Low fuel pressure, PA24-250

[Tom Deml]

Within the last year I overhauled my Lycoming O-540 A1D5, new carb, new engine driven fuel pump, amongst many other things. I've had a recurring problem with low to near zero fuel pressure on climb out(highest demand for fuel). Normal and rock steady pressure (4.5-5psi) in cruise. Electric pumps act normal before start up. During run up at around 1600-1800 rpm with engine driven pump only pressure drops to only 1psi. Turn on electrics and pressure comes up to normal. Back at idle, engine pump only, pressure normal. At idle before takeoff with electric now on, pressure normal. Full throttle for takeoff, at near rotation speed fuel pressure is dropping. During climb fuel pressure drops to .5 to 1 psi. Power to cruise, level off, pressure comes up, electric off, normal pressure. Engine has never stumbled on climb, thank goodness. My mechanics have ruled out the engine pump per replacing/testing/isolating. Have tripled checked for leaks/seals intake-side of engine pump. Next step is to isolate/bypass electric boost pumps. Given the unique nature of the dual electric fuel pumps and how they pump thru the engine driven pump rather than bypass direct to carb, my mechanics are thinking some sort of fault in one or both of the electric boost pumps. Anyone seen this before or ideas as to what the problem would be and respective solution.

[Timothy Poole]

[Zach Grant L1011jock]

First, your electric fuel pumps should bypass the engine driven pumps. This is both good and bad and I will get to that later. If they do not join the fuel system after the engine driven pump (and have the integral check valves in them), you are not in compliance with the type certificate. The electric pumps should operate at a nominal 4.5 psi and have a flow rate at that pressure of over 28 gph each. The engine driven pump should be set to about 4 psi and should pump upwards of 65 gph at that pressure. So as you can see, any one pump should be able to support full pressure at full power flow rates. With this in mind, you have to look at the common link to all the pumps. The supply side of the equation has several possible culprits. Any flexible hose that is under suction can entrain air if there is a bad connection, bad hose, loose connection etc. Additionally, the fuel selector/strainer can have air leaks. I would replace a section of the hose from the junction of where the engine driven pump gets its supply with a temporary clear hose and run the engine on the ground, observing the operation and fluid in the line. Being that you can get the pressure to drop at runup power, you should be able to find a culprit fairly easily.

Now about the pump plumbing....Look at the parts manual page 2H11, ( and 2H7 though a 180 with single electric pump it shows the routing of the hoses more clearly), it is obvious that the supply side of the engine driven pump is not the pressure side of the electric pump, but in fact shares a common supply from the firewall fitting. The pressure side of the electric pumps join with the pressure side of the engine driven pump prior to the carb. The fact that the engine driven pump operates at a slightly lower pressure than the EDP causes a stagnation of fuel in the EDP when the electrics are running, and has led to vapor lock in hot weather that was found when the electric pumps were turned off after takeoff and the pressure wasn't monitored. Solution is to turn on the electric pumps just long enough for pressure recovery and then off, and allowing the vapor to work through the system until the EDP pics up the full flow. Thats the bad part. The good part is that when the diaphram comes apart in the EDP and plugs the fuel flow, the electric pumps can still supply full fuel flow to the engine to get you safely to an airport instead of whatever you are over at the time of the failure!

-Zach

[Andrew Foster]

Tom,
We have a 61 250 with the identical engine an 0540A1D5. Ours has 1850 hours on it. We have owned it since 2000, overhauled in 97. I too had the same concerns when I first started flying it because on take off the fuel pressure was very low on the factory guage, perhaps .5-1 psi. Once manifold pressure is reduces then pressure increases. We do have a JPI 450 fuel flow as well. On take off at sea level we see app 24 or so gph at full throttle. This goes with the rule of thumb that take off FLOW should be about 10% of horsepower. here then is what I learned. Please keep in mind I am not an a&p, but am very active in the maintenence of our aircraft and have a mechanical background.
The 0540 only needs a min of .5 psi at full throttle. At full throttle you will have a high flow but low pressure as really there is no restriction to the amount of fuel to the carb. Think an open garden hose. Once you reduce throttle you decrease flow but increase pressure, think pinching the garden hose. This is very simplified, but remember, if you are getting at least .5 psi at full throttle you in an acceptable range..
More importantly has the reading changed from what you saw on the old engine? Yours sounds like it's operating just like ours. I hope this helps
If you don't mind could you tell us your experiences with the overhaul? I'm getting close and like all the knowledge I can get.

Andrew

[Tom Deml]

Hey guys. Thanks for the info.
We have verified it is not the gauge. They installed their own gauge and it mirrored the panel fuel pressure gauge.

[Tom Deml]

Zach, I had to read your post several times and reference the diagrams in the manual. I understand what you mean. Thank you for putting a lot of technical information in 2 short paragraphs. I will review with my A&P. The parts manual diagram seems more precise than the service manual diagram. Talking to my A&P today he is planning to put in some clear tubing to see if we are getting aspirated fuel flow. As for the physical plumbing itself we will be checking for leaks again.
One other issue that seems to have come along with this whole fuel pressure issue is the engine seems to want to stumble or hesitate a bit, particularly when it is not up to full operating temperature, when you transition from the idle circuit to run circuit. That range of 900 to 1200 rpms. My mechanic made some adjustment(s) on the carb and its better but not gone. if you pull the carb heat and then power thru that range, its gone. my mechanic has a suspicion that something else is not quite right because he said he's never had to adjust further than what he had to do on mine. so whether the low fuel pressure issue and this hesitation are related or not we have not determined. bottomline, before the overhaul, none of these issues existed. so, some thing or things are not right.

Andrew, maybe in the end, you are right and things are normal. I'm not comfortable though at this point when the needle is nearly on zero. I do not have a fuel flow gauge. as mentioned above, my fuel pressure always lived in the happy middle of the green prior to overhaul. so something has changed. hopefully we are not chasing a fake gremlin.

Andrew as for my overhaul, see a former post of mine under Engine Breather Tube. The conclusion to my overhaul was a very good outcome overall. I did have to replace my crankshaft which cost me an additional $8000 for an overhauled one with the full flange and could be ground again. My original didn't pass muster due to grooving in one or two journals and some heat cracking. it was caused by the old style rods that had been installed on the previous overhaul apparently. those rods had a narrower face and wore the journal. So when I got all done for my entire firewall forward including my prop I spent $48,000. About $10,000 more than I had hoped to spend , but I have confidence in everything up front.....except the fuel at the moment.....but it's fixable I'm sure.

[William Hughes]

I'd like to make a few observations. Our Comanche's originally came with a resistive style pressure gauge that is not particularly sensitive and can take a while (1/2 sec) to indicate low pressure.

Other Lycoming setups have a more sensitive large face round pressure activated gauge. Fuel line right into the cockpit - like a Lake LA-4. The fuel pressure gauge is a sensitive pressure type that was directly connected to the fuel. That needle flew all over the place. If the carburetor bowl was full the pressure was very high and when the needle valve opened to call for fuel the pressure dropped right off down to 1 psi and then bounced right back up to 8 psi when the needle closed. During cruise flight the pressure flopped back and forth between 3 and 10 psi. These pressure fluctuations are normal and due to the way the plunger style diaphragm pump works.

But the resistive type automotive gauges smooth all of that out and provide a sort of average reading. Do you have the original resistive gauge or has it been replaced with something more sensitive? An electronic engine monitor maybe?

Also, if there is air or vapor bubbles in the fuel it will still indicate the pressure properly. The bubbles are at the same pressure as the fuel they are in. Makes no difference to the gauge. Bubbles in the line may indicate that there is a restriction and leak somewhere upstream of the pump pulling air in (which is bad) but the pressure is the pressure whether there are bubbles in the fuel or not.

Fuel injected is different - but we are talking carburetors here, as I recall.

[Matt Bogard]

I had a very similar problem in my '59 250 a few years ago. If I shut off the boost pumps too soon pressure would drop, EGT would climb, engine would cough and fart.

Heritage Aero traced it to a mis-seated O-ring in the fuel selector that was acting as a check valve and allowing some air into the system at high fuel flows. Unfortunately this was after I assumed it was the engine-driven pump and replaced that...

[Tom Deml]

William, I have the resistor type gauge, but we put another inline and it mirrored my panel gauge.
Matt, We have not checked the fuel valves for an air leak....hhhhmmm? I have Osborne tip tanks and have had trouble with the left valve getting "stiff" despite both valves only a few years old. We will take a close look at those fuel valves. Maybe it's not "seating" properly.
I flew the airplane to the mechanics today. Hopefully the problem will be solved this week.

[Tom Deml]

Matt, By the photo of your aircraft, you have tip tanks as well. Are the JL Osborne STC setup with the Osborne selectors?

[Tom Deml]

Per isolating the respective electric boost pumps and installing some clear fuel hose where needed we have determined the source(s) of the fuel pressure problems. I have one electric that had failed. Probably has been failed for a while. Don't know for sure. And the other electric pump, pumps fuel and creates proper pressure at low to medium fuel demand, but it's internal check valve fails when it's turned off causing fuel to be able to re-circulate back thru the electric rather than strictly go to the carb from the outlet side of the engine driven pump. Hence low fuel pressure at medium to high demand for fuel with just the engine driven pump. When the electrics are totally taken out of the equation, fuel pressure is normal in all phases. Two new electric boost pumps on on their way to be installed.

[William Hughes]

Thanks for the follow up and this may be applicable to an issue in another aircraft type I've been poking around in.

[Zach Grant L1011jock]

This is a good time to bring up that on the carbureted 250, if you don't hear both pumps operating (they do not run at the same speed ever, so there is an uneven syncopated ruthenium's with two, and a steady click click with one operational), check them by turning them on, opening the right cowling door and put your hand on them individually. It will be obvious if one or the other is inop at that point.

Thanks for the follow up. It makes perfect sense.

Zach

[Hank Spellman]

Zach,

This discussion has brought another question to mind. Assume a 250 with a carburetor and one failed (unknown to the pilot) electric fuel pump on a takeoff roll at low altitude. After liftoff, and after their is too little runway left for landing, the engine driven fuel pump quits. Would reducing the throttle somewhat stop the surging and reduced power because of insufficient fuel flow? Would such a move actually increase the total power output over time? Where in the sequence of immediate actions would you put this?

Hank
Henry A. Spellman

[Matt Bogard]