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Low Oil Pressure Indication on Cockpit Gauge

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hgmckay

Joined: 23 May 2006
Posts: 397

Posted: Mon Jun 20, 2011 5:34 pm Post subject: Low Oil Pressure Indication on Cockpit Gauge

In the past there have been numerous discussions on this subject as well as  numerous suggestions as to what is causing low oil pressure readings on 912  engines. On April 4 thru April 5 of this year I posted a description of my drop  in oil pressure on my 912 UL, and my concern about it. Roger Lee suggested a  method to determine if the sender was functioning correctly, or if I had a  faulty gauge, or possibly a bad ground. I finally got around to installing  Rogerâ€™s suggestion and checked a mechanical pressure gauge against the electric  VDO pressure gauge in the cockpit. I have three pictures of the set up available  for those who are interested. Here is the sequence of events and steps I have  taken to find out what the problem is, and my conclusions at this point.

  

 1.    While flying in April I noticed a drop in oil pressure  as indicated on the VDO cockpit gauge. Drop at cruise was from about 3.8 bar  (55.1 psi) to 2.1 bar (30.45 psi).

 2.    Posted the issue on Matronics, and got numerous  suggestions as to the problem.

 3.    Replaced the oil pressure sending unit with a new one  from Lockwood Aviation.

 4.    Replaced the oil pressure regulator with the new Rotax  pressure spring and mushroom head (replaces the old spring and steel  ball).

 5.    After these changes, flew the plane. There were no  changes in the oil pressure as seen in the cockpit on the VDO gauge. On startup  pressure went to 3.8 bar (55.1 psi) â€“ cold engine, but within one or two minutes  pressure was back down to about 2.5 bar (36.25 psi), and continued to drop to 2  bar (29.4 psi). At cruise with oil temp normal the pressure indication was still  2 bar (29.4 psi). Pressure holds at 2.0 to 2.1 bar during flight.

 6.    Decided to check the pressure using a mechanical gauge  per Roger Leeâ€™s suggested method (see photos). I was in the cockpit, and a  second person read the mechanical gauge.

 7.    On start up, the cockpit gauge immediately went to 3.8  bar as usual.

 8.    Within a minute at 2200 rpm the needle on the  mechanical gauge was swinging violently (vibrating) between 55 psi and 75 psi.  The cockpit VDO gauge was initially steady at 3.5 bar (50.75 psi) and over about  a minute or two the pressure gauge in the cockpit gradually dropped back to  about 2.5 bar and then to 2.1 bar (at 2200 rpm).

 9.    Increased the rpm to 2400, pressure gauge in cockpit  held at 2.1 bar (30.45 psi). Mechanical gauge needle was still vibrating between  56 and 60 psi, averaging around 57.5 psi.  

 10.  Increased rpm to 3000, pressure gauge in cockpit held at 2.1 bar  (30.45 psi). Mechanical gauge needle was still vibrating between 56 and 60 psi,  averaging around 57.5 psi. 

  

 Conclusions at this point:

  

 1.    I do not have a low oil pressure problem.

 2.    Canâ€™t explain why I have a 27 psi difference in oil  pressure readings of 57.5 psi on the mechanical gauge and 2.1 bar (30.45 psi) on  the VDO gauge in the cockpit.

 3.     If I do not have a low oil pressure problem,  what is causing the low reading in the cockpit?

 4.     It is either a bad gauge, or a bad or partial  ground somewhere. 

 5.    Is there a way to check the VDO gauge to see if it is  bad. I noticed that the direct mechanical gauge needle constantly vibrated which  indicated hydraulic pressure fluctuations which must be coming from the oil  pumpâ€™s operation. If these pressure fluctuations are normal (I thought the oil  pressure regulator was supposed to dampen these out), the oil pressure sending  unit must constantly sense these pressure fluctuations. Why are they not seen on  the VDO gauge in the cockpit?

 6. If there is a way to check the VDO gauge itself, and it is reading  correctly, the problem must be a bad or partial ground.

  

 Any ideas where I should go from here????

  

  

 Hugh G. McKay III, P.E.

Senior  Consultant

Worldwide Engineering Inc.

4090 North NC Hwy. 16

Denver, NC  28037

  

 Allegro 2000

 Rotax 912 UL



Ph. 704-661-8271

Fax  704-483-5466

email hgmckay(at)bellsouth.net

http://www.wwegeo.com



 Hugh G.  McKay III, P.E.

Senior Consultant

Worldwide Engineering Inc.

4090 North  NC Hwy. 16

Denver, NC 28037



Ph. 704-661-8271

Fax  704-483-5466

email  hgmckay(at)bellsouth.net

http://www.wwegeo.com

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Roger Lee

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

Posted: Mon Jun 20, 2011 8:30 pm Post subject: Re: Low Oil Pressure Indication on Cockpit Gauge

Hi Hugh,



Now you know why I tell people with a low oil pressure issue it usually almost never a real low oil pressure problem in the engine, but an indication issue. The fluctuation on the mechanical gauge may have come from air in your mechanical gauge line if you did not bleed off the air down by the gauge after the engine was started, too large a copper line (should have been 1/16th") and didn't you take out the new oil pressure regulator cone and go back to the old ball bearing? The old style ball bearing caused these types of fluctuations / chattering.

My guess is still like before, a partial / poor ground, but it may be the oil pressure gauge itself.



I would connect a new ground to the gauge first and if that didn't work buy a new gauge.

_________________
Roger Lee
Tucson, Az.
Rotax Instructor & Rotax IRC
Light Sport Repairman
Home 520-574-1080 TRY HOME FIRST
Cell 520-349-7056

Dick Maddux

Joined: 24 Jun 2008
Posts: 516
Location: Milton, Fl

Posted: Tue Jun 21, 2011 3:20 am Post subject: Low Oil Pressure Indication on Cockpit Gauge

Hugh,

  Another reason you may be getting wild swings in your  mechanical gauge setup is that you might not have put a loop in the copper  line (if that's what you are using) feeding the gage from the sender. We do  this in aircraft (in addition to an internal line restrictor) in order to dampen  out any pressure pulses to the gage.(also to prevent work hardening of the  copper line) 

  In my case I had to remove the new "cone" system and go back to the  old ball because the new system was giving me large spikes in pressure  indications. It's fine now. I have an old early 90's engine .

                              Dick Maddux

                              Milton,Fl

 [quote][b]

Roger Lee

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

Posted: Tue Jun 21, 2011 5:03 am Post subject: Re: Low Oil Pressure Indication on Cockpit Gauge

I have never had pulsations in my mechanical gauge. If you use the smallest copper tube from the store and bleed the air at the gauge tip after the engine is running mine is rock steady. I have about 5' of 1/16th tube so it can come into the cockpit with me so I can compare both gauges.

_________________
Roger Lee
Tucson, Az.
Rotax Instructor & Rotax IRC
Light Sport Repairman
Home 520-574-1080 TRY HOME FIRST
Cell 520-349-7056

hgmckay

Joined: 23 May 2006
Posts: 397

Posted: Tue Jun 21, 2011 6:20 am Post subject: Low Oil Pressure Indication on Cockpit Gauge

Roger,



I was in a hurry to determine if my oil pressure had really dropped off that 

much, so I simply uses 1/8" npt pipe from the engine to the mechanical gauge 

which required a 1/4" NPT connection so I had to bush the 1/8" line up to 

1/4" to fit the gauge. I knew I would have needle vibration if I did this 

but wanted to see if I really had "low oil pressure". Explain how you get 

from 1/8" pipe down to 1/16" soft copper line and then attach a 1/4" NPT 

mechanical gauge at the end of the 1/16" copper line



Hugh G. McKay III, P.E.

Senior Consultant

Worldwide Engineering Inc.

4090 North NC Hwy. 16

Denver, NC 28037



Ph. 704-661-8271

Fax 704-483-5466

email hgmckay(at)bellsouth.net

http://www.wwegeo.com

--

moosepileit

Joined: 10 Dec 2007
Posts: 26
Location: USA

Posted: Tue Jun 21, 2011 12:58 pm Post subject: Re: Low Oil Pressure Indication on Cockpit Gauge

You changed too much at once.



Put the oil pressure regulator back to the way it was.  Take the oil sender and gauge as installed out of the loop.  I used a $15 dollar chinese kit from autozone, nylon line is long enough to go from the engine into the cockpit.



Let the oil bleed up before tightening at the gauge, per the instructions.



You are now ready to run and should have no problems, I bet.



My sender was totally all over the place when it went out, the electric VDO standard unit.  It rung out fine on an ohmeter on a bench, was worthless in use.



The $15 gauge was rock solid in indication.



Once you figure out how to see accurate indications, figure out if you want the mushroom pressure regulator vs. the ball.



Changing both at once makes for tough troubleshooting.

 	  hgmckay wrote: 	
		  Roger,



I was in a hurry to determine if my oil pressure had really dropped off that 

much, so I simply uses 1/8" npt pipe from the engine to the mechanical gauge 

which required a 1/4" NPT connection so I had to bush the 1/8" line up to 

1/4" to fit the gauge. I knew I would have needle vibration if I did this 

but wanted to see if I really had "low oil pressure". Explain how you get 

from 1/8" pipe down to 1/16" soft copper line and then attach a 1/4" NPT 

mechanical gauge at the end of the 1/16" copper line



Hugh G. McKay III, P.E.

Senior Consultant

Worldwide Engineering Inc.

4090 North NC Hwy. 16

Denver, NC 28037



Ph. 704-661-8271

Fax 704-483-5466

email hgmckay(at)bellsouth.net

http://www.wwegeo.com

--

Roger Lee

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

Posted: Tue Jun 21, 2011 6:12 pm Post subject: Re: Low Oil Pressure Indication on Cockpit Gauge

Hi Hugh,



Ace Hardware or probably any hardware store will having reducing fittings in copper. Use 1/8" NPT 27 copper fittings with a compression end that takes 1/16th copper. The 1/8 NPT will fit your oil pressure sending port. This small line and the bleeding of the out of the line will keep the gauge from bouncing. You should have a 1/8 NPT 27 copper fitting where your sender goes. One threads into the sender port and the other end is a compression fitting. The reason to use copper is to keep your ground to the engine. Run that first line out about 18" - 20" and put a tee in line with 2 more of the same fittings you used at the sender. Screw your VDO sender in the top of the tee. Run the rest of the copper line on out and into the cabin. Put a compression fitting on here too and then use an adapter to attach the gauge. Leave the gauge a little loose. Start the engine and allow the air to be pushed out by the oil. When you get 3-4 drops of oil the air is gone. Tighten the gauge. The gauge should now be very steady. The line is so small it may take a minute or two for the oil to actually reach the end of the copper tubing.

_________________
Roger Lee
Tucson, Az.
Rotax Instructor & Rotax IRC
Light Sport Repairman
Home 520-574-1080 TRY HOME FIRST
Cell 520-349-7056

Last edited by Roger Lee on Wed Jun 22, 2011 5:49 am; edited 2 times in total

rickofudall

Joined: 19 Sep 2009
Posts: 1392
Location: Udall, KS, USA

Posted: Tue Jun 21, 2011 7:48 pm Post subject: Low Oil Pressure Indication on Cockpit Gauge

Hugh, As suggested by one of the other responders to your question, put a restrictor in the line, too. You can solder up the end of the tubing and then drill a .040" or so hole in the solder. It will only effect the gauge in the time it takes oil to fill the tube when you bleed it. After that it won't have any effect. Should you have a line break the restrictor will keep the oil loss down to a small amount so you can get your plane on the ground. I had a line break when I put a gauge in a car once. You'd be amazed how much oil can be squirted out a 1/8" ID tube when it's under 40 PSI. 



Rick Girard



On Tue, Jun 21, 2011 at 9:12 PM, Roger Lee <ssadiver1(at)yahoo.com (ssadiver1(at)yahoo.com)> wrote:

 	  Quote: 	
		   --> RotaxEngines-List message posted by: "Roger Lee" <ssadiver1(at)yahoo.com (ssadiver1(at)yahoo.com)>

 

 Hi Hugh,

 

 Ace Hardware or probably any hardware store will having reducing fittings in copper. Sorry, It has been a long few days out at the hangar. I misspoke. Use 1/8" NPT copper fittings. This small line and the bleeding of the out of the line will keep the gauge from bouncing. You should have a 1/8 NPT 27 copper fitting where your sender goes. One threads into the sender port and the other end is a compression fitting. The reason to use copper is to keep your ground to the engine. Run that first line out about 18" - 20" and put a tee in line with 2 more of the same fittings you used at the sender. Screw your VDO sender in the top of the tee. Run the rest of the copper line on out and into the cabin. Put a compression fitting on here too and then use an adapter to attach the gauge. Leave the gauge a little loose. Start the engine and allow the air to be pushed out by the oil. When you get 3-4 drops of oil the air is gone. Tighten the gauge. The gauge should now be very stead!

   y. The line is so small it may take a minute or two for the oil to actually reach the end of the copper tubing.

 

 I apologize,

 I hope I didn't mess you up to bad when I said 1/16th.

 

 --------

 Roger Lee

 Tucson, Az.

 Light Sport Repairman - Maintenance Rated

 Rotax Repair Center

 Home [url=tel:520-574-1080]520-574-1080[/url]  TRY HOME FIRST

 Cell [url=tel:520-349-7056]520-349-7056[/url]

 

 

 

 

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dashwood

Joined: 24 Jan 2009
Posts: 73
Location: sw ontario canada

Posted: Wed Jun 22, 2011 4:55 am Post subject: Re: Low Oil Pressure Indication on Cockpit Gauge

i too have been chasing oil pressure issues in my 912. sourced a few problems with leaking oil lines and seals in gearbox...see posts over the last year by me about this and the gearbox failure.  i replaced the vdo sender 3 times. the pressure readings after 10 to 50 hours would start fluctuating or fall off too low...  i replaced the elec oil stuff with a steam gage style. very very small copper tube (1/16"). with a 2.5" loop at the engine outlet, another loop at the firewall and a "s" curve behind the gage. i did not change the relief valve to the new style. i did not bleed oil through the tube to the gage when i installed but the readings were rock solid from the time of first start with this system.. i would get a steady increase in pressure to about 70lbs on a cold start.. and very little fluctuation at any rpm settings. my ame said i should have put a slight crimp in the line close to the engine in case of a line break. to slow the oil loss. i did that in 2 seconds with a pair of pliers.

after about 8months of bliss running amsoil full synthetic 20/40 and 20/50 

 i changed oil type. used to have..... oil pres of 65lbs in all settings. slow pres rise when cold. rpm at climb out or 5280 and max 5300 in level cruise. egt of 1500 to 1570. oil temps around 220F

after changing to shell +4 semi syn the pressure rises quickly to 38lbs  and stays there at idle and taxi rpm. as rpm increases to full throttle the pressure increased to 65lbs and stays there solid.climb out is 5300 and level cruise starts at 5400 and quickly goes over 5520( the setting for my max over rev alarm) I now have to throttle back to maintain 5400. egt is constant at 1400 and oil temps stay close to 200F. fuel burn / hour has decreased slightly, and oil consumption / hours has increased slightly. ie no oil consumption at all till approaching time for a change (50hrs)now is 1/8 cup every 6 to 8 hours.  i have about 25 hrs on the newer oil in varied weather conditions. 

i feel the engine is performing better with less strain with this (shell airo +4)  and all the electric vdo oil sending units should be scraped. the system i now have was 1/3 the Price installed anyway.

_________________
Ross Alexander: CH701 driver 912ul 788tt
It only takes two things to fly, airspeed and money

hgmckay

Joined: 23 May 2006
Posts: 397

Posted: Thu Jun 23, 2011 2:28 pm Post subject: Low Oil Pressure Indication on Cockpit Gauge

OK guys, here are the results of the final (on the ground) test to try to 

reconcile the difference between the VDO electric oil pressure gauge 

readings in my cockpit (which reads in bars) vs. a direct mechanical 

pressure gauge readings. I had a 1/8" line coming directly from the engine 

port where the pressure sender is normally attached out to a tee where I 

attached a new (Lockwood ) sender and then reduced down to a very small 

nylon tube on to a mechanical gauge (range 0-100 psi). The conditions of the 

test were 455 hours on the engine, ambient temperature was 79 degrees F, Oil 

is Mobile 1 Racing 4T, plane level and tied down securely. Here are the 

results:



    ENGINE RPM        MECHANICAL GAUGE        ELECTRIC VDO GAUGE    CYL. 

HEAD TEMP.         OIL TEMP.

    AND TIME                        PSI                                bar 

PSI **                DEGREES   C                DEGREES  C



    2200    4:29*                    80*                              4.0* 

58*                        50*                                50* 

* Immediately after Initial start    ** bar converted to psi

    2200    4:32                      80                                3.5 

50.75                     50                                  50

    2200    4:34                      75                                3.0 

43.5                       50                                  50

    2200    4:35                      70                                2.8 

40.6                       50                                  50

    2200    4:36                      65                                2.4 

34.8                       50                                  50

    2200    4:38                      60                                2.1 

30.45                      50                                  50

Hugh G. McKay III, P.E.

Senior Consultant

Worldwide Engineering Inc.

4090 North NC Hwy. 16

Denver, NC 28037



Ph. 704-661-8271

Fax 704-483-5466

email hgmckay(at)bellsouth.net

http://www.wwegeo.com

--

hgmckay

Joined: 23 May 2006
Posts: 397

Posted: Thu Jun 23, 2011 2:33 pm Post subject: Low Oil Pressure Indication on Cockpit Gauge

Sorry Guys, I accidently hit the send button before finishing the data 

table.

I will resend a new email with all the data. Disregards the one you have 

just received.



Hugh G. McKay III, P.E.

Senior Consultant

Worldwide Engineering Inc.

4090 North NC Hwy. 16

Denver, NC 28037



Ph. 704-661-8271

Fax 704-483-5466

email hgmckay(at)bellsouth.net

http://www.wwegeo.com

--

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