Low Fuel Pressure

[fastfred]

I have a 2003 uls with a fuel pump not on the bad list. My 7 does not have the return fuel line either. 860 hours on it . What are the opinions on the return line and how many hours are the pumps good for?

[Dave S]

Fastfred,

I have an early S7 with the 912ULS also; and, no return fuel line.

Agreed the newest pump design has a better patent on the bleed hole so if a leak develops the stuff is piped away from the engine rather than dripping on the exhaust manifold. This has nothing to with the fuel return line.

What the return fuel line does is help prevent carb flooding in hot weather following engine shutdown. Say if a person is out cruising, its 90 degrees F you land and park the plane for 10 minutes come back and find you have a hot start problem (vapor/flooded). What can happen under this condition is the fuel in the lines between the fuel pump and the carburetors picks up enough engine heat to expand the fuel in these lines, increase the pressure and overpowering the carburetor needles - pushing enough fuel past the needle/seat to flood the engine when you go to restart. What the return line does is provide a small orifice in the line between the pump and carbs to bleed off the pressure after the engine is shut down and return the excess fuel back to the supply side of the fuel system.

I considered installing the return line; but have not because the flooding issue seldom arises - either re-start the engine a very short time after shutdown before the issue can develop, or let the plane sit long enough before restart that the fuel expansion/vapor deal goes away on its own and that is only under high ambient temperatures. If you are happy with the way the plane operates now - I wouldn't bother with the expense and installation time - just my opinion.

On the fuel pump. Rotax has what they call a 5 year rubber parts replacement requirement which includes replacing the fuel pump every 5 years. If you have the same pump on the engine that has been running since 2002 - i'd say it's time to replace it based on age. I don't see that Rotax has any hour replacement requirement - just time. Fuel pumps, at least the automotive ones of the same design (diaphragm type) also seem to suffer from a half live more so than mileage. I don't know that anyone can say exactly what the life expectancy of a fuel pump is, it also depends on how the plane is stored and flown; but, both the poppet valves and the diaphragm become less reliable with age.

[jiott]

I feel stronger than Dave on the necessity of the fuel return line. First, Rotax says to do it in the installation manual; I don't believe it is an option. Also I believe it is more important as a vapor-lock preventative than as a pressure relief. Vapor lock usually occurs on hot day shutdowns as Dave said and the return line will quickly recirculate cool fuel from the header tank. If you burn Mogas like many of us do, it is quite a bit more prone to vapor lock than 100LL, in fact there are cases where it has occurred in flight on hot days and/or high altitude. I personally feel much better following the recommendations and installing the return line.

[Bud Davidson]

Does this fuel heating problem exist when the engine is shut down by shutting off the fuel supply, ie, fuel pressure drop ? I always shut down engines in the past with mixture shutoff. Should I not be shutting down the 912 by shutting off the fuel supply ?

[jiott]

Not sure about the fuel heating effect, but that method of shut down will empty the carb bowls, so the next restart will be more difficult as the carb bowls have to refill.

[Esser]

I don’t think it will make it harder to start. As soon as you open your fuel valve, the bowls will fill. I like emptying the bowls so if you do get fuel vapour it won’t flood as the bowls can take fuel if there is an expansion

[kmach]

If you have all your engine compartment fuel lines in fire sleeve (recommended), your chance of vapor lock decreases, allot.

In summer, I usually open the oil cap access door after shutdown, it lets allot of heat out.

I did install the fuel return line when installing the new engine ( as per Rotax's instructions), but I never ever had vapor lock with the old engine with no fuel return line.

[jiott]

I also believe firesleeve helps a lot, and have started opening the oil access door after shutdown. However, I do this mainly to avoid cooking the expensive ignition modules.

Vapor lock is a funny thing; I have the fuel return line and never had a vapor lock incident for about 3 years. Then one hot summer day (about 95F) I shut down for maybe 10 minutes on a gravel bar. I restarted OK and immediately started my T/O roll. Suddenly the engine lost most power and started coughing but didn't die. I aborted the T/O and shortly the engine was running fine again. I am sure it was vapor lock due to the short, hot shutdown (had not opened the oil door), but the fuel return line soon circulated cool fuel to kill the vapor lock. Lesson learned: after a short hot shutdown I need to let the engine idle for a while after restart before attempting an immediate T/O. This has never happened again, but that one time it could have had a very bad outcome.

[fastfred]

Thank you for that information. The plane is new to me so I am unlocking its secrets all the time. Trucked it to the airport today and hope to get it up soon but I can't find anyone to check me out in it yet.

[Bud Davidson]

OK. I am now able to run my engine. Shutting down the engine by shutting off fuel does not work well for normal shutdown. Engine stops so quickly washing oil off cylinder walls by turning off the ignition is likely no issue. I stand corrected and will shut down with ignition switch.
I have a lot to learn about the Rotax.