water in rear engine/pressurized case

[atsiii]

I'll try to keep this brief, but will provide more details if anyone thinks they can help. I have a 1969 T337D with a JPI engine analyzer. I am based at 6200 MSL, and constantly fly in very cold air.

I had been seeing evidence of water in the rear engine (rust and water on dip stick) for some time. I was also fighting oil leaks from places where there should not have been oil pressure (through bolts at the top of the case, etc.). The front engine was dry and showed no evidence of water. My typical cruise is 28” and 2400 rpm, at which time the JPI indicates front oil temps around 150, and rear around 180. This agrees with the Cessna gauges. Cylinder and EGT temps are all normal (300 - 340, 1410 - 1510). The engine has GAMI injectors installed. I cruise at peak plus 75.

A M-20 Model 300 Air/Oil separator was installed a few years ago on the rear engine, and attached to the side of the oil filler pipe directly in the path of the rear engine intake air flow. No oil separator was installed on the front engine. I began to suspect that the case was not venting properly through the oil separator, and thus retaining water vapor. I theorized that if this moisture was then freezing inside the separator, it might further restrict airflow and proper venting through the separator, thus creating pressure in the case resulting in my strange oil leaks. The only whistle hole seemed (please excuse my ignorance!) to be mis-located-- as it was downstream from the separator at about push rod level. Thus, all venting had to go through the oil separator, which was getting hit with all the cold intake air. There did not appear to be any other means vent if the separator froze or created back pressure.

To test this theory, I removed the oil separator and ran hoses from the breather on the oil filler pipe and the breather coming up out of the case, to the rear vent hole by the rear prop. I flew one hour and landed to find all sorts of water and thick mustard colored oil/water mixture dripping down from the rear vent hole. However, I did not have oil leaking from places where there should not have been oil pressure, and there was no water on the dipstick. I cleaned everything up and another one-hour flight produced the same results. I therefore thought my theory might be correct, and that more flying would burn off the remaining accumulated water.

However my next flight was 3.5 hours in very cold air. Upon landing it appeared that the vent hoses had once again frozen and pressurized the case. As I took things apart, I literally found tablespoons of water and the thick mustard colored oil/water goop in the breather hoses. Pulling the rear breather up out of the case, I can look down into case and still see suspended droplets of water in the oil. The oil itself looks normal, both in the case and on the dipstick.

My quesitons: Have you ever seen such a thing? Does any of this make sense? Can my theory about freezing and back pressure be true? Or might this have to do with the rear oil temp control? How best can I get rid of the residual accumulated water in the engine? Has this likely done great damage to the 850 hour engine?

Thanks in advance, Allen

[Kevin McDole]

I don't know much about this subject, but in the CPA class, they said that oil breather lines are at risk of freezing closed - and that a well know fix is to cut a notch in the hose a few inches from the end (such that the notch is just inside the cowling. The notch serves as a backup vent.

As I recall, the notch looks as though it was created by simply sawing about 1/4 of the way through the rubber tube. I asked my mechanic about this - and he acted as though it was a well known issue and the solution was common practice. See if your mechanic knows anything this - or you might drop CPA a line.

A different approach would be to "debug" the M20 oil air separator. Since those work based on gravity - the path from the separator back to the engine must be as "downhill" as possible. Oil is returned to the engine a drop at a time, so there must be a clean path for this to flow (no flat spots and minimum turns and distance). If you're returning the oil into a valve cover, the line must enter the valve cover above the oil level inside the cover (something like 2" up from the bottom of the cover).

If the M20 was misbehaving, I think you'd simply see excess oil on the belly - so this is probably not the root cause of your problem.

Bill Sandman at M20 is a very nice and helpful guy. You might consider sending this question to him. In the past, I sent him a digital picture of an M20 installtion that was causing trouble, and he very quickly had a solution. Maybe he'll have some ideas on this.

http://www.m-20turbos.com/

[Bob Cook]

Kevin

The problem appears to be engine temperature. You need at least 170 deg oil temp to rid condensation from engine. any colder and condensation is going to occur. 150 degrees is really totally unacceptable.

Need to check the thermionic valve in the engine cooler and make sure the oil is being bypassed from the cooler. secondly check to see if you can find winter baffles for the engine.

Thirdly you probably need to insulate the oil fill tube on the rear engine as this is a good place for condensation to occur.

The oil temp is different from the crankcase and your engine temps on the crankcase (top) is probably going to be colder!

Not sure if you have an oil seperator but this might cause some problems with venting of the engine. I would suspect this needs to be insulated or it can cause freezing. (mine is insulated with the black type foam) (just a consideration).

My vent hose does have a slit (relief) inside the cowling that would allow venting. It appears the excess moisture is being vented then freezing which is adding to the problem.

Bottom line............ engine temps are just too low. perhaps cruising at a higher power setting would help to some extent. What are the cht's ?

bob

[Kevin McDole]

Bob,

I agree with your comments, but from Allen's posting, he's says he's getting 180F on the rear engine, where the water problem is occuring. His front engine oil is very cold (only 150F).

He said his CHT's were in the 300-340 range, which strikes me as being on the cool side, but not terribly so.

Your idea about getting the winter baffles very good.

[Bob Cook]

Kevin

The rear usually runs warmer but when you take into consideration temps you need to look at the crankcase vrs the oil temp. Condensation occurs with difference in temperatures and existing moisture.
Either there is excessive moisture or there is a large difference in temp and condensation is occuring due to very low upper crankcase temperatures.

The cold oil filler extension can cause moisture and the later models have this insulated (in the p versions at least).

I tried to find the baffle information in my manuals and couldn't find part numbers. I have been told they do exist.

Bob