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Rotax 912ULS Low Oil Pressure after start


ozzietriker
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Hi Folks,

 

Every now and then when I start my engine, the Oil Pressure reading on FlyDat reads zero (or very) low for a split second but long enough for the FlyDat system to flag it with a red status light.

 

I have no issues with pressure indication say approx 1-2 secs after the engine fires and during total flight duration - it's only the first second or two after it fires.

 

My engine is 18 months old fitted with original oil pressure sensor and yes I gurgle the tank prior to engine start and oil level is half way up the dipstick & changed regularly.

 

Has anyone ever had this issue before? i'm thinking that I have a dodgy pressure sensor.

 

Thanks in advance,

 

Andrew

 

 

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We were talking at Aerokits about this problem recently in regard to the restricted diameter of the flexible metal oil lines now supplied with Savannah kits, they are significantky thinner than the rubber hoses that used to be supplied, but are more durable in the area around the exhausts in the engine bay. I like the idea but Reg Brost suggested they would increase the issue of oil pressure drop after throttle increase, apparently the oil pressure actually drops momentarily after the engine revs up. Most engines pump more oil when the revs increase, immediately increasing pressure. Might pay to check for sharp bends in your oil hoses, look for flattening which might be causing restriction.

 

 

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I have recently seen exactly the scenario you describe. I'm no Rotax expert but - I'm thinking this is probably quite normal.

 

When you 'burp' the engine, the slow turning creates an imbalance where the blow-by gasses force more oil out that the pump takes in. It winds up in the sump so we can check the level. At normal running speed I expect the blow-by is much less effective as a scavenging mechanism. (and in fact burping is best done slowly holding the compression stroke for as long as possible.)

 

Then when you start the engine you return to a 'normal' state where the return oil in and oil out must reach some equilibrium. It might just take a few seconds for a full flow of pressurised oil to get from the sump through the oil pump up to the sensor. This may also depend on the length of the oil hoses in different installations.

 

Two things you might try.

 

A cold start without burping. See if the pressure comes up quick enough to miss the warning. (not suggesting you fly without checking the oil level :) )

 

After burping crank a few times with the magnetos off so the system is primed. See what happens.

 

 

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I can't follow the logic of some of that. The pressure oil pump gets oil from the oil tank so while ever the tank even has a small amount of oil there should be no interruption to flow . Burping the oil just makes sure that oil collected in the sump is returned prior to checking level. If you topped up without doing it you could overfill and some oil would be vented overboard.. If you turn the engine backwards it would introduce air into the oil system, but no one has suggested that is a factor. The engine is a dry sump system except that it is fairly unusual in returning the oil to the tank by crankcase pressure from blowby gases. New engines have less blowby and may have to be turned over slowly to allow time for the compression to leak past the rings when "burping" or it takes longer. Cranking an engine over without firing it up to get a pressure indication is a good idea if the battery isn't unduly taxed, by doing it..Nev

 

 

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We were talking at Aerokits about this problem recently in regard to the restricted diameter of the flexible metal oil lines now supplied with Savannah kits, they are significantky thinner than the rubber hoses that used to be supplied, but are more durable in the area around the exhausts in the engine bay. I like the idea but Reg Brost suggested they would increase the issue of oil pressure drop after throttle increase, apparently the oil pressure actually drops momentarily after the engine revs up. Most engines pump more oil when the revs increase, immediately increasing pressure. Might pay to check for sharp bends in your oil hoses, look for flattening which might be causing restriction.

Thanks for your input & tip - i've just checked my oil hoses and they're mickey mouse.

 

 

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I have recently seen exactly the scenario you describe. I'm no Rotax expert but - I'm thinking this is probably quite normal.When you 'burp' the engine, the slow turning creates an imbalance where the blow-by gasses force more oil out that the pump takes in. It winds up in the sump so we can check the level. At normal running speed I expect the blow-by is much less effective as a scavenging mechanism. (and in fact burping is best done slowly holding the compression stroke for as long as possible.)

 

Then when you start the engine you return to a 'normal' state where the return oil in and oil out must reach some equilibrium. It might just take a few seconds for a full flow of pressurised oil to get from the sump through the oil pump up to the sensor. This may also depend on the length of the oil hoses in different installations.

 

Two things you might try.

 

A cold start without burping. See if the pressure comes up quick enough to miss the warning. (not suggesting you fly without checking the oil level :) )

 

After burping crank a few times with the magnetos off so the system is primed. See what happens.

Thanks for those tips - I will give them a try!

 

 

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I can't follow the logic of some of that. The pressure oil pump gets oil from the oil tank so while ever the tank even has a small amount of oil there should be no interruption to flow . Burping the oil just makes sure that oil collected in the sump is returned prior to checking level. If you topped up without doing it you could overfill and some oil would be vented overboard.. If you turn the engine backwards it would introduce air into the oil system, but no one has suggested that is a factor. The engine is a dry sump system except that it is fairly unusual in returning the oil to the tank by crankcase pressure from blowby gases. New engines have less blowby and may have to be turned over slowly to allow time for the compression to leak past the rings when "burping" or it takes longer. Cranking an engine over without firing it up to get a pressure indication is a good idea if the battery isn't unduly taxed, by doing it..Nev

Thanks Nev, I will hand crank it and see what the oil pressure reads :)

 

 

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Thanks Nev, I will hand crank it and see what the oil pressure reads :)

Not sure that Nev meant by hand. I think he meant crank it over without the mags on so it doesn't fire, hence the reference to making sure your battery wasn't overworked.

 

 

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