High oil pressure mystery

Anyone care to take crack at the following mystery? Thanks for even idle speculation, since I'm stumped!

O-320-D2A with 500 hours, running with Dynon EMS-D10 the whole time. The Dynon has been set with a redline of 100psi oil pressure the whole time, which was never hit until recently, despite it being summer. First on the takeoff roll, and now even during the runup, just barely touching 100 psi. Cruise pressure sometimes seen >90 psi (manual says redline is 90 except during start and warm up, then 100)

What I've tried:
- Replaced old-style pressure sender, no change
- Checked pressure with steam gauge, matches Dynon within 2-3 psi
- Adjusted pressure relief valve to all the way out, did drop cruise pressures a bit to just below 90
- Oil change, no visible metal found (waiting for oil analysis), psi still high after oil change
- Inspected relief valve and probed the drain passage, no trouble found

With hot oil (running W100 for past two years), the pressures I guess are still (barely?) within limits, but something seems to have changed. What else should I look at?

While it would seem odd to fail with high pressure, I would look at replacing the oil pressure control spring. Could also be blockage in the oil pressure relief circuit (the overflow drain to the pan) on the overflow side of the ball, but would struggle to see how that could happen. I would pul of the relief assembly and visually inspect.

Yep, I pulled the pressure relief assembly. The only slightly odd thing I saw, in retrospect, is that the spring had a somewhat speckled appearance due to some coating flaking off. I fed a wire down the drain hole to the sump as described in the Lycoming manual, it seemed clear and came back up clean. Ball and seat seemed in good shape, shiny and smooth.

to the books

i would get the service manual to that engine and read. look for all the parts involved that produce pressure and possible diagrams of said engine. i have only helped put 3 engines together and honestly had no idea but the IA in charge was kind enough to allow me to hold parts and hand him tools and he also lets me bolt stuff on with his supervision,. i recall my 1st year in a and p school we went over pumps and their operational functions. you may get some direction from the 8083 or the ac43.13-1b. hope this helps, i will not learn if i dont ask and i mean you no harm. cheers

Since you said that you could drop the pressure by adjusting the spring, but couldn't get it where you want it, you know that the ball and seat (and spring) work - just not in range. I'd have to suspect the spring as well. At least it's easy to change to try it! Of course, you have to get one.....

Oil pressure

Had that problem once, changing the spin on filter fixed it.

8 ball said:

Had that problem once, changing the spin on filter fixed it.

Click to expand...

In most installations, the oil pressure sender is tapped in the channel between the filter assembly and the relief assembly (i.e. downstream of the filter). In installations with this arrangement, the filter cannot cause high pressure readings. Nothing can cause pressure increases in the downstream direction, only upstream pressure.

I guess this begs the question to the OP: where are you getting your oil pressure from? Taps upstream of the filter will be sensitive to filters, coolers, etc. and may have no correlation to what your pressure relief is doing to the oil pressure fed to your oil galleries.

In the end, systems should be measuring the pressure that the engine's oill galleries will see otherwise a plugged filter can trick you into thinking you have good pressure when your gallery pressure is dangerously low and you are eating up your bearings. This is why Lycoming is starting to tap the oil pressure in galleries downstream of the pressure relief mechanism. When the oil pressure is captured before the pressure relief, you have no idea when your bearings are worn or other pressure robbing maladies are occurring. In the auto world, the pressure is regulated in the pump and pressure is measured in the galleries. In those cases, oil pressure can be directly correlated to bearing health.

Larry

lr172 said:

In most installations, the oil pressure sender is tapped in the channel between the filter assembly and the relief assembly (i.e. downstream of the filter). In installations with this arrangement, the filter cannot cause high pressure readings. Nothing can cause pressure increases in the downstream direction, only upstream pressure.

I guess this begs the question to the OP: where are you getting your oil pressure from? Taps upstream of the filter will be sensitive to filters, coolers, etc. and may have no correlation to what your pressure relief is doing to the oil pressure fed to your oil galleries.

Click to expand...

Right, I'm getting it from the Van's-recommended location below and to the right of the vacuum pad, which geographically would seem to be downstream of the filter and upstream of the pressure relief. Photo/discussion here, I used the port described in that thread.

Diagram showing this further: http://www.vansairforce.com/community/showthread.php?t=45548

Thinking about it in that light, it seems there are two possibilities given the placement of the sender between the filter and the relief:
1. the relief is not working properly (spring magically got stiffer? drain blocked?) OR
2. something beyond the relief is plugged up so badly that the relief can't keep up.

Right? I guess if I were to cut down the relief spring and see no change, that would prove #2. Also when the oil analysis comes back it might strongly imply #2 (hopefully not).

Hard to disprove #2 unless there is a port downstream of the relief, like lr172 mentions... there is a tapped hole right above the oil relief port, I guess that would still be upstream?

Paul Eastham said:

Thinking about it in that light, it seems there are two possibilities given the placement of the sender between the filter and the relief:
1. the relief is not working properly (spring magically got stiffer? drain blocked?) OR
2. something beyond the relief is plugged up so badly that the relief can't keep up.

Right? I guess if I were to cut down the relief spring and see no change, that would prove #2. Also when the oil analysis comes back it might strongly imply #2 (hopefully not).

Hard to disprove #2 unless there is a port downstream of the relief, like lr172 mentions... there is a tapped hole right above the oil relief port, I guess that would still be upstream?

Click to expand...

You are partially correct here. Properly working, the relief can bleed off a lot of the pressure, regardless of what is happening downstream (in theory, full blockage downstream could still show 80 PSI before the relief if it can pass the volume of oil necessary). However, I don't know how much volume it can accomodate. I suspect it is possible that it cannot handle the entire oil pump volume and as volume increases (i.e. volume that used to flow in the galleries), the ball hits more spring resistance and drives the pressure up. Clearly, you want to hope it is the pressure relief spring, as a downstream blockage is cause for grounding and could mean a tear down to find it. A blockage there means that oil isn't getting to parts of the engine at the volume it should, if at all.

The starboard side of the engine has the main oil gallery running aft to fore, about 1/4 down from the top of the block. There is a clean out plug at the extreme forward end of this gallery. It points forward. I believe it is a 1/4" NPT plug, but could be 1/8" NPT. Pull the plug and replace with an oil pressure gauge. If you have blockage, it is very likely downstream of this point, as this is a relatively large gallery. This will tell you what is going on downstream of the pressure relief mechanism and help you to diagnose. If you also have 100 PSI at that sender, you likely have some type of blockage. In most cases, this sender should report less than the 70-80 PSI normally read at the standard sender location. This is especially true if your relief is generating 100 PSI (output volume goes down as pressure goes up, therefore feeding less oil to galleries and less volume then equals less pressure downstream of the relief). Others, like Mahlon, with more experience can provide some insight into how much pressure the Lycoming oil system can maintain, post relief, under various conditions after the relief valve. This tougher to assess, as the pressure will be a direct function of the volume delivered.

Let us know what you find and we'll help diagnose.

Larry

Oil Pressure

The oil pressure reading at the front of the RH gallery will be approximately 10# less than at the location near the right upper engine mount. Cessna has been using the front location since the restart of single engine production.
so if your oil pressure in a late model 172 reads 80 psi it is really 90 psi. The higher oil pressure is helpful for valve lubrication.
The oil pressure maximum red line has been 115# for start and warm up for many years.

lr172 said:

If you also have 100 PSI at that sender, you likely have some type of blockage.

Click to expand...

Are you sure about that? If the pump/regulator is leaving the pressure too high, won't that carry through to the post-regulator pressure?

Me too!

Paul, interesting post. Did you ever solve your high oil pressure problem? My TMX-360 oil pressure is also running high, sometimes. I too have a Dynon D10 EMS. During level cruise, 2500 rpm, my oil pressure changes from a normal 82 psi then climbs to around 105 psi. After a few minutes, the pressure returns to 82 psi, then back up to 100+. Changing engine rpm (down to 25% power) does not affect the high pressure indication. I have a VDO pressure transducer remote mounted on the firewall fed by a hose from the 'normal' oil pressure sender location (per my A&P). I have a new pressure transducer on order and I haven't tried removing and inspecting the pressure regulator yet.

Crankcase vent

I just went through this problem in my 8. My Skyview also warned me at 100 psi. It usually runs 90 on takeoff and 75 once its at operating temp(185ish). I saw 105psi and that got my attention. I have a Skydynamics 4 into one exhaust that vents the crankcase gasses in the center of the collector. Carbon deposits built up in the pipe until it was about 80 percent clogged.
Cleaned it out with a metal brush (that looked like a pipe cleaner) and a drill.
330 hours on the engine. I think i'll clean it out at every annual now.

Bud K said:

Paul, interesting post. Did you ever solve your high oil pressure problem? My TMX-360 oil pressure is also running high, sometimes. I too have a Dynon D10 EMS. During level cruise, 2500 rpm, my oil pressure changes from a normal 82 psi then climbs to around 105 psi. After a few minutes, the pressure returns to 82 psi, then back up to 100+. Changing engine rpm (down to 25% power) does not affect the high pressure indication. I have a VDO pressure transducer remote mounted on the firewall fed by a hose from the 'normal' oil pressure sender location (per my A&P). I have a new pressure transducer on order and I haven't tried removing and inspecting the pressure regulator yet.

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Those symptoms point to a failing sensor and I would replace that as a first step. Those VDO sensors are not known for long-term robustness.

Larry

Sensors

That's what I thought too Larry. I'd still check the breather. If it gets plugged it could push the prop seal out and then you'll have a real emergency.

Interesting!

lr172 said:

Those VDO sensors are not known for long-term robustness.

Larry

Click to expand...

Interesting comment about VDO sensors. I have also been having an occasional problem with my Fuel Pressure reading high. When it reads high, it doesn't matter if the boost pump is on or not. I thought that it was a bad sensor ground but the sensor is mounted to a manifold bolted to the firewall. It too is a VDO sensor. That might be my next sensor replacement.

edsong said:

That's what I thought too Larry. I'd still check the breather. If it gets plugged it could push the prop seal out and then you'll have a real emergency.

Click to expand...

In order to increase your oil pressure by 20 psi, your crankcase pressure would have to be either 20 PSI or 100 PSI, not sure which. Either way, it seems HIGHLY unlikely that your front oil seal could handle that much pressure. You would also see problems with fuel pressure/delivery with this type of crankcase pressure.

Larry

Solved, simply

Well, my mystery has been solved. It was indeed the VDO oil pressure sender unit. It was a simply fix.