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Oil Pressure fluctuation video


Roger Lee

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After nearly an hour in the air yesterday, my oil pressure jumped up to the low 70's then gradually worked it's way back to the 50's, the gradually moved up/down never going above 73 or below 55. Didn't seem to be RPM specific until I got down to pattern altitude/RPM's. Once I pulled back to idle, pressure was high 40's/low 50's. It was 44 on the ground while taxiing. I did a runup to 4500rpm and kept it there for awhile. Oil pressure stayed at 44. I'm guessing my VDO oil pressure sender just bit the dust after a meer 65 hours. I'll check the grounds when I can get out to the hangar early in the morning before the Texas heat makes it unbearable. Any other likely cause candidates? All other reading were normal: oil temp - 230, RPM - 5000, CHT - 194 EGT - 1250.

 

Also, have any of the firewall mounted senders failed (mine is still on the engine)?

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After nearly an hour in the air yesterday, my oil pressure jumped up to the low 70's then gradually worked it's way back to the 50's, the gradually moved up/down never going above 73 or below 55. Didn't seem to be RPM specific until I got down to pattern altitude/RPM's. Once I pulled back to idle, pressure was high 40's/low 50's. It was 44 on the ground while taxiing. I did a runup to 4500rpm and kept it there for awhile. Oil pressure stayed at 44. I'm guessing my VDO oil pressure sender just bit the dust after a meer 65 hours. I'll check the grounds when I can get out to the hangar early in the morning before the Texas heat makes it unbearable. Any other likely cause candidates? All other reading were normal: oil temp - 230, RPM - 5000, CHT - 194 EGT - 1250.

 

Also, have any of the firewall mounted senders failed (mine is still on the engine)?

 

If your oil psi sending unit is on the oil pump it will give erratic readings and eventually fail. There is a relocation kit and LOA to move onto the firewall. I recommend doing it and have it done by someone that knows what they are doing. There is a restictor in the fitting that goes into the oil pump and it is the only thing that will save your engine in case of a hose malfunction to the sender.

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After nearly an hour in the air yesterday, my oil pressure jumped up to the low 70's then gradually worked it's way back to the 50's, the gradually moved up/down never going above 73 or below 55.

I've seen this before. Can never find the source of the problem. It is frustrating.

 

Doug

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My mechanic and I went out to debug the oil pressure anomoly noted earlier is this thread on Saturday. We found no bad grounds which is really what I expected given the gradual changes in the pressure. We then went to reproduce the issue and got similar behavior, in reverse by doing an extended run-up to 4000rpm. The oil pressure gradually went down to 30 psi then bounced up (in one step) to 40 psi on it's way to 60. We replaced the VDO sender and got pretty much the same behavior.

 

We had previously done the 2000 TBO SB (-057) at the last oil change so we went down that path and went back to the original ball/spring/plug combination. Runups were fine and today's 1.2 hr flight at multiple altitudes and RPM's had oil pressure happily steady in the 50's. Guess I didn't need the extra 500 hrs TBO ;)

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I (my A&P) did the 2000TBO SB mod about 25hours ago and now have been seeing strange oil pressure readings. Like the pressure going down as I rev the engine up! I see reading in the 50's while warming up at 2400 rpm, but it drops down to 37 as I rev up to 4000+ on run up and first takeoff. It jumps back to 48-54 after i level off at cruise. It seems to get a little better as the oil gets fully up to temperature. I was going to see if a different gage would read the same, but now I'm wondering if anyone else is seeing strange readings after doing the 2000 hr TBO spring and bolt change?

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Hi Bruce,

 

The drop in pressure as you rev up can be normal and it may go back up as you level off in rpm. I have seen and read many post from other people in other aircraft that have seen the same thing and it isn't anything to worry about.

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FWIW, my pressure indicates 50+ psi during warmup and initial flight and then shows being around 37 psi after stabilized flight. I have steam gages and have had the 2000 TBO pressure regulator installed. I am currently using Mobil 1 Racing 4T full syntetic oil and change this every 50 hours. The Rotax manual appears to indicate 27 - 73 psi is a "normal" range for oil pressures.

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Hi Bruce,

 

The drop in pressure as you rev up can be normal and it may go back up as you level off in rpm. I have seen and read many post from other people in other aircraft that have seen the same thing and it isn't anything to worry about.

I agree, totally normal for the 912 to see a small drop in pressure when going from idle to part or full throttle.

 

My OP was much more stable after going to the cone style regulator, although they were slightly lower.

 

Doug

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  • 4 months later...

My mechanic and I went out to debug the oil pressure anomoly noted earlier is this thread on Saturday. We found no bad grounds which is really what I expected given the gradual changes in the pressure. We then went to reproduce the issue and got similar behavior, in reverse by doing an extended run-up to 4000rpm. The oil pressure gradually went down to 30 psi then bounced up (in one step) to 40 psi on it's way to 60. We replaced the VDO sender and got pretty much the same behavior.

 

We had previously done the 2000 TBO SB (-057) at the last oil change so we went down that path and went back to the original ball/spring/plug combination. Runups were fine and today's 1.2 hr flight at multiple altitudes and RPM's had oil pressure happily steady in the 50's. Guess I didn't need the extra 500 hrs TBO ;)

 

S3flyer, I am diggin up this older thread due to some strange oil pressure readings which I think started after installation of the new mushroom oil pressure regulator. I seem to see oil pressure readings very much like the ones you talk about in your post above. Today, I had around 48 psi during cold start. Pressure went down to "low 30's" after warm up. After take off, I saw readings from 25 psi to 35 psi with occasional pressure readings that ran up to 48 psi. During flight the readings slowly move around between mid 20's to mid 30's and this fluctuation seems unrelated to engine rpm. Once back on the ground, I would see 35 psi at idle and then see the pressure drop to 25 psi when I throttled up to 3000 rpm. I consistently saw these readings each time I went to idle then back up to 3000 rpm. Your comments about getting good pressure readings by re-installing the old regulator indicates the mushroom regulator is suspect. I will re-install the old ball style regulator and see if this gives me proper oil pressure readings similar to yours. If there is anyone who may have more recent comments or observations regarding this, I would appreciate hearing from you.

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Hi Dick,

I've gone the last 5 months with the ball regulator and no oil pressure irregularities. I know I'm in the minority but I'm staying with the current config.

Roger, did you ever find anything conclusive with your 2 customers who had similar behavior?

 

 

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I have had these swings in oil pressure and usually it was a bad sender. I think I am on my fifth one in 700 hours. The sender is mounted on the engine and I have always replaced it with the $40 VDO version. Since I went to the conical regulator about 200 hours ago, I am on my second sender. The pressures have been more stable but very different with the two senders. First sender I would get 50 psi on start up and settle down to about 32 in the climb, then back up in cruise as a function of altitude. With the current sender it jumps to 75 or so on start, down to about 50 for takeoff and settles in cruise to 45-55, depending on altitude (higher readings at higher altitude). So, I have to conclude that there is a lot of variation in the readings obtained across the VDO senders. Let me also guess that slight pitting or variations in the cones could matter.

 

WF

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Thanks for the replies. I'm leaning towards temporary installation of a mechanical pressure gage in place of the sender and then monitor pressure during static runup on the ground after oil temp stabilization.

 

WF, have you considered getting the remote sender "kit" that allows placement of the sender "off engine" and on the cowl? I have been wondering if this would be warranted and due to your history of broken senders, this seems like something I should consider. Your history of bad senders seems high compared to what I read on the forum but I'm wondering if other owners just put up with weird pressure readings and actually have senders tha are NFG? I really suspect the sender but S3flyer's comments really sum up my situation and he is now "good" with just a change back to the old ball regulator. I think a mechanical gage is warranted to see what's what.

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Before you go to all the trouble of relocating your sender, unscrew the connector, clean terminal on the sender and on the wire and put it back together. It took about 10 minutes and fixed the problem for me.

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Jim, I see your suggestion to check the wire connections. That's the first thing I did and no changes found but I will repeat this since I've been fooled before by wiring. Acutally, today an A&P friend stopped by and asked if I had added a ground wire to the sender. He said that engine senders are notorious for not making good ground thru their threads. Hadn't done this. Maybe I can just hose clamp a temporary ground to the sender body and see what happens.

 

UPDATE: A very interesting development today. I flew my plane to my A&P and asked him to install a mechanical pressure gage. The plumbing was temporary for this so I was limited to a static runup on the ground. On initial start with oil cold, the instrument panel gage read 50 psi and the mechanical gage read 75 psi. After initial runup and rise to operating temperature, my instrument panel gage read 35 psi and the mechanical gage read 60 psi. There was a 25 psi difference. I believe that my instrument panel pressure gage is defective or the sender is NFG or the ground is bad. Or, I may find that this innacurate pressure reading is all that can be expected from the CT's miniture panel gage and automotive pressure sender. My next step is to properly plumb the mechanical gage so that I can fly with this and monitor the oil pressures. If this 25 psi difference is found to be consistent thru the range of pressures I now see with my instrument panel gage, I should have actual pressures that are not lower than the low 40's and see high pressures in the high 60's.

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It's always best to use a mechanical test gauge to check against the normal sender. The sender is usually the culprit 90% of the time or a mechanic putting something on the threads.

 

I still think it's the sender. It should not be a grounding issue at the sender threads unless someone put something on those threads. The engine should be grounded just fine.

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Another addendum to this thread. I was going thru some postings and saw this photo that or Safety Officer put up. This shows the oil pump housing hi pressure "plug" that I used to temporarily install a mechanical pressure gage. The plug is a standard 10mm bolt with 1mm thread pitch with a a head that accepts metric (6mm?) hex wrench. This same plug can be found on the top of our oil reservoir. I purchased an "engine oil pressure kit" from Harbor Freight which has a bunch of fittings. One of the fittings happened to be an adapter fitting which is 10mm X 1mm on one end with 1/8" NPT female on the other. This fitting allows me to connect my mechanical gage to the oil pump without removing the oil pressure sender. This way, the instrument panel gage readings can be compared along with the temporary mechanical gage readings. Attached is the photo which shows the oil pump housing and plug. Originally the red arrow was there to show the sender but it points to the plug also.

post-24-0-07269200-1323015083_thumb.jpg

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Hi Roger. The plug is a straight 10mm X 1mm thread that is not tapered so I would think that a 1/8" x 27 NPT fitting will work here. I know that the adapter I used was straight thread 10mm x 1mm and it fit this port. The port where the sender goes is 1/8" NPT.

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Hi Roger, we're apparently crossed up with the ports I'm talking about. The port for the plug is a straight threaded port with M10x1 treads which is different than the oil pressure port which has 1/8" x 27 NPT threads. The plug port uses a M10x1 plug which also has a copper washer under it's head to seal things. I have removed the plug and checked this with a thread gage and this definitely is straight threaded with 1mm pitch and has a copper washer. The sender port, since it's a tapered thread, relies on the taper of the 1/8" x 27 NPT to seal things and this is why one must use caution when tightening the sender so that the housing isn't cracked by overtorqueing the tapered threads. Guess it's better to just refer to the Rotax parts catalog. Rotax parts catalog Ed2 Rev0 page 96 shows the plug as item #36. This page also shows that a M10x1x30 bolt can also be used here which connects a banjo fitting used to supply oil to the turbo charger on 914 series engines.

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