I’ve started a thread on MooneySpace a while ago and I’ve got some great input on what could be the matter with the turbo on my Mooney. Somewhere in the turbo, oil leaks in the exhaust, leaving the starboard geardoor well lubed. Most oil is lost at start up and not so much in cruise, so it pools somewhere when the engine isn’t running en then it get’s blown out at start-up. My question for this forum is not so much on what could cause this leak, although any input would be appreciated.
But what I would like to know from other owners: how does one keep an eye on maintenance costs when fixing such an issue? In this instance, what kind of deal would you make with the repair shop? They tend to see this as a ‘fun project’ and they have all sorts of ideas, but to this date they have not produced a fix for my turbo and the costs are mounting.
I am well acquainted with this problem, but don’t know anything about the Mooney turbo installation.
In my experience the oil supply to the turbo shaft bearings is in the centre of the turbo housing. The oil is then forced out to a bearing on the turbine side and a bearing on the compressor side.
If the seals on the bearings are worn, or the oil pressure is too high, oil will be forced through the bearing seals into the hot exhaust gas and into the air in the intake system. On the exhaust side, the oil will be burned/blown away, but on the intake side, it will be ingested into the engine (fine if it is a tiny amount, but not good if it is a significant problem).
I was part of a design team for a turbo charged race car engine and we ended up with lots of oil pooling (from the turbo) in the intake pipes. At low RPM/low throttle the pool would build up. At high RPM the oil would mostly be sucked through and burned. If a pool of oil was present in the intake and then the throttle opened you could suck in a significant quantity, with risk of damage to the engine. We were forever draining the engine pipework before start up to ensure that there was no risk.
The problem was solved with setting a correct oil pressure supply to the turbo (ours was set too high) and replacing the turbo bearings.
I also had this happen in a Volvo XC90 at about 100k miles… I sold it before sorting it out, but it likely need a turbo rebuild.
Best of luck. Better fix it properly before you get a ‘slug’ of oil ingested into the engine.
Thanks! Oil pressure could be too high. Ten to twelve years back the turbo was replaced, but that did not fix the problem satisfactory.
As I said, I don’t know anything about the Mooney installation, but as you mention that the oil is blown out the exhaust on start-up, you probably need to establish with certainty:
1) That the oil is coming from the turbo. If it is coming from the turbo then it is likely coming from the turbine shaft seal.
2) Is the seal in good condition?
3) Assuming that it is from the turbine seal, is oil weeping past the seal while the airplane is parked for days/weeks/months? Can oil gravity feed into the turbo while the engine is not running? Are there check valves to prevent this?
4) Is oil being pushed past the seal when oil pressure is applied, i.e. the engine is running. In this case, is the pressure too high, or is there is restriction in the drain line, or insufficient scavenge vacuum?
Edit: A quick internet search turns up this result which seems to fit your symptoms (which is similar to my comment 3 above).
“Turbocharger Scavenge Pump: The turbocharger scavenge pump gears we talked about earlier draw oil out of the turbocharger and lift it up to the top of the engine where it is dumped back into the crankcase. If the oil is not scavenged from the turbocharger it will fill up the bearing housing cavities and flood the turbocharger with engine oil. This results in oil entering both the intake and exhaust systems. Worn scavenge pump gears that can’t draw a suction, can’t scavenge oil from the housing cavity. Also turbocharger check valves that are improperly installed or worn can flood the housing cavity.
All the M20K aircraft have two turbocharger check valves which allow oil to flow through the housing but prevent oil in the scavenge pump line from draining back into the housing after engine shutdown. An air leak below the scavenge pump can also cause an oil-flooded housing. The inlet check valve opens when there is oil pressure and at a higher pressure than the outlet check valve. At idle (low oil pressure) the outlet valve will be open allowing the scavenge pump to draw all the oil from the inlet valve out into the scavenge pump and back into the engine. This is one of the reasons why you should always allow the turbocharger to spin down and cool off at idle for four minutes after landing. This gives the scavenge pump enough time to draw all the oil out of the lines. Assuming the valves are not leaking, the turbocharger will not fill back up with oil. Because the turbocharger is on the low point of the engine, oil will drain back to the turbocharger if the outlet valve is leaking or installed incorrectly. At the next engine start, the oil is sprayed out the tail pipe. The two turbocharger check valves frequently leak as they get hours on them or if they get held open with debris. Two new check valves can cost as much as $1,500 not including the labor to trouble-shoot and R & R"
Unfortunately, I am all to used to the puddle of oil produced under a radial engine after it has been parked for a period… The sump in a Yak is obviously higher than the lowest cylinder and can drain down when it is parked.
Best of luck.
For those who are interested: It appears letting the engine idle for a few minutes to let the turbo cool at say 1200 rpm prohibits the scavengepump to do its job properly. I let the engine idle at a faster pace at around 1.400 rpm the last 30 secs before idle cut-off and the exhaust stays clean (for now…)
Bobo wrote:
It appears letting the engine idle for a few minutes to let the turbo cool at say 1200 rpm prohibits the scavengepump to do its job properly. I let the engine idle at a faster pace at around 1.400 rpm the last 30 secs before idle cut-off and the exhaust stays clean (for now…)
OWT : Idling turbo engines on the ramp to supposedly cool them off was de-banked years ago, yet pilots continue to do this completely useless practice.
It’s very simple: the turbo is usually at it’s lowest temp at about touch-down. Once on the ground with little to no cooling air coming through the cowl, the turbo heats UP and any additional ground running is not cooling it off.
As far as oil leaking out the tail pipe after shut down and cool off, prob 90 that it’s a faulty check valve as suggested above.
Michael wrote:
OWT : Idling turbo engines on the ramp to supposedly cool them off was de-banked years ago, yet pilots continue to do this completely useless practice.
I thought idling turbos was supposed to allow the turbos to slowly run down, meaning that when the motor is switched off, the turbo isn’t turning that fast without oil to lubricate it. The reason I thought this was due to a particular occurrence a colleague had with his BMW 530d. In those days, using the telephone in the car was a no – no. He was driving, pretty fast (German Autobahn) and his boss rang him. He needed to take the call (it was about his bonus) so he pulled over immediately and switched the engine off to make sure his boss would know he was stationary.
A few seconds later, the turbo had cooked itself…. at least that was the explanation of the BMW mechanic…..
This is, as always, type specific. The DA42 specifically states, twice (once as a checklist item, once as a caution box):
Before shut-down the engine must run for at least 1 minute with the power lever at 10% to avoid heat damage of the turbo charger.
Your turbo will have plenty of time to slow down during approach and taxi.
I have a Rotax turbo 914 and if I don’t let him cool down for 2 min after a high power run-up, I will get a “bang” at shut-down and the oil will be full of carbon debris.
A descend at low power and further taxi to the ramp will have the same effect as running 2min at low power.
