Poor Starting 912iS

Good News!!!

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For those that are questioning whether to start with Both Pumps or just One, consider this...

If running one pump draws, let's say, 10 amps, running both pumps does NOT draw 20 amps.  More like 14.

Notice that the pump speed increases when you energize the second pump.

The second pump takes the load off the first one and they both share the load.

When starting the battery voltage is significantly lower than the Generator voltage while running. <12 vs >14 volts.

The lower battery voltage, especially while cranking makes it difficult for a single pump to maintain fuel pressure.

Running both pumps ensures that the fuel pressure is good even when the voltage drops to 10v or lower for a moment.

The small current penalty is a small price to pay.

As another one posted earlier, does anyone know why Rotax cautions against starting with both pumps on?

Bob Yanniello wrote:

As another one posted earlier, does anyone know why Rotax cautions against starting with both pumps on?

 

.....I guess not.  When I read it I thought it was logical - to unload the battery as much as possible for the start.  If there is another reason, it would be nice to know.

Substituted another ECU unfortunately it didn't fix the starting problem, but it is another thing eliminated. It was also a simple thing to try before stripping the engine down to look at the Sprag clutch. Our UK Rotax dealer is still convinced the Sprag clutch is the cause of the problem. I still have some other things in the Fuel System to try first. The weather in the UK has been reasonably good for the time of year, hence the aircraft is needed most days for Sailplane towing so its difficult to get to work on it, it pays for its keep when it's flying. 

Malcolm,

It is highly unlikely to be a Sprague clutch problem.

If you keep reporting that it doesn't start on the first attempt.  This might be confusing for your dealer.

It DOES START in the First Attempt.  What it doesn't do is keep running.

And because it DOES Start, it is turning fast enough, so it is NOT a Clutch problem; And 1 second later it is turning fast enough for the generator to bring the battery voltage above 14v, so it is also not a voltage problem.

The ECU is not detecting any faults because any Air in the hoses and/or the Fuel pressure are not visible to the ECU.

It still feels like a fuel problem.

Try my modified checklist from a few posts back. it worked for Jim.

Ii would look at getting a bypass orifice installed.