Hello John, I'll do the best I can with your questions, but I'm rather new to this airplane and don't know how it has behaved in the past.
I believe the electric pump is in parallel with the mechanical pump because you can switch off the electrical pump and there is little or no change to the fuel pressure. We have been debating whether the electric pump should be left on during cruise flight, or just used during take-off and landing. I leave the pump on. Normally, from what I can observe, the fuel pressure gauge normally reads about 4 psi, give or take, which I understand is about right. There is a red fuel pressure warning light that is supposed to be off when fuel pressure is adequate. It is common for this light to "flash" or blink from time to time with no noticeable change in pressure indicated on the gauge. I do notice that when the electric fuel pump is on, there is less flashing and extended periods when the light is off, so there is a net increase in fuel pressure when both pumps are operating. I have no idea where the pressure sensor for this light is, or if it is a separate one from the one for the fuel pressure gauge. I pretty much ignore the light as long as the gauge is reading about 4 psi of fuel pressure, although it is disturbing that the light will flash when the pressure gauge seems to indicate normal fuel pressure. I don't like things I can't explain. The carb was recently (May this year) completely rebuilt. This was before I owned the airplane. I do not know if it has the right float valve seat, but the A&P who did the work is very knowledgeable about Ercoupes. At the same time, all the fuel lines were replaced. Over all, the engine compartment is very clean, neat and organized. The plane has had an on-going chronic problem with "siphoning" gas out of one of the gas caps. We have gotten into the habit of not filling the tank all the way to the top, as leaving an inch or so seems to stop the siphoning. We have debated purchasing new gas caps. We recently measured the exact fuel capacity of each of the tanks by completely draining them, then adding fuel one gallon at a time. Our wing tanks are exactly 8 gallons each. I noticed that the fuel caps have the number "9" stamped on each one. I wonder if the caps are intended for the 9 gallon tanks? Perhaps we have the wrong caps. I've not been able to find out if there are different caps for different capacity tanks. When I had to make my emergency landing after the engine shutdown, I found that the left tank was empty (Put 7.4 gal in it) and the right tank still had 6 gallons in it. The right wing fuel cap vent was found to be completely clogged. I believe now that the clogged vent caused a vacuum in the right wing tank that could not be overcome by the fuel pumps, so all the gas was sucked out of the left tank. By the way, both tanks are interconnected and should drain equally. When we drained the fuel from the tanks to measure their capacity, the fuel level remained equal as we drained from one side, but then both caps were off. I think that clearing the vent hole in the right cap will allow the fuel to equalize between the tanks. An A&P is looking at the plane tomorrow, and if he finds nothing else wrong, I'm going to do some pattern work to verify that the fuel is being drawn from both tanks. If it is, I'll consider the plane safe to fly and will add checking the vent holes to the pre-flight checklist. Tom From: John Cooper Sent: Tuesday, August 17, 2010 5:34 PM To: [email protected] Subject: Re: [ercoupe-tech] Re: Fuel System Problems Part 3 fingers crossed.... As far as your fuel pressure problem goes, is this a new development? Does it go away if you turn on the boost pump? If neither pump can produce adequate pressure, since it is unlikely they both are bad, I suspect you still have a fuel supply problem (or a bad pressure sensor), otherwise I suspect one of the pumps is weak. If you have a Stromberg carb, the pressure feed version requires a different float valve seat. Make sure you have the right part. If your boost pump is in series with the engine driven pump, and the engine driven pump fails due to a leaky diaphragm, use of the boost pump can result in a lot of gas being pumped into the engine compartment. You are better off plumbing the boost pump in parallel with the mechanical pump. That way, you need a failure of both the diaphragm and the outlet check valve before you have fuel leakage. -- John Cooper Skyport East www.skyportservices.net
