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Fuel in oil 912ULS

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peterlthomson(at)gmail.co
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Posted: Sun Jul 05, 2009 4:04 am Post subject: Fuel in oil 912ULS

I have found my oil analyses consistently show 1.2-1.5% fuel in the oil sample.



Other oil properties (Shell Advance Ultra full synthetic, changed at 50 hrs on

unleaded gas) are fine. Engine runs quite cool, but no moisture in oil.

 

Any experiences here ?



Peter

Zenair CH701SP

Rotax 912 ULS 380 hrs since new

OIl analysis by Blackstone

  [quote][b]

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K Dilks

Joined: 22 Sep 2008
Posts: 108
Location: UK

Posted: Sun Jul 05, 2009 10:56 pm Post subject: Re: Fuel in oil 912ULS

Use a oil thermostat and get that oil hot....thats the reason for the hot oil temp is to evaporate fuel and water out. 

Oil dilution is not good for high load areas such as the cam and tappets.

Cruise at 90c is good.



Kev

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Thom Riddle

Joined: 10 Jan 2006
Posts: 1597
Location: Buffalo, NY, USA (9G0)

Posted: Mon Jul 06, 2009 3:10 am Post subject: Re: Fuel in oil 912ULS

Peter,



I think of only two ways that fuel could get into the oil supply.



1)The engine is running so rich that there is raw fuel going into the combustion chambers and mixing into the oil on the cylinder walls.



2) You clean the oil tank with fuel and don't drain it and let it dry completely before refilling with fuel.



If there is any other way, it does not come to mind.

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rampil

Joined: 04 May 2007
Posts: 870

Posted: Mon Jul 06, 2009 8:25 am Post subject: Re: Fuel in oil 912ULS

I don't know if 1% fuel contamination is actually a problem or outside

the expected range for Rotax 9xx.   It would seem an inconsequential

amount in terms of physical dilution of oil and reducing lubrication but

I have no idea as to whether it is relevant chemically.



Having an oil thermostat might treat the symptom, but as Thom

said the common way gas gets into oil is via leaky rings.  The

first step in diagnosis would be a compression test to test the rings.

It is a required component of the annual inspection anyway.

Rotax advises that the compression test should be performed when the

engine is close to it usual warm operating temperature so that the rings,

cylinders and pistons are in expanded thermal equilibrium.

(I tried my engine cold and it tested fine, but others recently observed

that their engine leak mightily when cold)



Ira

(in the midst of my 5yr annual)

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Ivor Phillips

Joined: 16 Jan 2006
Posts: 253
Location: London UK

Posted: Mon Jul 06, 2009 8:41 am Post subject: Fuel in oil 912ULS

Check the fuel pump Diaphram, if its weeping it can allow fuel to enter the gearbox/engine

regards

ivor

---- Peter Thomson <peterlthomson(at)gmail.com> wrote: 

 	  Quote: 	
		   I have found my oil analyses consistently show 1.2-1.5% fuel in the oil

 sample.

 

 Other oil properties (Shell Advance Ultra full synthetic, changed at 50 hrs

 on

 unleaded gas) are fine. Engine runs quite cool, but no moisture in oil.

 

 Any experiences here ?

 

 Peter

 Zenair CH701SP

 Rotax 912 ULS 380 hrs since new

 OIl analysis by Blackstone

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K Dilks

Joined: 22 Sep 2008
Posts: 108
Location: UK

Posted: Mon Jul 06, 2009 9:09 pm Post subject: Re: Fuel in oil 912ULS

The fuel dilution is a feature of using blow by gas to return the oil to the oil tank, there will always be some unburnt fuel in this. The designers take this into consideration so hence the need to get the oil temp up to evaporate the  contamination out of the oil. 

1% is ok but if you know you are running cool then do something about it. Long runs at low power / idle will accelerate the dilution situation. Worst case is when folks fill up with Jerry cans that have a small amount of diesel fuel , this goes strait to the crank case and in worst cases will increase the volume seen in the oil tank.



Kev

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Roger Lee

Joined: 10 Jan 2006
Posts: 1464
Location: Tucson, Az.

Posted: Sun Jul 12, 2009 5:45 am Post subject: Re: Fuel in oil 912ULS

Hi Peter,



Kev and Ira both make good points. I get an oil analysis done at each oil change and never have fuel trace in the oil. I will see if I can't post one of my analysis a little later. Doing an analysis every 100 hrs. or the annual is good. Getting the oil temps up does keep impurities cooked out of the oil. The thermostats only go to 180 degrees. Many people use a 2" strip of aluminum tape to accomplish the same thing over the radiator, but you can tailor it to your needs if you need more temp or less. Shoot for at least 212-220. I do a compression test and carb balance every 100 hrs or annual on my plane and every plane that comes through my shop. Doesn't make any difference what the manual says here for me. It only takes about 20 minutes on a hot engine for a compression test. I do it right after the carb balance because the engine is still hot. Never do a compression check on a cold 912. Test pressure is 87 psi and not 80 like I see test coming from A&P's. Typical test results are 87 test and 83-85 psi differential.

The other thing believe it or not that will contribute to this issue is out of sync carbs that never get balanced. (I won't get into the full explanation here) The engine just shakes too much, even if you can't feel it, and causes a host of issues over time. Do this every 100 hrs. or every annual. Carb balance is extremely important for your engine. You should be keeping on top of carb rebuilds too. Waiting 800 hrs. is not good.



Between better oil temps, compression test and carb care you will have a good handle on what the engine is doing and have a healthier engine.

p.s.

I hope you are doing oil changes at 50 hrs. for 91 octane and 25 hrs for 100LL. Plugs at 75 hrs. and not 100 hrs.

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