Today's Message Index:
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1. 06:34 PM - Low Oil Pressure Indication on Cockpit Gauge (Hugh McKay)
2. 09:32 PM - Re: Low Oil Pressure Indication on Cockpit Gauge (Roger Lee)
Message 1
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| 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 Roger=99s 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) =93 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
Lee=99s 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. Can=99t 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 pump=99s 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@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@bellsouth.net
http://www.wwegeo.com
Message 2
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| 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 afetr 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.
--------
Roger Lee
Tucson, Az.
Light Sport Repairman - Maintenance Rated
Rotax Repair Center
Home 520-574-1080 TRY HOME FIRST
Cell 520-349-7056
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http://forums.matronics.com/viewtopic.php?p=343614#343614
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