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Onan L423 diesel won't start

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Old 12-03-2011, 09:22:21 PM
dalmatiangirl61 dalmatiangirl61 is offline
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Default Re: Onan L423 diesel won't start

Originally Posted by David C View Post
The last test would be to pressurize each cylinder with air and listen for air leaks out of the intake or exhaust manifolds.
This is usually one of the first tests I do on an engine that has been sitting. Quite frequently I find the valves are not seating, so with cylinder pressurized, i use a soft brass hammer and tap on valve stems, this lets air flow over seats removing debris, and pressure helps seat the valves. Tap away till it seats tight. This has worked numerous times when others said it needed a valve job.
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Old 12-03-2011, 10:02:50 PM
bobbyz72 bobbyz72 is offline
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Default Re: Onan L423 diesel won't start

As parts are no longer avalible..... I need the Radiator and the Light test switch on the control panel...... If you part it out that is.
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Old 12-04-2011, 01:27:45 AM
David C David C is offline
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Default Re: Onan L423 diesel won't start

The engine shouldn't be worn out but I did buy it used 8-10 years ago from a friend who had connections with Onan and it had only about 400 hours on the hour meter then (but who really knows if the meter was correct or if it had been changed). The generator has been run very little since I have owned it. It was always a little hard to start but with a couple squirts of ether and a little patience, it would usually start.
Gunny, Power

If the statement above had not been made, I would feel better about the condition of the engine. It sounds like it has not started very well for some time.

If you can get your hands on a leakdown tester, it might give you a better idea of the condition of the rings or the cause of the low compression. Google "leak down tester" and you can get some interesting info. It may be a good investment if it would save you from tearing the engine down.

Everyone has suggested some good ideas, so you just have to start eliminating one possibilty at a time.

David C.
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Old 12-04-2011, 05:53:37 PM
fromthefifties fromthefifties is offline
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Default Re: Onan L423 diesel won't start

Originally Posted by Power View Post
I agree with Gunny - follow the proceedure I laid out before you open it up- as soon as you do, you are in for time and the cost of gaskets at a minimum. My proceedure takes very little of your time and has negligible cost - bit of penetrant & oil, may solve your problem, and get you a saleable unit.
I did the procedure that you recommended. Penetrating oil in all cylinders, set overnight and then rock the crank back and forth, did it again for the 2nd night. Then put motor oil into each cylinder and left overnight. Cranked out excess oil and re-installed injectors. I used my shop kerosene heater to blast heat at the engine for 30-45 minutes to warm it up. Used hair dryer in intake manifold and tried to start the engine while spraying diesel fuel down the intake. It puts out white smoke and sounds like it is close to starting but it doesn't.

I may try ether one more time before I take the manifolds and head off.

Thanks for all the suggestions and help.

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Old 12-05-2011, 09:40:14 PM
fritzrr1104 fritzrr1104 is offline
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Default Re: Onan L423 diesel won't start

If you put oil in the cyl and the comp. goes up then the rings are washed dry this will increase the comp by sealing the rings to the cyl a broken ring will not increase with oil

a few pumps from a oil gun to much could cause a hydro lock and bend something turn eng over befor trying a comp. test agin

i didn't see if you tryed to start the eng after you put the oil in
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Old 12-05-2011, 09:46:00 PM
EnginePat EnginePat is offline
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Default Re: Onan L423 diesel won't start

More Heat!!My father used to used a propane torch in the intake of his old Rio to start it.It worked! I don't do that but a 2200 watt heat gun work great.

edit: PB blaster sprayed into the cylinders will help breakup carbon buildup. it works. Also I use Super Clean straight to remove carbon on pistons,heads ect, I melts the carbon some how. but you have to wipe/rinse it off.
5GO 3/RA
7.5 jb's x4
6.0 DJE X 1

Last edited by EnginePat; 12-05-2011 at 10:08:50 PM.
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Old 11-15-2012, 10:47:26 AM
fromthefifties fromthefifties is offline
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Default Re: Onan L423 diesel won't start

This is a delayed update to this thread regardiing the Onan L423 diesel that wouldn't start last year.

Had some more health and personal problems that delayed working on this generator until a few days ago.

If you will remember I tried all of the suggestions that were provided in response to my original thread. None of these suggestions resulted in getting the generator started.

So, a couple of days ago, I decided to go ahead and pull the head to see if there was anything obvious keeping the compression below 200 on all cylinders. The head looked as if it had very little usage since it was last rebuilt/reworked. The reason that I say that is that there was hardly any carbon buildup on the valves or the top of the pistons and no rust or evidence of any kind of leakage on the head or head gasket. The head gasket was in excellent condition and looked new, no blowby evidence anywhere and no obvious damage. No bent pushrods, and the intake manifold and exhaust manifold did not have any blockage.

The top of the cylinder walls did have some carbon buildup but not that much. The cylinder walls did appear to be fairly dry. This may be the problem, stuck rings or too much ether cleaned the cylinders.

Since I no longer need the generator, I have now reached the point that I don't want to throw any more money into it to get it running. I have close to $1000 in rebuilding injector pump, injectors and misc other expenses.

I am going to try to sell the generator as is or sell parts off it and thought I would give you folks first shot at it as you'all did work with me trying to get it started.

I'll take $1200 for the generator as is. If no one buys it, I'll start parting it out.

Thanks, again, for all your help.

Neil Williams

My phone number was incorrect in that last post, it is 615-828-3589.

Neil Williams
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