Poor Starting 912iS

In doing some diagnostic tests on my fuel flow “red cubes”, I think I’ve noticed that the return line flow does not drop to zero when the pumps are switched off nearly as fast as in the inlet line. I have an orifice across the lines per rotax instructions. I suspect what I am seeing is air trapped somewhere in the system expanding. On test engine runs, I have also noticed gurgling noises for a few seconds accompanied by erratic fuel flow and pressure indications  which suggest that there are air or vapor bubbles in the system at rest. Fuel feed is from the bottom of a header tank, so no bubbles from that source.  The tanks are above the header, so there is positive fuel pressure at the pump inlet. The fuel pressure and pump noises settle down after perhaps ten to fifteen seconds. I always hesitate and listen to the pumps before starting.

What that suggests to me is that air, or vapor, is either trapped, comes out of solution or leaks back into the high pressure manifold when the engine is stopped and the fuel pumps are off. Perhaps this happens when the engine is hot and is allowed to cool - you might then  even get a slight vacuum in the manifold since the vane pumps don’t pass fuel nor does the return line because they both have non return valves. Perhaps under vacuum there is an injector air leak?

Anyway, if you allow your fuel pressures to stabilize for a minute as Bill suggested before starting and the problem then goes away, it’s was caused by air in the system in my opinion.

Hope this helps. Good luck.

The primary source of the vapor bubbles is the Butane in the gasoline.

This is the "Starter Fluid" component of gasoline.

It has a Boiling point at ambient Pressure of just a hair under 0°C.

It acts similarly to dissolved Carbon dioxide in a soft drink.

You will get a lot of "Fizz" as it exits the Pressure regulator or the Bypass orifice.

You would get the fizz in the fuel pump but the increase in pressure keeps it in solution.

Gasoline in the northern climates get an increased dose of it in the fuel during the winter month partly because it helps with cold starting but mostly because it does no harm as long as the fuel remains below ~25°C (~80°F) and mostly because it is cheaper than the other components in gasoline.

Notice that Gasoline is less expensive during the winter months in areas where tropical temperatures are unlikely.

While it readily vaporizes above freezing at ambient pressures, it vaporizes vigorously at even lower pressures.

The MAP pressure in an idling engine routinely approaches 10"hg. 

At his pressure Butane "Boils at ~ -20°C (~0°F) making winter starting easier.

This also explains why a frozen engine will idle but will not rev-up; opening the throttle raises the pressure and halts the evaporation.

Once the intake manifold warms-up the engine runs as if it were summertime.

The next time you find yourself warming up the car, consider what you are really want to warm up is the intake manifold as the car does not really care how comfortable you are.

The Sprag clutch I am referring to is part of the Starter Motor system NOT the one in the Gearbox. It allows the Starter Motor to engage and crank the engine, when the engine fires up it disengages. Rotax UK think this is slipping and not allowing the engine to spin up fast enough. If the Cranking speed isn't reaching a certain speed then the ECU won't initiate a start sequence and the engine won't start. This all seems possible, I am just trying to verify that the Sprag clutch is the main cause and not the Starter motor, wiring (voltage drop), solenoid etc. Some more tests to do. I might get chance this weekend looking at non-flyable weather in the UK. Many thanks for everyone's input.

Malcolm Huddart wrote:

The Sprag clutch I am referring to is part of the Starter Motor system NOT the one in the Gearbox. It allows the Starter Motor to engage and crank the engine, when the engine fires up it disengages. Rotax UK think this is slipping and not allowing the engine to spin up fast enough. If the Cranking speed isn't reaching a certain speed then the ECU won't initiate a start sequence and the engine won't start. This all seems possible, I am just trying to verify that the Sprag clutch is the main cause and not the Starter motor, wiring (voltage drop), solenoid etc. Some more tests to do. I might get chance this weekend looking at non-flyable weather in the UK. Many thanks for everyone's input.

I understand what you are saying, but the issue is a start then stop.  

Malcolm wrote:

The Sprag clutch I am referring to is part of the Starter Motor system NOT the one in the Gearbox. It allows the Starter Motor to engage and crank the engine when the engine fires up it disengages. Rotax UK thinks this is slipping and not allowing the engine to spin up fast enough. If the Cranking speed isn't reaching a certain speed then the ECU won't initiate a start sequence and the engine won't start. ...

As Jim and I have mentioned numerous times, you Do NOT have a starting problem!.  It STARTS! It just doesn't keep running After it Starts!!!

You do Not have a Starting problem. You have a Stopping problem! And, your engine Stopping problem has Nothing to do with the Sprague clutch!