Wildly Fluctuating Oil Pressure

[mike.smith]

I could use some collective brain power for my AeroVee.

Several weeks ago I suddenly, on one flight, began getting fluctuating oil pressure readings. Normally I run around 48 psi or so. I was getting readings that would stay up in the 40's, then suddenly drop to the low 20's. Then up into the 30's, etc. There was no rhyme or reason to the fluctuations. It stopped for a very short while after I polished the pressure relief valve plungers, but then came back pretty quickly. I've tried everything I can think of. Today I did my first run-up and taxiing after replacing the oil pump. I got the same fluctuations while taxiing, with readings as low at 17 psi.

Background:
- AeroVee 2.1
- 8 years old, 530 hours
- MGL Xtreme Mini
- No previous OP issues, ever
- Recent changes: none

I've tried all of these, with no change in behavior whatsoever:
- New OP sender.
- New wire from OP sender to RDAC.
- Replaced oil pump.
- Checked oil screen and pickup tube. Nothing there, and no obstructions in the tube (stuck a very long pipe cleaner up there).
- No oil leaks.
- Removed both pressure relief plungers. Polished the outside of the plungers, and polished the insides of the orifices. No obstructions or abrasions felt inside. Ground/polished anything on the springs that might possibly be grabbing. The springs are still incredibly strong (difficult to compress when reinstalling).

Unless there is something else I'm missing, my money is on the OP relief system, though I thought I've tried all the possible solutions, and don't know what else to try.

I suppose it's possible the RDAC could be bad, but everything else is nominal, so it would have to be a problem with just the OP in the RDAC. I'd ask MGL, but getting any responses from them these days is hit or miss, and mostly a miss.

I could put a manual gauge on the engine and run the engine with the top cowl off, but I really need to be able to taxi while watching the guage. You can't "just run the gauge into the cockpit" without cutting holes in things. If anyone has a solution, let me know. A guage with a long hose, taped to the windshield? What size are the threads for the OP sender? Anyone have a link to something suitable?

Any input would be appreciated.

[Bryan Cotton]

How about a cheap car backup camera?

https://www.ebay.com/itm/403873190906

[mike.smith]

Keith Macht emailed me and posited:

"The only idea I had that you might not have checked is to make sure your oil pickup tube is still secure to the case. It sounds like the oil system is maybe getting air in somehow. It is just one nut holding the pickup tube in place and the other side is just a press fit into the case."

That sounds plausible, though I've been in the bottom of the engine a couple of times lately and didn't notice anything. But it's worth checking.

[gammaxy]

I believe the case is M10x1 which makes it difficult to find replacement gauges that work without an adapter.

I'm currently using one of these adapters to 1/8 NPT: https://www.amazon.com/gp/product/B00062YVNE

If I were you, I'd strongly consider using a mechanical gauge like this: https://www.summitracing.com/parts/ATM-2604. It comes with a fairly thin 10' long tube that you may be able to jam through an already existing hole in your firewall or route around the cowl and in an air vent. Maybe there's a gauge that won't require an adapter. I haven't personally used one because I'm not satisfied with the thin tube as a permanent solution, but I have one on hand just in case :-)

Another electronic thing to check is whether you have a good connection between the body of the sender and the RDAC ground for the other leg of that resistive circuit.

I'm pretty skeptical of those VDO senders after having a couple fail, so while I think it's unlikely, I wouldn't completely discount the idea of the new one being a dud.

[GraemeSmith]

Mike,

I've replaced the VDO sender to cure that symptom. Vibrations seem to make them go "sticky". I posted the other week about difficulties finding stock on the VDO 360-001 sender - which has the M10x1 thread for the case. But I did just find some at JBugs this week.

You can also use a 360-006 which has the 10mm x 1 thread and an extra pole for a warning light - which you can ignore and not wire.

[sonex892.]

Th best way to be sure the sender is good or bad is to check it by applying pressure and use an ohm meter from ground to the terminal post. Now this is from memory so my figures may be a little off. But with a good sender, the resistance will rise about 10 ohms per 5 PSI. It is a very linear rise in resistance. Air pressure and a regulator work fine.
Steve

[XenosN42]

Th best way to be sure the sender is good or bad is to check it by applying pressure and use an ohm meter from ground to the terminal post. Now this is from memory so my figures may be a little off. But with a good sender, the resistance will rise about 10 ohms per 5 PSI. It is a very linear rise in resistance. Air pressure and a regulator work fine.
Steve

I agree - use an air compressor. Then you'll be able to cross check the reading on your MGL device against the compressor's gauge.

[Bryan Cotton]

Or an adapter like this:

[mike.smith]

Mike,

I've replaced the VDO sender to cure that symptom. Vibrations seem to make them go "sticky". I posted the other week about difficulties finding stock on the VDO 360-001 sender - which has the M10x1 thread for the case. But I did just find some at JBugs this week.

You can also use a 360-006 which has the 10mm x 1 thread and an extra pole for a warning light - which you can ignore and not wire.

It's definitely not the sender. The original, a spare, and a new one, all give the same results.

[GraemeSmith]

To eliminate the engine - you could plumb a manual OP gauge into the cockpit to watch what is going on.

https://www.jbugs.com/product/V150030.html
plus
https://www.jbugs.com/product/V150851.html

At the same time you could then confirm the RDAC and Mini Extreme stays at no pressure and doesn't randomly wander around. Actually - without adding the manual sender/gauge - what happens with the OP sender/RDAC wire disconnected? Does it stay at zero?

If you want to try a different RDAC - I think I currently have a spare I could let you try.

I suppose there is also the possibility the Mini head is going bad. I had an ENIGMA give weird numbers prior to going stupid bad on me and showing garbage. It was like the calculation or programming chip was on the way out.