SonexBuilders.net

Hi Everyone!

I just purchased a Sonex TD, and I just finished my first cross country, 2:20 in length.

Super fun bird to fly.... nice and fast.... but I've got a problem with oil consumption.

The prior owner had no issues with oil consumption; the engine is a 2.1 with a top oil cooler upgrade. It does not have an oil filter.

The plane has zero oil leaks, at least to the visible eye.

The engine has <600 hours and was rebuilt to a 2.1 about 4 years ago. In that time, it's logged another <200 hours with zero issues with prior owners.

Anyway, on my cross country flight, it apparently drank more than a quart of oil. Very shortly before touchdown, I encountered dipping oil pressure; thankfully I was on the ground just a few minutes later.

Before the flight, the prior owner and I both checked that oil was full.

After the flight, the oil was below the lowest notch on the oil dipstick. Adding a quart raised it to mid marks, and the oil pressure issue disappeared on a ground test run.

I need help, that's for sure. I'm in the middle of a cross country ferry flight home.

Oh yeah, I was running 3300 RPM with around 1220 on the EGTs and low 300's on the CHT. The flight was a little bumpy, but I've had far worse over my flying career.

After landing and parking the plane, there is no significant oil residue on the underside, and only a couple spots of oil out of the breather.

Last compressions were excellent, high 70's.

Thanks!

James Wiebe

James,
Welcome to the forum and congrats on your new Sonex. My initial thought would be the breather - that's where most of the oil comes out in my experience. I find more oil on the belly when I run higher RPM. I cruise 3150-3200. Also I have an air-oil separator. That won't help now, but something to consider.

Is your breather blocked? That could do it. Check that it is clean and flowing ok.

Did the prior owner put anything under the breather fitting on top of the case? That usually causes issues.

If there is truly not much coming out of the breather, it has to be coming out somewhere else. Check behind the oil cooler on top of the case. Other common leak spots are pushrod tubes, front and rear main seal, and oil pump.

If there are no leaks (would be making quite a mess if it were) it must be burning oil. Unlikely to be intake valve seals at high throttle settings so more likely through the cylinders. I am not sure if 3300 rpm is a high setting for the aerovee because I have never flown one but you could try a lower power setting and monitor how that effects oil consumption. Maybe others could comment about RPM range.

Welcome to the forum James!

Thanks for all the great comments! I will be reducing power settings and monitoring the oil consumption carefully. I'll let you know what I find. -- James

Follow on question: the stated total oil is 2.75 qts; however the manuals I have say this is "without oil cooler".

My engine has both bottom and top oil coolers; what is the actual oil total qty with this setup?

thnx...

James,
That is 2.75 quarts in the sump. To calibrate the dipstick, you add minimum oil (2.25 qt) and mark the low line. Then add another half quart and mark the high line. So that will not reflect oil in the external systems.

You didn't mention the condition of the spark plugs. Burning that much oil will carbon up a cylinder and plug and in worst cases can foul a plug. My experience was exactly the same as yours and it was caused by a broken oil ring on one cylinder. The compression didn't show it. Only the plugs. It allowed oil to get up to the compression ring and the top of the cylinder. The oil ring lays on top of a spring type ring that keeps it against the cylinder wall. On mine, the original installation, the ends of the spring were turned up to prevent it overlapping. One of the turned-up ends had broken off allowing it to overlap and eliminate the pressure on the oil ring. The new replacement set of rings had a molded-on boss on each end of the ring that were colored, and the instructions said if you couldn't see the color when you installed it you had a problem.
If your RPM reading is correct, you are running wide open, which I didn't do, and I don't know if that has anything to do with the oil consumption.

Good post Onex107

Onex107 wrote:If your RPM reading is correct, you are running wide open, which I didn't do, and I don't know if that has anything to do with the oil consumption.

The RPM depends on the prop. I can exceed 3500 WOT.

After adding oil, I did a test flight at the airport this morning, 30 minutes in the pattern and at 3000 RPM. No significant oil consumption.
I resumed my flight home. I'll go through everything in the next few days (inspection, compression check). Everything you all have noted, I will take a look at and share what I find.
I'm now safely home. So glad the bird is now nestled in its new home as well.