36kw unit power fluctuations under heavy load

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richard
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Thu May 30, 2019 9:52 pm

Hi. Just found your site - hope I don't ask a question you have already addressed. I have a Generac model 0056401 QS 36kw unit (2.4l turbo engine running on natural gas) that was installed about 10 years ago. Other than small maintenance items (idler pully, charger, oil pressure sensor) the unit has worked well - able to keep us cool in the regular power outages we have on the Gulf Coast. Very recently, I've noticed the unit's voltage output will fluctuate enough you can hear fans speed up/slow down while the generator is under load. It starts fine, idles smoothly and seems to work just fine while there is low loads on the unit (lights, TV, computers, etc). It seems to support one central a/c unit (3.5 ton) without too much variation in output but when any other heavy load come on the power will fluctuate, enough to trip battery backups on the computer, LED lights flicker, etc. I shut the mains off today and the engine begins to slow down (stumble?) and return to normal speed, over and over while maybe at 50% or so load and up. It's serviced regularly, changed the plugs and checked all the wiring, all looks good. There are no abnormal lights on the panel. Any help/ideas would be greatly appreciated.
Chris
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Thu May 30, 2019 9:56 pm

So it sounds like the variance is audible with the way the engine sounds? If so, I'd check fuel pressure and see what it looks like... Almost sounds like a fuel issue.
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richard
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Thu May 30, 2019 10:07 pm

Thanks, I did look at the fuel lines, all looked fine but I don't have a nanometer to check the pressure. Nothing has been done to the fuel lines since it was installed so I suppose they could be out of tolerance. Will see if I can find a nanometer tomorrow and will check pressures.
richard
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Thu May 30, 2019 10:08 pm

Sorry, yes the RPM level changes are auditable. Almost sounds like it wants to die and then picks back up. I'd guess it does this about every 5 or so seconds while under load.
Chris
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Fri May 31, 2019 11:59 am

Yep, you need a manometer to check fuel pressure at the unit. Sounds a LOT like fuel issues.
Generac tech for over 12 years. I'm here to help!
"The only source of knowledge is experience" -Albert Einstein
Looking for Generac parts? Gentek Power offers the full line of Generac replacement parts! Shop Gentek Power Generac Parts
A list of my favorite Generator & Electrical Tools! https://www.amazon.com/shop/gentek_power
78buckshot
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Sat Jun 01, 2019 8:24 am

I agree with it sounding like the fuel side of the system. First thing I would check would be the atmospheric vents at the gas regulators. Depending on the gas system you may have one or more regulators from the meter to the generator. If insects have blocked the vent the regulator cannot control the pressure.
richard
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Sun Jun 02, 2019 1:03 pm

Quick update. Got my nanometer yesterday evening and set off this morning to see what's going on with the generator. Sure enough, the gas pressure was only about 7.5" with the unit off and dropped a bit more than 1.3" when on. All the vents on the various regulators appeared clean. I adjusted the regulator that feeds the generator to 12.5". Solved the problem. When I think back, I've always had some fluctuation since it was installed when under full load so maybe it has always been low - anyhow seems to be fixed now. Currently still running on generator to load test it (getting hot in Texas) with all three a/c units running (about 9 tons) with various lighting, washing, etc. and holds 119 - 120 volts inside the home. My wife did turn on her vacuum and it seems to be about 118 volts and a bit of fluctuations to 120 but I'm probably getting close to the max output from the unit.

Thanks to Chris and Buckshot for sharing your expertise - much appreciated. With hurricane season just around the corner it is comforting to know the family will be with power if we have another storm come through - last time we were without power for 15 days - not fun if you don't have a/c down here.


Again, thank you for helping - love your blog.
78buckshot
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Sat Jun 08, 2019 6:59 pm

Good to hear it's running smooth but I think 12.5 in.w.c. it out of spec for that unit, if your happy with it then let it run.
Chris
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Sat Jun 08, 2019 9:08 pm

I forgot to ask what fuel it was on, but for natural gas, smaller liquid cooled units like this one are rated from 5-14" WC and LP is 11-14". If it's running well, like Buckshot said, I'd leave it!
Generac tech for over 12 years. I'm here to help!
"The only source of knowledge is experience" -Albert Einstein
Looking for Generac parts? Gentek Power offers the full line of Generac replacement parts! Shop Gentek Power Generac Parts
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richard
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Mon Aug 05, 2019 10:14 pm

36KW older (2009 or so vintage) w/2.4L turbo engine.

Well, I thought I had it fixed. Today, at about 98F outside, the electric company decided there was too much load and we lost power for about 4.5hrs. The generator went back doing the same fluctuations as before - everything seems fine until a heavy load is put on it and then power output isn't stable - I can hear the RPMs changing. There are no fault lights showing up, everything looks fine from the aspect of the onboard diagnostic lights. So, the last time, I did change all the normal wear items, including the plugs, belts, filters, etc. One thing I had done last time when I was adjusting the gas pressure (NG) I took the black cover off the throttle body and did a little cleaning. Anyhow, after that round the thing seemed to be working fine - until today - it's run about 10 hours total since the last hiccup, 6 hours it did fine, the last 4+ not good at all. I am certain the gas pressure from the regulator to the generator is ok, today confirmed it was just north of 12WC when running (this measurement is outside of the generator, right before going to the generator's regulator). I also removed and cleaned the RPM sensor while I was in there today just for grins (there was a good bit of metal shavings/rust but came right off with a paper towel, Ohms at 1115). Cleaning the RPM sensor didn't change anything, still running smooth until under load and then starts to act like it is starving for fuel. I'm thinking it may be the throttle body (BOSCH 0280750149), or possibly the gas regulator and assorted bits (OG612300689). All the hoses from the turbo, air cleaner assembly to the throttle body look fine, no leaks. I would just start throwing parts at it but they seem to be way expensive. Any ideas? Thanks in advance.
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