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Onan Propane House Back-Up Won't Cold Start

soundtechmike

Registered
Engine model is P224G-L/116148. When it trys to start on its exersize cycle it fails. If you reset and try manually. still won't start. while its cranking the throttle lever on the throttle body hooked by a rod to the solenoid fluctuates back and forth. But if you hold the throttle against the idle stop screw it fires right up and runs fine. Also starts right back up on its own with only short warm up. but cold does not start. Any help would be greatly appreciated!
 

Andrew Mackey

Moderator
Last Subscription Date
05/14/2017
Re: Onan propane house back-up won't cold start

Cold start on propane was a prblem on propane powered sky lifts i used on jobsites. Cold gas, cold engine - no start! We used to keep a little gasoline in a squirt type oil can. One shot of gasoline, and the engine would start - no matter how cold it ws outside (-10F one day). You might try and adjust the throttle selenoid so the throttle stays closed for cranking, and opens after the engine starts.
 

cornbinder89

Subscriber
Last Subscription Date
05/11/2020
Re: Onan propane house back-up won't cold start

When we ran manlifts on LP, we found that the spark-plug gap needed to be reduced from the gasoline setting for best results. Look in the manual if they list a setting for LP.
 

dkamp

eMail NOT Working
Re: Onan propane house back-up won't cold start

The reason why it's not starting with throttle wide open, vs, throttle at idle, is simple: Dynamic compression with throttle at full open is too high for the ignition to fire off a hot enough spark to ignite the fuel/air mix.

Shortening the spark plug gap will increase arc current, perhaps enough to make it fire. Clean the points good, check the condenser and coil.

Also make sure that the diaphram in your fuel controller hasn't dried out and become hard.
 

kwil

Registered
Re: Onan propane house back-up won't cold start

you should be able to adjust selenoid close your plug gap 5 thousands and advance timing 2 to 3 degrees LP is 140 octane it needs a hot spark kwil
 

dkamp

eMail NOT Working
Re: Onan propane house back-up won't cold start

you should be able to adjust selenoid close your plug gap 5 thousands and advance timing 2 to 3 degrees LP is 140 octane it needs a hot spark kwil
No, it's closer to 112 RON... but it DOES need a very hot spark. More than likely, his demand regulator's diaphragm has degraded enough so that it's developed a slight leak or too stiff to respond.

Closing the throttle would, under normal conditions, reduce the regulator's demand flow, but the fact that it fires with the throttle closed clearly identifies that mixture while cranking at WOT is too lean to get ignition. Closing the throttle decreases dynamic compression which increases spark energy intensity, thus, igniting the lean condition.

If it was running on propane previously (and it sounds as if it has been), then altering the timing is not a proper solution here... something degraded and needs to be replaced.
 

grub54891

Registered
Age
63
Last Subscription Date
06/08/2010
I'd go with regulater also. I've rebuilt many with the same issue. One place I worked the owner had the kohler specialist up twice to find the issue.
The no start persisted. I offered to rebuild the regulater, and it's been fine for the last 9 years.
 

zuhnc

Subscriber
Last Subscription Date
12/08/2019
I know this is an older thread, but the OP did not respond with the fix. This might help, if the OP still has the generator set. What is the model and spec of the complete generator? Search the forum boards for Onan generators. If your unit is a 12RS (I think) there is a discussion on the Onan forum as to the reasons for not starting cold. Nothing new parts will fix. It is a design issue with the electronic controller. zuhnc
 

dkamp

eMail NOT Working
...But if you hold the throttle against the idle stop screw it fires right up and runs fine....
If there's a design issue with the electronic controller, I doubt it would have any bearing on the situation he describes... clearly, he's getting fuel, air, compression and spark.

Mike... what did you find out?
 

zuhnc

Subscriber
Last Subscription Date
12/08/2019
He is NOT getting proper fuel under normal remote/local unattended operation. The governor is programmed to pulse the throttle, I believe, but the programming was not done properly to enable reliable cold starts. Manually closing the throttle enables proper fuel flow when cold. Since this unit (if it is a 12RS) is no longer in production, and had a limited production span, no fix is available from Onan/Cummins. There is an individual, who posts on the Onan generator forum, that has one of these units, and is a programmer or some such Guru. He has examined and adjusted the programming for his unit, but has not come up with a reliable fix. Since it starts properly with manually closing the throttle, and runs properly after that, both with and without load, I do not believe there is anything wrong with any of the regulators or gas supply. A fix would be to use a solenoid to pull the throttle closed upon startup, and have that solenoid drop out upon running (oil pressure switch or voltage sensitive relay, for instance). zuhnc
 

dkamp

eMail NOT Working
I'll reiterate, because I've dealt with this exact same problem in 4 of the 9 propane fueled engines in my collection, and a half a dozen other guys' that suffer the same:

IF closing the throttle results in the engine firing and starting, the problem is EITHER weak ignition (not strong enough to fire the mixture COLD under high compression) or a worn-out demand regulator diaphram that is not accurately responding to fuel demand.

Messing with programming is nothing more than a band-aid. When set up right, you can hand crank them to life by stepping on the crank three times with the ignition on (or magneto ground lifted) AND the gas valve on.
 

EICBob

Subscriber
Age
62
Last Subscription Date
07/13/2019
...snip
There is an individual, who posts on the Onan generator forum, that has one of these units, and is a programmer or some such Guru. He has examined and adjusted the programming for his unit, but has not come up with a reliable fix. Since it starts properly with manually closing the throttle, and runs properly after that, both with and without load, I do not believe there is anything wrong with any of the regulators or gas supply. ...snip
I am the the individual that zuhnc is referring to. I have 8 Onans that all are propane fueled. 2 are the RS12000 series that both have exhibited the cold hard/no start that the OP here has mentioned. I have used my primary regulators and hoses from 2 of my other sets that do run fine and that has not corrected the issue.
I have in the course of troubleshooting this used a decent manometer to verify proper WC values going into and out of my regulators.
I have accessed the programming and did find errors in the base control programming. These values may or may not have been changed at some point in time to correct this issue. The changes I made come from the manufacturer of controller.

******************************************************************************************************************************************************

I'll reiterate, because I've dealt with this exact same problem in 4 of the 9 propane fueled engines in my collection, and a half a dozen other guys' that suffer the same:

IF closing the throttle results in the engine firing and starting, the problem is EITHER weak ignition (not strong enough to fire the mixture COLD under high compression) or a worn-out demand regulator diaphram that is not accurately responding to fuel demand.

Messing with programming is nothing more than a band-aid. When set up right, you can hand crank them to life by stepping on the crank three times with the ignition on (or magneto ground lifted) AND the gas valve on.
Onan, and I can only speak to what they have done, has issued service bulletins, cold weather modification kits and different governor controllers to address this. One of the RS12000 units uses an Onan engine and the other is a Honda engine.
Both have had their ignition systems worked on and set to manufacturer specs.
On the Onan engine I have replaced the demand regulator and it still acts up. The Honda powered set uses a slightly different type of demand regulator that I have not dived into yet. Yet as mentioned once either of these sets warms up they both run well, hold the load and generally act as you would expect.

That being said, it is entirely possible I have missed something along the way and I will admit that to the end of time.
Just wanted to add my experiences with cold/no start propane units that rely on electronic control versus electro mechanical controls like my older Onans.

Interesting conundrum and it would be nice to know if the OP of this thread ever got his issue resolved and what was the fix??

-Bob C
 

dkamp

eMail NOT Working
Hmmm... that's fascinating, Bob, thank you for including that tidbit.

Okay, so, there are SOME gaseous engines that use electronic fuel injection, but I doubt this is the case... it sounds like a typical system, but with electronic control, perhaps the governor is electronic, perhaps the fuel cutoff is too.

The basic recipie for a running engine is fuel+oxygen+compression+ignition. Most wrench-holders are familiar with the character of a gasoline engine, but fail to realize that on cold start, the fuel entering the combustion chamber is frequently in non-evaporated state.... only a tiny fraction of it actually evaporates enough to mix with oxygen, so there's free oxygen floating around when the spark ignites. It's because of this, that a gasoline (liquid) engine needs enrichment (substantial over-enrichment, actually) in order to start (lots of unburned fuel goes out the pipe). When this happens, it's possible for the spark plug to get wet, thus, shorting the plug, and most gearheads refer to this as 'flooding'.

Gaseous engines don't have the non-evaporation issue... it's gaseous... all they need, is to mix with oxygen... and of course, the oxygen/fuel mix needs to be close enough to a hot-enough spark to ignite. Gaseous engines can flood very easily... just pour in so much NG or propane, to displace all the oxygen, and you have a flooded engine.

My point being, is that as odd as it seems, it's actually easier to flood a gaseous IC engine, than a liquid-fuel IC engine.

I don't think I mentioned it here, but I've mentioned to others, in newbie-type threads, that Gaseous engines act just the opposite of liquid engines with respect to mixture- if you're working on a gaseous, and it's acting like a gasoline engine running LEAN, then the gaseous is running too rich... and if it acts like a flooded gasoline, then the gaseous is running lean. In a gasoline engine, to clear a flood, you open the throttle all the way to draw in plenty of air, and the least amount of fuel. To clear a flood on a gaseous, you CLOSE the throttle, which reduces fuel demand, in the venturi.

For a variety of reasons, gaseous fuel engines benefit from more compression, more ignition advance, and more ignition energy. The ignition advance is due to flame speed difference... but compression and ignition energy are intertwined. Higher compression forces the mixture of molecules closer, which increases the flame's propgation rate, just like cramming more people in a small area increases the velocity of virual transmission. Working against this, however, is the fact that higher combustion pressure raises the voltage required to incite corona-effect on the spark plug gap. Lower compression will allow a weaker spark, but having a poor mix of fuel and air (stratification, etc), means a spark is more likely to NOT result in ignition event. Higher compression, by sheer physics of space, makes it more likely that the spark plug will be in the middle of a good mix... but less likely that a weak ignition system will do the job. Voltage starts the arc, current maintains it, and it's the arc current and duration that creates the heat sufficient to light a fire.

When a naturally-aspirated engine's throttle is wide open, the engine ingests air with the least amount of restriction, so it draws in the greatest volume of air, and thus, it's compression level is the greatest. When you CLOSE the throttle, the throttle poses an input restriction, lowering manifold pressure substantially, and imposing throttling-loss, and compression pressure falls. The paradigm here, is that it takes a certain amount of compression to run... but too much compression, with too weak of a spark, will cause it to falter as well.

The propane/NG engine starts with a closed throttle because throttling losses lower combustion chamber compressiion pressure, and the ignition system finds enough oxygen and propane mixed, to touch off a cylinder or two.

So if they actually did a software change that closed the throttle for cold start, then there's something fundamentally wrong with how the parts were spec'd and set up. If they opted for a software change, I'd be willing to bet that the decision was made simply because they could 'avert' a crisis of customer faith through a software hack, then actually fixing the 'real' problem. I don't doubt for a moment any of what you've said, Bob, and I'm confident in it, as I've had many propane engines that'd crank-to-the moon without firing, and simply forcing the governor/throttle linkage to the idle-stop screw for a moment would start it... and each time, cleaning and adjusting the valves or tuning up the ignition resolved it.

I think there's cause for deeper investigation here...
 
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