an easy to understand walkthrough on the process, check out our YouTube video on the subject!  Fuel pumps are common failures. Thankfully they're cheap and easy to replace. Watch our YouTube video on the subject to see for yourself! Getting really technical As the weather got hotter, the generator began acting up again. We were having the same symptoms as we did when the first fuel pump failed: stalling, Code 36, then failure to prime and restart until the unit cooled. Naturally, I replaced the pump again only to find it was not the issue. A quick test of the fuel pump wiring revealed that there was no power going to the pump but this was only happening when it got hot. Once the generator cooled, the pump would prime again. I then traced the wiring back to the control board, which is located inside the gen cover and under the shroud which houses the switch and breakers. Upon removing the shroud (four bolts), I was able to retrieve the computer and quickly see what the problem was! The computer board sits in a housing with one side open, embedded in a clear epoxy. The board looked clear and healthy, with the exception of one circuit in the corner of the board where the epoxy was burned. This was the circuit for the fuel pump and it was evident that when the board was hot, the solder was coming apart opening the circuit. Once cool, the connection touched again and allowed the pump to prime and run. Contacting Cummins for a replacement board revealed that I probably wasn’t the only one having this issue. The part was on backorder for at least six months, a situation surely compounded by the fact that we were right in the middle of the COVID-19 pandemic. A search for used parts turned up empty but thankfully, just as I was about to give up, I found Flight Systems, Inc.! Although the name would have you believe they specialize in aircraft components, over half of their 50-year existence has been focused on RV generator controls and circuit boards. We ordered a replacement board from Flight systems which arrived in just a few days. Once plugged in and hooked up, the genny ran great in the spring air! There are some differences with this aftermarket board you should be aware of: First, the FS unit has a larger heat sink to help dissipate heat better than the Onan unit. Hopefully this prevents the type of failure we experienced with our original module. Second, the FS computer will only shut the genset down for what it considers to be major concerns - overheating, low oil level and stator voltage errors. Lastly, the unit will not generate fault codes (for better and for worse). Some people will see this as a big issue, I personally do not. After all, the codes displayed by the original unit were of no use to me in my troubleshooting. We’ve put about 700 hours on this Flight Systems board and while we’ve had plenty of other issues with the genset, the computer has operated flawlessly. Now, about those “other issues”...  See that dark spot in the upper right corner of the board? Cooked. Spewing fuel Having now replaced the fuel pump multiple times and with a new control board installed, the genny was running semi-reliably when the weather was below 80 degrees (although still stalling in higher temps). Whereas priming the generator had been a concern in the past because there would be air in the lines or it simply wouldn’t prime (due to the previously defective control board), one particular day in Wyoming we got the opposite effect while priming the system: a distinct fuel smell which led us outside to find fuel POURING from the generator! A quick removal of the gen set cover revealed that the fuel was pouring from the airbox and bowl of the carburetor. The carburetor is the part on top of the engine which mixes gasoline with air and gives the proper ratio for ignition inside the engine. Obviously, the fuel should stay inside the carb and it should certainly never be puking out of the air filter box. There were a couple of possible reasons that this could be happening: First, the fuel overflow sensor could be malfunctioning. Second, the float could be sticking inside the bowl. I removed the carb, cleaned and inspected all of the float components and reinstalled it only to have the same results. At that point, given that we needed to be in Pittsburgh just days later and it was hot outside, I knew we needed to find a quick solution. When these types of things come up during your travels, it’s a tricky balance. In our case, we had an extended warranty however we didn’t have time to be without the RV and we needed to get back on the road. I also didn’t have the tools or time to diagnose the sensor nor order an inexpensive aftermarket carburetor online. With this in mind, we opted to purchase a new Onan carburetor (complete with overflow sensor) at a local dealer and install it before getting back on the road rather than putting it in the shop for a “free” warranty repair. This cost about $450 and took a couple hours but it really was our only option. Thankfully, the generator ran great with the new carb and we continued on our journey with power and house air conditioning! For a little while, anyway. Grasping at straws One thing most people learn with the Onan QG5500: There is no escape from Code 36 and stalling when the temp gets over 80 degrees! Even after the new carburetor was installed we found ourselves without power and A/C when we needed it most, as the QG stalled and failed to restart until it had cooled. Once cool, we might get ten minutes of run time out of it. With the mechanical (and electrical) fuel components now having been almost completely replaced, there should have been no reason for this. Time for more testing! Fuel pump? Good. Control board? Good. Carburetor? Brand new, and set properly. One thing I noticed was that when the engine was warm, if we tried priming the system with the hose removed from the carburetor there was a lot of air in the system and the fuel sort of spit out rather than flowing freely. Whereas before I had attributed this to a fuel pump weakening when hot or the pump impeller spinning on the shaft, I began to have suspicions about the 10-year-old rubber fuel line that ran about 20 feet from the fuel tank to the generator carb. As fuel lines age, they can fail in multiple ways. Hairline cracks can form, introducing air bubbles into the fuel system or the inner wall of the hose can collapse, choking the fuel line and keeping the carburetor from getting the proper amount of fuel. I climbed under the rig and pulled the protective sheathing from the line the entire length of the rig, inspecting it fully. Along the way I discovered that the technicians at Thor had clamped the fuel line into a tight bundle alongside the wiring harness and hydraulic lines in two places, really pinching the line! I removed the clamps, freed and rerouted the fuel line and reattached everything but unfortunately this did not solve our issue. I decided to replace the entire fuel line, which was honestly not as big a job as I expected but still quite tedious and dirty! With 23 feet of new high-quality fuel line installed, we set off for our next destination with fingers crossed and hopes high… Only to be let down yet again.  Replacing the entire fuel line was not only a pain in the butt, it didn't fix our issues. Excuses for upgrades At this point, I was at a complete loss. With temps in the high 90’s, our generator simply would not run even at highway speeds (which should keep airflow moving enough for sufficient cooling). Stall. Code 36. Failure to prime. Restart 30 mins later. Run for 10 minutes. Repeat cycle. Again traveling across the northern United States in a fairly remote area, there wasn’t much relief or hope for repairs. I hung out the window shirtless as I drove down the highway, trying to stay somewhat cool as additional heat was radiating from the engine cowling between the front seats. It was like choosing to stick my heat in an oven as opposed to a furnace. Thankfully, Tosha, the kids and the rabbit were in the Sprinter van which was cooling nicely. After a few hours of this, I couldn’t do it anymore. I pulled off at the first sign of civilization and decided I was going to fix this generator on the side of the road, come hell or high water. A little research determined that I could likely install a low-pressure fuel pump in-line back towards the fuel pump to help “boost” the fuel all the way up to the Onan pump, which is located directly under the generator waaaay up the line. In theory, if the fuel pump was indeed weakening due to heat it would be remedied with this helper pump. I purchased a Posi-Flow 7-10 PSI pump from Napa, got some appropriate male fuel line fittings and 20 feet of both red and black wire then climbed under the rig and got to work. I found a safe, sturdy place to mount the pump just ahead of the rear axle and attached the pump to the frame of the motorhome. I then cut an opening in the fuel line, installed the fittings and ran wire from the factory pump back to the secondary unit so they would run fully in tandem (I had some concern that this second pump would overload that circuit but this has not proven to be the case). Once back on the road, I found this did actually help the issue! We were running for upwards of an hour before another stall and Code 36 faults, however the system would prime sooner and run longer after these stalls. This told me the issue was indeed fuel related and I was onto something but I hadn’t quite solved the puzzle yet. As it turns out, the smallest fix was exactly what this thing needed the whole time. Either this works, or I set it ablaze As you might imagine, I was beyond the end of my rope with this QG5500. We had spent thousands of dollars and countless hours diagnosing, repairing and upgrading the unit only to be let down and disappointed time after time. It was time for some outside-the-box thinking and more thorough diagnosis. Since I determined once again that there was air in the line when hot despite the new fuel lines and auxiliary fuel pump, this issue had to be heat related. I began my sleuthing with an infrared thermometer in hand. I ran the genny until it stalled, then removed the engine cover and checked the temps at the fuel pump underneath the generator, the exhaust behind the generator, the fuel line alongside the back and exhaust, the cooling fan below, and - most notably - the fuel line inside the genny cover as well as the carb float bowl. What I found was pretty surprising: The temps were obviously highest at the exhaust, but the second-highest temps were right inside the generator cover and around where the small fuel line ran from the fuel pump to the carb! Immediately following a stall, I applied ice bags to the fuel pump and fuel line running the length of the genny. Once cool, I restarted the genny and ran it for about fifteen minutes until it stalled once again. The temp gun showed that the fuel system was completely cool all the way up until the fuel went into the generator cover (because, ya know… it was encased in ice). The temp of the hose inside the genny was - once again - in the second hottest area in the bay. I determined this wasn’t just air getting in the lines, this was fuel being vaporized by heat as it made its way through the final leg of its journey to the engine! Fuel vapor lock is a phenomenon that occurs in internal combustion engines when the liquid fuel in the fuel lines or carburetor transitions into a gaseous state before reaching the engine. This vaporization disrupts the fuel flow, resulting in engine performance issues or even engine stalling. Essentially, the fuel boils. When this happens, the pressure caused by the gaseous boiled fuel can push the liquid fuel further back down the line, thereby causing pockets of air to form. Hence, lots of spitting fuel when we tried priming our hot generator. I was quite pleased yet simultaneously annoyed at these findings. After all, I’d had the evidence the entire time and knew this might be the case, I just hadn’t ever taken the time to go through this process of diagnosing the issue with a temp gun and process of elimination. Now I just needed to figure out a way to keep that little bit of fuel cool on its 8-inch trip through the hot spot. I first drilled a hole in the outer cover of the generator and ran a line outside the cover, through the hole and directly to the carb. Again, this helped but it only increased runtime by about 30 minutes and still left several inches of hose in the hot area behind the cover. In addition, it made the cover very hard (and messy) to remove since the fuel hose had to be completely disconnected. It was a temporary solution for trial purposes only, and it didn’t really work. I then considered cutting a hole in the cover and installing a mesh grille to allow heat to escape that area before realizing the cover is designed a certain way to dissipate heat from the engine and I shouldn’t disrupt that air flow.  Testing my overheat/vapor lock theory by running the fuel line outside the generator box. Finally, I settled on the simplest and cheapest solution: Another stop at an auto parts store for a roll of reflective heat shield sleeve. This stuff may not look like much, but it can greatly reduce the effect of heat on things like fuel and oil. There was actually a sleeve of some sort installed by Onan at the factory but it was not a heat shield. Instead it is a mesh cover that is simply intended to keep the hose from chafing on nearby components. Why they didn’t just install a proper heat shield and accomplish both goals is beyond me. At any rate, I cut a strip of sleeve to cover the line completely from the pump to the carb, installed it and said a prayer.
Hallelujah! Since installing the heat shield sleeve, we have experienced only one stall which did not appear to be heat related and the generator restarted immediately without issue. Something else to note, I gave the leftover heat shield sleeve to two friends of mine who were experiencing stalling and Code 36 issues with their QG5500s (one of which was brand new). They both installed the sleeve and said they have not had any heat-related stalling since! The two “hacks” every Onan QG5500 owner should do I know this was a somewhat long blog with a lot of info but I felt it necessary to put all of this stuff in one place. The truth is, almost every QG5500 owner will experience Code 36 and stalling issues at some point during ownership (if not right off the lot) and any combination of these things could be the source of the problem. With that in mind, here are two things I would recommend every Onan gas generator owner do right off the bat:
With these additions, your Onan generator should run much more reliably especially when the temperature rises and you’re running your air conditioners! While the QG5500 (and its sister models) are indeed very common, Cummins engineers pulled up a little short when it comes to the little details that would truly make this a reliable unit in all conditions. Hopefully these additions prove to be difference makers for you! Have you experienced issues with your RV generator stalling? Did you do something else to fix the issues? Comment below and tell us what you did!
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