Onan 4K issues.

[bagged97taco]

I’m trying to figure out this damn onan 4K generator. I replaced the carb with an oem carb (not an Amazon special). It cranks but won’t fire. I have fuel pressure and bowl is full. Verified that fuel cut off solenoid is engaging. It has spark and obviously is getting air. I sprayed starting fluid in intake while cranking and it doesn’t do anything. Coach battery is brand new. Any help would be great.

 

[grumpy88]

Check the oil . They have a low oil switch

 

[yard dog]

Where are you located at ? , my 4k has the same problem. In Fort Mohave there is Gibson generator repair l have an appointment next week to have him troubleshoot mine . He is pretty knowledgeable on these generators. Here’s his phone number. If you want to give him a call and maybe he can give you some insight on what to look at 928 542 3470

 

[rivrrts429]

Check the oil . They have a low oil switch

^^^ This

 

[spectra3279]

Swap the plug wires. Maybe 180 out

 

[4Waters]

Check the oil . They have a low oil switch

^^^ This

The low oil switch shuts off spark, he says it has spark. It's definitely worth checking but wouldn't make sense if it's low and still has spark.

 

[rivrrts429]

The low oil switch shuts off spark, he says it has spark. It's definitely worth checking but wouldn't make sense if it's low and still has spark.

That’s true but worth checking. He has everything… spark, fuel and air. Makes no sense why it won’t fire. Has to be some stupid safety switch like a low oil sensor.

I’m trying to keep it simple in my thinking based on what the OP has shared.

 

[bagged97taco]

It has oil (filled to correct level) I read online that these newer models don’t have the low oil cut off switch anymore. I’m stumped. No codes at anytime. Only the over crank warning (4 blinks) if I hold the start button for too long.

 

[Mototrig]

While cranking can you feel compression from the exhaust pipe? Could be too much back pressure from a clogged muffler if spark and fuel are present. Long shot though.

On those onan 4k's the book says at certain intervals to remove the drain plugs from the muffler and run the unit to cycle out any suit or rust buildup from condensation at startup.

 

[bagged97taco]

While cranking can you feel compression from the exhaust pipe? Could be too much back pressure from a clogged muffler if spark and fuel are present. Long shot though.

On those onan 4k's the book says at certain intervals to remove the drain plugs from the muffler and run the unit to cycle out any suit or rust buildup from condensation at startup.

Yes has good compression. Every once in a while it will backfire thru carb while cranking. Also verified that the carb linkage is in correct.

 

[Bigbore500r]

I’m trying to figure out this damn onan 4K generator. I replaced the carb with an oem carb (not an Amazon special). It cranks but won’t fire. I have fuel pressure and bowl is full. Verified that fuel cut off solenoid is engaging. It has spark and obviously is getting air. I sprayed starting fluid in intake while cranking and it doesn’t do anything. Coach battery is brand new. Any help would be great.

How did you confirm it has spark?

If it wont cough or fire even with starting fluid, this leads me to believe it doesn't have ignition.

That, or your new carb's mixture is Waaaaaaaaay off and its being flooded - in that case, starting fluid, brake cleaner or the like wouldn't help.

Does it have an altitude knob? My friend's motorhome wouldn't fire the generator on his last trip, and I had to play with the altitude knob it get it "happy". We finally got it fired after some adjusting.

 

[OCMerrill]

Possible your compression is to low to fire. You might have cooked a valve so a leak down or at least a compression test is in order.
I have a dead Microlite 4.0 in my motorhome because years ago a carb stud broke and it sucked a bunch of sand out in Glamis. I ran it another 500 hrs and now its dead with 25lbs compression.

In most mechanics eyes they are disposable. SIngle cylinder, 3600-3800 rpm.

 

[Mototrig]

Yes has good compression. Every once in a while it will backfire thru carb while cranking. Also verified that the carb linkage is in correct.

Double check spark consistency.

The last thing I can add is check valve clearance. It's not difficult....I can probably get you the numbers in a couple of hours when I get home.

 

[bagged97taco]

How did you confirm it has spark?

If it wont cough or fire even with starting fluid, this leads me to believe it doesn't have ignition.

That, or your new carb's mixture is Waaaaaaaaay off and its being flooded - in that case, starting fluid, brake cleaner or the like wouldn't help.

Does it have an altitude knob? My friend's motorhome wouldn't fire the generator on his last trip, and I had to play with the altitude knob it get it "happy". We finally got it fired after some adjusting.

Pulled plug (new) hooked plug wire to plug. Grounded plug and engaged started button. Blue/white consistent spark.

 

[Bigbore500r]

Pulled plug (new) hooked plug wire to plug. Grounded plug and engaged started button. Blue/white consistent spark.

Then its mixture related. Let it air out and see if it will cough or fire with a little dribble of gasoline right off the bat. You might also be able to give it a little throttle on cranking by manually opening the linkage, if that makes it come to life it needs some adjustment. Just don't manually tweak that linkage while its running, cause that's the governor

 

[CLdrinker]

This is why I run mine every couple weeks. This things hate sitting

 

[braindead]

If it's mixture related hold the carb wide open and crank for a couple seconds to clear it out, then it should fire even with the carb wide open.

 

[Dr Rob]

Yes has good compression. Every once in a while it will backfire thru carb while cranking. Also verified that the carb linkage is in correct.

These 4.0 Onans were also known to break valve springs.

 

[Willie B]

…I hate to be a Debbie downer but you may have an internal electricle problem with the generator. I’ve had this happen to my 2.5 Onan propane and my 6.5 Onan propane… I hope you have not been shutting the generator down while it is under an AC load???…

 

[Icky]

Does it have an altitude knob? My friend's motorhome wouldn't fire the generator on his last trip, and I had to play with the altitude knob it get it "happy". We finally got it fired after some adjusting.

I was thinking this as well

 

[Mototrig]

…I hate to be a Debbie downer but you may have an internal electricle problem with the generator. I’ve had this happen to my 2.5 Onan propane and my 6.5 Onan propane… I hope you have not been shutting the generator down while it is under an AC load???…

Dammit don't tell me this

 

[bagged97taco]

I don’t know any history on this rig. It’s a friends moms that I’m helping out Thanks for all the reply’s. I’m going to work on it more later tonight when it’s only 105 outside

 

[Dalton]

Check the rocker arm, may be loose

 

[ElAzul]

Is it mounted in a RV? Does the RV have more than 1/4 tank of fuel? Mine won't let me start my Genny under a 1/4 tank

 

[bagged97taco]

Is it mounted in a RV? Does the RV have more than 1/4 tank of fuel? Mine won't let me start my Genny under a 1/4 tank

Yes mounted and that is always the first thing I check for!

 

[JDKRXW]

Pop of the valve cover and look for a valve that's not moving when cranking.

 

[PlanB]

This is why I run mine every couple weeks. This things hate sitting

Smart. The Onan in our toy hauler was the same way. All kinds of carb issues.

 

[bagged97taco]

Pop of the valve cover and look for a valve that's not moving when cranking.

That’s my next plan. Just need to order a gasket, everything online says it’s gonna rip when I remove it.

 

[bagged97taco]

Pop of the valve cover and look for a valve that's not moving when cranking.

. Found the issue.

 

[bagged97taco]

So I pulled the push rod out ( not bent). Valve was stuck open. Soaked it with some Kroil and messed with it and now it’s fixed. Adjusted the valves and now it runs like a champ. My guess is carbon build up cause the valve to stick open. It moves freely now. Ran the unit for 20 minutes with ac blasting. Didn’t skip a beat. Should I worry about anything else. I’m going to run some sea foam thru it just to try and clean it up some.

 

[Dr Rob]

So I pulled the push rod out ( not bent). Valve was stuck open. Soaked it with some Kroil and messed with it and now it’s fixed. Adjusted the valves and now it runs like a champ. My guess is carbon build up cause the valve to stick open. It moves freely now. Ran the unit for 20 minutes with ac blasting. Didn’t skip a beat. Should I worry about anything else. I’m going to run some sea foam thru it just to try and clean it up some.

I have a good engine for this generator that was replaced for the very same thing under warranty. If you or anybody else needs it please let me know.

 

[timstoy]

Start the generator at least once a month with a load, AC is a good load and keeps the RV cool for awhile! I run Lucas fuel stabilizer in all my vehicles.

 

[bagged97taco]

Start the generator at least once a month with a load, AC is a good load and keeps the RV cool for awhile! I run Lucas fuel stabilizer in all my vehicles.

This isn’t mine. But have a check list of things for the owner to do on a regular basis in the future. I run my diesel gen monthly just in case.

 

[SoCalDave]

I have a good engine for this generator that was replaced for the very same thing under warranty. If you or anybody else needs it please let me know.

Now that's a very generous offer...

 

[Dalton]

Check the rocker arm, may be loose

View attachment 1256439 . Found the issue.

my buddy is a Cummins employee and told me that’s a very common problem

 

[rrrr]

I have a 4K Onan in my shop I bought three years ago for $100 because it didn't run. It turns over OK. Maybe it's time to do a little work on it. Like, I dunno, pulling a valve cover.