Wrong Oil Pressure of the Rotax 912/914 Engines
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| Pressure sender installation |
Blowby pressure in the crankcase then returns the oil from the engine sump to the tank.
Oil pressure and temperature are measured at the oil pump, to be displayed on the cockpit instruments.
What can possibly go wrong, and how can failures be diagnosed? This post addresses that question.
Oil Level
During preflight, the propeller is repeatedly pulled through until a burping sound coming from the opened oil tank indicates that engine blowby has pushed all oil in the engine sump back into the tank.
At that time, the dipstick of the tank correctly indicates the oil level.
If a hot engine was stopped for an extended period, say at least 12 hours, then typically 10 or more blades must be pulled through before the burping sound occurs.
Pull the blades slowly so that the compressed air in the cylinders has time to bleed past the piston rings into the crankcase.
We always pull a flew more blades after the initial burping sound. That way, a bit of additional oil is forced by the oil pump into the galleys of the engine and into the bearings, thus reducing start-up wear.
We check the dipstick before turning the propeller, just to see how much oil has pooled in the engine.
The level shown on the dipstick should be 1/2 to 1 pint below the maximum value when the engine has been stopped for at least 12 hours.
An unexpectedly high oil level corresponds to far fewer blades that need to be pulled through before the burping sound occurs.
If an engine has been stopped for a long period such as several days, this is a warning sign that things may not be well.
Indeed, the oil hose at the oil tank may have an air leak. Thus, this should be carefully investigated. For details about air leaks, see below.
The warning case does not apply if you regularly pull through the cold engine just to move the pistons and circulate the oil. In that case, a much smaller amount of oil may pool in the engine sump.
Unexpected Oil Consumption
Suppose you made an oil change yesterday, flew for an hour, and shut down. For a final check, you pulled through a few propeller blades till you heard the burping sound, and verified that the oil level was right at the top mark of the measuring stick.
Today it is quite cold, say 38 deg F. You check the oil level for another flight. When the burping sound occurs and you check the oil level in the tank, you are surprised to see that almost half a pint of oil is missing. A worrisome question: Did you burn so much oil in just one hour of flying?
No, you did not burn that much oil. Due to the low temperature, the oil is a bit thicker, and the burping process does not return all oil in the engine sump to the tank. Instead, the supposedly missing oil coats the inside of the oil pan.
Keep this in mind whenever you check the oil at low ambient temperatures.
A Beneficial Side Effect of the Oil Tank System
When a hot Rotax 912/914 engine is shut down, within 30 minutes some oil moves from the oil tank to the engine.
That oil pools in the bottom of the engine and closes off the port where the return hose to the oil tank is attached.
Since the hose from the oil tank to the engine is filled with oil, the crankcase of the engine is then completely closed off to outside air.
Suppose the engine now sits for a while, say two weeks.
If during that time temperature and humidity in the hangar vary and cause condensation in the hangar, none of that moisture gets into the crankcase and contaminates the oil, affecting viscosity and lubricating properties.
This is very different from aircraft engines where a vent tube is always open and admits air to the crankcase. Cycling of temperature and humidity then produces condensation in the crankcase.
Engine Startup
Immediately after the engine starts running, check the oil pressure. It should move almost instantly from zero to around 60-80 psi and stay there without fluctuating.
There are two ways in which things may go wrong: The oil pressure may stay at zero, or it may go to some low to medium value, typically accompanied by fluctuations.
We cover the two cases separately, beginning with a zero pressure indication.
Startup Problem 1: Zero Pressure Indication
There are three possibilities:
1. The engine indeed does not develop oil pressure. This is a nasty situation since running the engine for anything beyond a short period, say beyond 30 sec, will ruin the engine.
2. The sending unit at the oil pump is stuck.
3. The oil pressure gauge in the cockpit is stuck at zero.
How can we decide which of the three cases applies? We first test the oil pressure gauge since this is easiest.
Test of Oil Pressure Gauge
For the test, we use the fact that a broken wire from the gauge to the sending unit will produce a max indication of the instrument.
Thus, we disconnect the wire at the sending unit, turn on the master, and watch the gauge. If it stays at zero, then it is stuck and must be replaced.
For further confirmation, we tap the gauge vigorously with one finger. It may well happen that the gauge flips up to max indication, thus giving us another confirmation of the diagnosis. At one time, we had exactly that case.
We replace the gauge, making sure that we get a replacement with correct max range. In our case, the range is 0 to 80 psi, and the gauge is VDO 350 104.
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| VDO Gauge 350 104 |
Test of Pressure Sender
We do not have a sophisticated test for the sender. In our installation, it fails every few years. So we always have a spare on hand for installation.
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| VDO 360-003 pressure sender |
Lawyers have convinced VDO to rule out any aircraft use, but this is just a game to avoid liability. The fact is that the sender works fine for several years, and then fails.
If the new sender results in proper oil pressure shown by the gauge, we are done.
Otherwise, it's time to connect a gauge that directly measures oil pressure. It should show zero oil pressure, thus proving that the oil pump has failed.
For investigation of the oil pump, consult the Rotax manual for oil pump removal and so on.
Startup Problem 2: Low to Medium Fluctuating Pressure
A trickier problem is at hand if at startup the oil pressure fluctuates around low to medium values. It may well be caused by an air leak of the oil hose at the oil tank.
The remedy is immediate shutdown, followed by detailed investigation.
To check for an air leak, disconnect the oil hose at the oil pump. If there is a substantial air leak, some oil will exit, but then the flow will virtually stop.
On the other hand, if there is no substantial air leak, the oil will continue to flow and eventually will try to empty the oil tank.
We had this problem many years ago.
The Fix ...
... was easy. We replaced the hose from the oil tank to the oil cooler.
Unfortunately, the oil cooler has a 1/2 inch OD fitting, while the oil tank has a metric 12 mm OD tube. Thus, the oil tank connection is a bit smaller, to be precise, by 0.7 mm.
We used a quality 1/2 inch ID oil hose as before, but tightly wrapped the metric 12 mm tube of the oil tank with electrical insulating tape to increase the OD to 1/2 inch.
It took some effort to get the hose onto the enlarged OD, as desired. Then we added clamps for both connections.
Oil Pressure Problems in Flight
You are flying, when suddenly the oil pressure gyrates. Regardless how the oil pressure fluctuates, to higher or lower than usual values, this calls for landing at the nearest airport and careful investigation.
If there is enough oil in the tank, no sign of an oil leak, and if the fluctuation was from the accustomed oil pressure value to higher ones, the odds are that this is a failure of the oil pressure sender that has no bearing on engine performance.
So, install the new sender you have always with you, and the problem most likely will be gone.
If you have fluctuation to lower values, this may indicate a serious problem. It may be caused by a substantial air leak of the oil hose at the oil tank.
The earlier described test for low to medium fluctuating oil pressure upon startup applies here as well.
Note that any substantial amount of air drawn into the oil pump is not good for the engine: It potentially harms the hydraulic lifters and plays havoc with lubrication in general.
One pilot told us that air pumped into the galleys and lifters eventually caused in-flight engine failure. He then opted to replace the entire engine.
Note that the false air does not cause any oil leak showing at the tank. The only indication will be lower than expected, fluctuating oil pressure.
Max Oil Pressure
Upon engine start or in the air, oil pressure may go the max value and stay there.
This may be due to cold temperatures and oil with too-high viscosity value.
Barring that, it most likely indicates that the pressure sender is no longer connected to the pressure gauge in the cockpit.
This feature was designed into the oil pressure sender/instrument system so that an interrupted connection does not trigger emergency action.
Thus, this likely is a minor problem that can be analyzed upon landing at the nearest airport.
Zero Oil Pressure
If oil pressure indication suddenly drops to zero, you must assume that the engine has no more oil lubrication and thus will seize shortly.
Shut off the engine and prepare for an emergency landing.
You can see that this is very different from the earlier discussed zero oil pressure after start of the engine.
Recommendation
On cross country trips, always carry a spare oil pressure sender.
It's one of two must-have items you should have along. The other item is a cheap and lightweight voltage tester to analyze possible failure of the engine voltage regulator/rectifier.
Some Words of Caution
1. If more than a small amount of false air has been drawn into the oil hose connecting oil tank and oil pump, or if for any other reason that hose contains a nontrivial amount of air, you must eliminate the air from the hose before starting the engine.
Rotax Service Instruction SI-04-1997 has details about the purge operation. For related bulletins, for example, covering oil changes, go to the top menu for service bulletins.
2. When you order a replacement pressure sender, make sure that the pressure range matches that of the cockpit instrument.
In our case, the pressure range is 0-80 psi, and VDO sender 360-003 is correct. If you don't know the part number, go to the VDO website to identify the correct sender.
Similary, if you replace the pressure gauge, get one with matching max range. For us, this is VDO 350 104.
3. The pressure sender is exposed to the high frequency harmonics of the nearby gear box. Same for the sender unit for oil temperature.
For this reason, the wire connectors for both units tend to fail. This happened twice to us.
To prevent this, attach both wires with tie wraps to the pressure sender, or to nearby equipment that is directly connected to the sender, before proceeding to the engine frame.
In our case, the intake hose for the heater is attached to the sender, so we chose that as attachment point; see above photo.
That way, the wires and the connectors vibrate with the same frequency, and the connections will not break.
Have any questions or feedback about wrong oil pressure of Rotax engines? Please share your thoughts in the comments.
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