912 ULS Weak Cyinder

I hate to even post this, but I have to start somewhere. This is a 100hp engine in my RV12. First, I noticed that my oil was extremely dirty after only 25 hours on it. Total time of the engine is 152 hours, and each oil change has found very clean oil until this one. The oil seems loaded with carbon. Pulling the engine through with a totally cold engine exposed 3 nice compressions, and one soggy one. The next step will be a compression test to locate the weak cylinder, but then it will be exploratory from there. My guess is some broken rings are a bad piston. Does anyone have any experience with the above symptoms? I think that I am going to really need some help, and support with this one.

Thanks in advance.................Tom

Sorry to hear of this Tom. In addition to doing the compression tests be sure you get an oil sample to have analyzed. You will want to know what is in that oil.

What oil are you running ?

Fuel .... AVGAS OR MOGAS?

did you run the engine prior to the compression check to operating temps?

What was the result of the compression tests .... Pressure wise?

I'll be following your thread with interest. At my last condition inspection, I had one cylinder with lower differential compression than the other three, but still within limits. I need to pull the cowls and check it again. I was hoping that the ring gaps were just aligned.

Ethanal Problem

If you use mogas and it has a high percentage of ethanal you have probably had some detonation issues and or corrosion and damaged a piston .

A Few Answers

To answer of the questions:
I'm using ethanol free premium auto fuel, except for one tank of 100LL while on a trip.
The only oil used has been Shell Sport
I changed oil at 25, 75, and 125 hours. All were good, and everything seems fine, until now.
I may have notices a very slight decrease in performance, but I can't say positively. If it was, it was very subtle.

Thanks for the tip on oil analysis. I really appreciate all of the support guys

Tom .

Oil purge

Per Rotax, did you do the oil purge before before the engine was first run and after the first start did you check the firmness of each of the rocker arms? Just a idea, but I've come across a few 912 owners who were not familiar with that requirement. If I recall correctly, if you have a soft rocker arm the engine might initially appear to run normally, but you might begin to see serious problems many hours later from the lack of adequate tappet lubrication (ie >100h) I had one myself at the beginning that took several additional purges before it firmed up.

todehnal said:

Pulling the engine through with a totally cold engine exposed 3 nice compressions, and one soggy one. The next step will be a compression test to locate the weak cylinder........

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Pulling the prop through to detect low compression can be/will be misleading, especially with a cold engine. The compression test is a must. Oil WILL get quite dark.

It is not uncommon to see low compression on a Rotax cylinder when cold. Just a short run up will usually "cure" it. Just did a condition inspection on an Aircam where hand propping on the engines showed one cylinder on the left engine felt low and the other engine felt fine. The compression check showed a low cylinder on the right engine and the other engine was fine. After a short engine run both engines tested fine (all above 82/86).

You?re pulling the prop by hand and back-driving the engine through a 2.4:1 ratio gearbox. This will not give you the ?feel? of compression that you would get with a direct drive engine such as Lycoming or Continental.

Advice as per above is to warm engine and do differential compression leak down test.

PilotBrent said:

Per Rotax, did you do the oil purge before before the engine was first run and after the first start did you check the firmness of each of the rocker arms? Just a idea, but I've come across a few 912 owners who were not familiar with that requirement. If I recall correctly, if you have a soft rocker arm the engine might initially appear to run normally, but you might begin to see serious problems many hours later from the lack of adequate tappet lubrication (ie >100h) I had one myself at the beginning that took several additional purges before it firmed up.

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I did do the purge and rocker arm firmness test. It all went well and the first 125 hours was well. This is a new wrinkle.........

Tom,

Was the engine ever in an overheated condition? Mine got almost up to the CHT and oil temp limits once last summer. I was concerned about the effect on compression, but managed to dodge the bullet fortunately.

Rich

Never overheated. I monitor it vigorously. In fact, I did the cowl mod to add a bit of clearance for the oil cooler. Never seen anything above 230. CHT are always around the 180 mark. The EGTs do run a bit high from my thoughts, but not according to the op manual. I've ordered an oil analysis kit. As soon as it gets here I will warm it up run the compression test, then change the oil and get the sample sent off. As long as the compression is reasonable, I plan to fly it with fresh oil, and wait for the lab report. I typically cruise at about 5200, and it seems pretty happy there...............Tom

I have had a soft cylinder on a few occasions but after giving the engine a run it has always had very good compression
I have 450 hours up and change the oil at about 25 hours and change the filter every 50 hours

Well, I'll sleep better tonight!! My oil analysis kits came in yesterday, and I went to work this morning. First, the engine has been sitting for the past 3 months while we were wintering in Florida. After a 10 minute warm up to get the oil up to temp before draining, I found during the post run gurgle step that all of the cylinders felt pretty good. I drained the oil, took my sample, changed the filter and filled with fresh oil. I gave the old oil the finger rub test, and it felt pretty good. Also, the oil was pretty black, but it didn't really show any signs of contamination. When I did the compression leak test, I found 1 cylinder with a 2 pound loss, and the other 3 with about a 4 pound differential from the input air pressure. I was quite pleased to see that. From the results of todays actions, I feel confident that Marty was correct from the start, and that dark oil after 25 hours is nothing to worry about. I will post the lab results on the oil, but I really don't expect to hear anything bad. Thanks to all that took an interest in my perceived problem. It's terrific to have such a fantastic support group....Tom

Everything you wanted to know about COMPRESSION CHECKS by Mike Busch, the Savvy Aviator.
Quote from Article:

When an owner asks me whether to be concerned about low compression readings, one of my very first questions are "How's the oil consumption?" and "What do the top plugs look like?" If a cylinder isn't burning oil or pressurizing the crankcase or leaking past the valves, it's probably just fine regardless of what the compression test numbers are.

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Update: Received results of my oil analysis from Avlab.
All was normal.......Wheeew!!

Tom