Oil / Fuel In Manifold Pressure Guage

by Chris Adams in 912 / 914 Technical Questions 12 years ago

Has anyone had a symptom of a fuel / oil mixture filling the manifold pressure guage. It has us baffled as this should be negative pressure ? Application 912F3 installed in a DA-20.

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Re: Oil / Fuel In Manifold Pressure Guage

by Tony Garrett » 12 years ago

Tony Garrett

20 37 11

I have the same problem - oil in the MP gauge. After some oil accumulated in the bottom of the gauge, it stopped. The attached photo shows my MP gauge after about 50 hours.

P1030212.JPG (You do not have access to download this file.)

Thank you said by: Chris Adams

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Re: Oil / Fuel In Manifold Pressure Guage

by Mike Goodrich » 12 years ago

Mike Goodrich

8 123 0

Tony & p reino
I'm in the process of fitting a manifold pressure gauge so I'm interested in your problem.
I assume you've got a simple tube from the manifold to the gauge and that's how the oil/fuel is getting to the gauge. Where have you connected the tube to the manifold and is the gauge lower than the manifold?
In theory (reality is often different) oil/fuel should not go down the tube because there shouldn't be any airflow to help it along (assuming that the tube is a small bore tube). That liquid manages to get along the tube suggests that there is a leak at the gauge end and that as the air flows down the tube to escape via the leak it takes some liquid with it. I don't think your suggestion that this gauge/tube should be negative pressure is correct, looking at the manifold pressure curves in the operator manual the pressure is positive and above atmospheric pressure above about 5000 rpm (with a fixed pitch prop). What I find interesting is that according to the book the manifold pressure of a 914 can rise to at least 38 inches of mercury (Hg) yet your gauge only goes up to 35. From that we could assume that perhaps (seems unlikley) that the overpressure has damaged the gauge and caused a leak. I say unlikely because the diference between 35 and 38 inch Hg is very small and I doubt that the gauge isn't strong enough to take it.
The other interesting thing is why oil, fuel would be more understandable. I once made the mistake of turning the engine over by hand for a long peroid without the plugs in and as a result the oil remained in the crankcase and flooded the turbo. Luckily I realised before I started up but it makes me think that the most likely way of getting oil into the manifold is via the turbo so perhaps check the little spring loaded ball check valve in the oil supply line to the turbo.
These are just some ideas.
Last question how did you manage to insert your photo, I've always wanted to do that?
Mike G

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Re: Oil / Fuel In Manifold Pressure Guage

by Mike Goodrich » 12 years ago

Mike Goodrich

8 123 0

Tony, I've just realised that you may have a 912, in which case your 35 inch Hg would be OK.
Mike

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Re: Oil / Fuel In Manifold Pressure Guage

by Tony Garrett » 12 years ago

Tony Garrett

20 37 11

Mike
Insert photo using the add file button at the bottom left.
I have a new 912. I installed two new MP gauges using vacuum hose from where the crossover tube was. I removed the crossover tube so that one MP gauge measures one intake manifold pressure and the other gauge measures the other side manifold pressure. I keep the MP on each side within 0.2" of each other. Yes my gauges are lower than the connection point. I could never detect any oil or gas inside the hoses. I spent months messing with the gauges and talking to Lockwood and LEAF mechanics. Our primary conclusion was that the oil gets there by magic. I have not disassembled a MP gauge. Maybe there is some oil inside them as part of the mechanism. The level of oil in my gauges stopped rising after about 15 hours. I currently have about 300 hours on the engine and gauges and the gauge oil level has not changed.
Tony

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Re: Oil / Fuel In Manifold Pressure Guage

by Chris Adams » 12 years ago

Chris Adams

1 5 0

Tony, Your pic is interesting....same but diffrent in color.(your fluid looks green) What type of coolant are you using ? Just a thought but perhaps it is coolant and not oil / fuel. As to how it is getting there...we have everyone baffled ? We run evans and the fluid in our gauge looks beige. We have tried 2 new gauges to the same result. The negative pressure in the sense line (off the balance tube between manifolds) is at its lowest when throttle is at full.....but still -1 ( confirmed with a pressure / vacuum gauge ). Diamond has told us that there is a small hole (1/64") in the manifold pressure elbow at the firewall P/N 20-7170-01-01.....it could be blocked. We are now looking into this as we have checked and found none ? We operate 4 Katanas and this is the only one with the problem. All are variable pitch and the MAP reads below 30" on max throttle.

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Oil / Fuel In Manifold Pressure Guage

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