jrwilson Posted May 28, 2011 Report Posted May 28, 2011 I have a '63 M20C and just had the engine driven fuel pump replaced. The old pump would run at about 5 psi on the ground, then on takeoff, drop to about 4 to 4.5. As I climbed through 6,000', the pressure would drop to 2.5 to 3 and the pressure needle would be a little "jittery." When I leveled off at around 9,500', the pressure would go immediately back up to 4.5 psi, with no power changes. I didn't like the jittering, and it was getting time to replace the pump. The new one does the exact same thing, with about the exact same readings. The pressure drop starts on the new one at about 5,000'. Any idea why this would happen? I hit the boost pump and it jumps about 1 pound. The readings are in the green arc, but I'd like to know why the drop happens, and why it goes back up when I level off. Any ideas?
Piloto Posted May 28, 2011 Report Posted May 28, 2011 The fuel pressure is also a function of the amount of fuel in the tanks, fuel differential height (pump vs fuel) and fuel tank venting. Check the fuel caps O rings. Any leak on the caps will cause a negative pressure in the tank. Check fuel tank vents. You can check engine pump health with the following test. With engine off 1. Mixture full rich, throttle open. 2. Turn electric pump on 3. After getting pressure close mixture and throttle 4. Turn electric pump off. 5. Verify fuel pressure still indicated for at least five minutes (longer is better). If pressure drops right away you have a perforated diaphragm in the pump or a bad bowl needle valve in the carburator. José
jrwilson Posted June 1, 2011 Author Report Posted June 1, 2011 Thank you. I think it might have something to do with the venting or fuel level...Pretty sure my caps are tight. I tried the fuel pump/carb test, but I don't think it will work with mine, doesn't the 63' C have independent lines for the boost pump and engine driven pump?
Piloto Posted June 1, 2011 Report Posted June 1, 2011 Quote: jrwilson Thank you. I think it might have something to do with the venting or fuel level...Pretty sure my caps are tight. I tried the fuel pump/carb test, but I don't think it will work with mine, doesn't the 63' C have independent lines for the boost pump and engine driven pump?
Kwixdraw Posted June 1, 2011 Report Posted June 1, 2011 Although Piloto is very experienced and I am still working on my A&P I think you might also check your gauge. According to my recent classes the fuel pressure gauge in an older plane may have an orfice in it to smooth out fluctuations in pressure. You changed the pump already and no change. Thoughts Piloto?
Piloto Posted June 1, 2011 Report Posted June 1, 2011 Quote: Kwixdraw Although Piloto is very experienced and I am still working on my A&P I think you might also check your gauge. According to my recent classes the fuel pressure gauge in an older plane may have an orfice in it to smooth out fluctuations in pressure. You changed the pump already and no change. Thoughts Piloto?
Kwixdraw Posted June 2, 2011 Report Posted June 2, 2011 Yes I see what you are saying. Wouldn't it be possible that there was a fault in the fitting for the orfice and though the pressure Indicated a change it was just a faulty gauge indication? Was thinking the easiest think might be to hook up a test gauge to see if it shows the fluctuation.
jrwilson Posted June 2, 2011 Author Report Posted June 2, 2011 I need to check the vent. Assuming it still might be the fuel pump (even though new, and with same symptoms) or carb, would there be other indications? I have a JPI engine moniter, and EGT/CHT is stable in climbs and cruise, so if the carb or pump is dumping more/less fuel, wouldn't there be some type of engine indication? Why would the pressure increase when you level off due to pump or carb? The other thing is there used to be a little kinked off piece of tube off the back of the fuel pressure gauge, which the mech removed a few years ago. Could that have done it. The problem seems to be altitude/pressure related (at least it seems so to me) so could that little piece of tube done something? It wasn't connected to anything, just a thin piece of metal tube, kinked off. That is when I started noticiing the issue. Not that it might not have been there before, I just started noticing it then. Thanks for the help!
Piloto Posted June 2, 2011 Report Posted June 2, 2011 Quote: jrwilson I need to check the vent. Assuming it still might be the fuel pump (even though new, and with same symptoms) or carb, would there be other indications? I have a JPI engine moniter, and EGT/CHT is stable in climbs and cruise, so if the carb or pump is dumping more/less fuel, wouldn't there be some type of engine indication? Why would the pressure increase when you level off due to pump or carb? The other thing is there used to be a little kinked off piece of tube off the back of the fuel pressure gauge, which the mech removed a few years ago. Could that have done it. The problem seems to be altitude/pressure related (at least it seems so to me) so could that little piece of tube done something? It wasn't connected to anything, just a thin piece of metal tube, kinked off. That is when I started noticiing the issue. Not that it might not have been there before, I just started noticing it then. Thanks for the help!
jrwilson Posted June 3, 2011 Author Report Posted June 3, 2011 hmmmm....I tried the static pressure test, but the pressure dropped immediately. There was no fuel dripping or anything, but the fuel pressure did not hold. I asked the mech and he said he thought he remembered my plane had a boost pump that bypassed the engine pump, which wasn't the same as newer mooneys (he wasnt sure though, going from memory). So I don't know, i need to check the parts manual. You mention leave the pressure gauge alone about the transducer. Do you mean leave the tube off, or put it back on? I really appreciate your help.
Piloto Posted June 3, 2011 Report Posted June 3, 2011 If the pressure didn't held and you found no drip, and your pump configuration is parallel it looks like a bad check valve on either pump. The check valve keeps the fuel from coming back to the tank after pumped. If defective or intermitent some amount of fuel will return back to the tank after each stroke thus causing a fuel pressure drop. On some fuel systems the check valve is separate from the pump or the pump may have an integral check valve, check yours. The check valve would be in the fuel line between the pump outlet and the carburator. They are cheap, about $20. José
Urs_Wildermuth Posted June 18, 2011 Report Posted June 18, 2011 While we are on the subject of fuel pressure: My engine driven pump gives off pressure right in the middle of green, however if I add the electric one (which got installed new), pressure will rise well above red line into 6.5-7. This was so with the old pump and is the same with the new one. Any thoughts? Does anyone of you see that too and is it normal or not? My maintenance people, who've been dealing with this plane for years (at least 15 before I got the plane) tell me it's alway been like this. But does that make it right?
Piloto Posted June 18, 2011 Report Posted June 18, 2011 Urs, I would not worry about the high pressure, is not that high. Excessive high pressure (over 20psi) on the carburator will cause overflooding of the bowl because the float valve will not be able to close. This will cause engine roughness in idle but not at high power settings. José
Urs_Wildermuth Posted June 18, 2011 Report Posted June 18, 2011 Thanks José. That is what I thought too Then again, a red line is a red line, so better ask.
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