Onan 4000 generator runs but surges

[Thedigdug]

I have an onan microlite 4000 generator that pulses with or without load. I have replaced the carb, fuel pump, fuel filter and spark plugs. Any help would be appreciated. Please look at the links below to see what I'm talking about.

https://youtu.be/lPaOfxyNxPU

https://youtu.be/fTf_s4uc0Sw

Tom

 

[SkateBoard]

Looks like your spark is gone when it happens. I would replace the coil

 

[kdbgoat]

It does sound like a loss of ignition, not a fuel issue. There are two things I know that shut down an Onan's ignition, one is loss of 120 volts AC, and the other is low oil level. Loss of 120 volts is unlikely in your situation. Start with the easy/cheap stuff. Check your oil level, and make sure its at the maximum. If it is, buy a new oil level switch. They can have intermittent contact issues. They don't cost that much, and aren't too hard to change out. If you can change all the fuel system stuff, you can certainly handle the switch. If that doesn't fix it, I would say SkateBoard may have called it right with the coil.

 

[Bobmws]

In theory you should be able to check the switch by bypassing it. It's either a normally open circuit that closes at low level or normally closed that opens at low level. Upplug the switch & see it the gen starts and runs steady. If no start, jump the leads and test again.

 

[RoyM]

I would try an inline neon spark tester available for cheap at most auto parts stores. Watch the bulb when the engine falters, if the color changes to yellow or quits the problem is ignition. If nothing happens it could be fuel delivery, possibly a control board or wiring problem that is momentarily closing the fuel cutoff.

 

[john owens]

I am no expert but...its a fuel problem. Looking at your list of replaced items..you need to check the fuel line from the gas tank into the genset. I bet you have a cracked rubber hose and fuel delivery is being interrupted just enough to cause the hesitation...

 

[SkateBoard]

If you watch the video the test light goes out. It's an electrical problem and I say it's the coil.

 

[Thedigdug]

Thank you for the replies. I have traced the fuel line to the tank. It seems only the last 20 inches to the fuel filter is a rubber hose with no visible leaks. The rest of the line to the tank is a hard line which runs along the chassis. The line does not seem to have any issues. I haven't checked the fuel pressure as I do not have a gauge. It does seem to be getting fuel. I was leaning more towards an electrical issue as well. If someone can post me to a picture of the ignition coil so I know know what I'm looking for. Also the location of the oil pressure switch. I will try both.

 

[SkateBoard]

Here's a pic of an Onan coil. Yours may look different. It's on the other end of the wire that goes to the spark plug. There is a little can attached also which is a capacitor that should be replaced.

 

[Thedigdug]

My gneset is a microlite 4000 ky spec. I'm not sure it has an oil pressure switch. They might have done away with them on this model.

 

[SkateBoard]

I just looked at your owners manual for the coil. Just follow the other end of the spark plug wire. It plugs into the coil

 

[john owens]

In Defense of my previous comment...I thought.....the second video was showing me an inline fuel filter!!!!!  Today I looked and there is a spark plug wire attached to the fuel filter!!! Sumting wong here!LOL!!!!!

 

[youracman]

My gneset is a microlite 4000 ky spec. I'm not sure it has an oil pressure switch. They might have done away with them on this model.

I wonder if your Onan has been uupgraded/replaced from the original.  From 1st video, it looks a LOT like my 10 yr old unit.  I have been told that the last character in the alpha-numeric model number is the "spec'; e.g., my model number is 4KYFA26100K so it is a "K" spec (for parts ordering or whatever).  In the event yours is somewhere near this vintage, here is a link to the service manual (in case you don't have it.)  http://www.rvwithtito.com/download/Onan4000MaintenanceManual.pdf

Mine does not have a "coil" per se.  Oh, it has a coil, but it is part of the magneto assy.  There is a good depiction of all that in the service manual referenced above.  I need to replace my magneto because of a weak spark but I am "putting it off" until fall cuz I need to drop the unit down and the whole family is using the heck out of the coach right now.  In order to get it to run, I had to gap the plug clear down to about .010" and it is a "finicky" starter now.  Way different problem from yours, though.  BTW- A mag assy complete with molded plug wire, etc is just about $50 online.

Good luck with the fix.........lotsa folks will want to know how it all turns out.

Regards, ed s in Denver

 

[Charlie 5320]

Just because you have replaced the fuel pump, don't mean you have a steady supply of fuel. There is a rubber line that goes from the steel line at the tank and if it is getting bad the pump won't hold pressure on the line. If this unit doesn't have points it has a mag, and they either work or don't, no in between. Take the inlet hose off the pump and using another length of hose, stick it in a can of gas near the generator and see if that don't clear it up. It may be airiating the fuel if the pump is sucking air along with the fuel.

 

[john owens]

) What Charlie said....

 

[youracman]

) What Charlie said....

Agree........good way to prove (or disprove) a fuel supply problem.  On the  OP's 2nd video though, I see his spark checker light (in series with the plug) go out as the stall  occurs then it comes back on immediately as  the engine recovers.  Don't see how a fuel problem ties into that.  Could be, I suppose that the control module makes the kill switch to the mag if the rpm slows due to a fuel problem........but that seems like a stretch.  Will be interesting to see how it all pans out, for sure.  These things can drive ya nuts.

 

[Thedigdug]

i bought a new coil and will be replacing it next. At the same time i will replace all the rubber fuel lines i can. I do recall that when the rv is on level ground it does not surge as often as it does when its at a slight incline. It almost seems that gravity affects the frequency of the surges. Im not too sure how this factors into the original problem. I will keep everyone posted.

 

[Charlie 5320]

i bought a new coil and will be replacing it next. At the same time i will replace all the rubber fuel lines i can. I do recall that when the rv is on level ground it does not surge as often as it does when its at a slight incline. It almost seems that gravity affects the frequency of the surges. Im not too sure how this factors into the original problem. I will keep everyone posted.

that's probably because it takes more to pull fuel up hill, and it will pull more air too. You may have to drop the tank to replace the fuel line. Try putting a new line in a gallon container right by the generator before replacing the coil.

 

[namumac1]

How fresh is the fuel?

 

[djordan57]

I have an onan microlite 4000 generator that pulses with or without load. I have replaced the carb, fuel pump, fuel filter and spark plugs. Any help would be appreciated. Please look at the links below to see what I'm talking about.

Tom

Too bad the vids were removed. It's hard to know if the problem being reported might match the problem I'm having.