fuel system troubleshooting.

Interesting project. wonder if anyone has run across this problem.
Aircraft: Cessna Agtruck IO520D fuel injected engine
Last year, at end of season engine would die when rpm was under 1500 without boost pump on
Cleaned Dist valve- no fix. Replaced engine fuel pump. Seemed to fix problem.
This year boost pump went out middle of season
replaced with serviceable unit.
immediately afterward engine wouldnt run below 1000 rpm without boost pump on..

Now suddenly engine is back to last years scenerio of not running on ground without boost pump on.
Engine runs fine in the air until turning at end of the field and then boost is needed. Further testing shows fuel pressure is directly associated to the number of G's being pulled. pull back on the stick and the fuel pressure continues to drop, push forward and the pressure continues to climb.
Trying to figure out what would cause this. before I start replacing parts

water upline of pump... to thick to make it through injectors...?

Low wing, right? And I assume turns at the end of the field are a combination of lower speed and higher g's?

When pulling g's more pressure is required to raise the fuel from tank to engine. And it's on the "vacuum" side of the engine driven pump. Crud in a pump check valve, or leaky diaphragm, or loose fuel line fitting or a leaky fuel hose inside a fire sleeve?? As to in-flight vs on the ground, are the tanks pressurized with gas cap elbows facing into the wind or similar? If so that would provide more of the required pressure at higher speeds.

Is the fuel supply change related to Gs or attitude? Pulling up would be similar to the static position. I'd look at a supply issue upstream of the pump and maybe ground test it by raising the tail to see if that helped it idle. Restricted supply line, loose obstruction, vent issue..... I know squat about an Ag Truck fuel system so I'm just guessing. But I just spent a week chasing a fuel supply issue in my 180 so I'm sympathetic.

If you're confident about the pumps it sounds to me like the system is air locking on the supply side, regardless of weather this system has wings or is on a skid in a plant somewhere I'd be looking at the tank venting. Run it on the ground with the tank caps loose or off as a test.

Be safe!

Mike's reference to water is valid, too. I little bit of water in the wrong place can interfere with fuel delivery and it may be more prevalent in one attitude that another. And it can be hard to find. A couple of bottles of isopropyl would be worth trying before removing parts. In fact that'll be my first course of action for any subtle or occasional fuel delivery issue in the future. End of season, cooler temps... could condensation be the common factor?

Those turns at the end of the field wouldn't happen to be down wind, would they....?

John Scott

I would think that when the boost pump failed it sent something down stream that hurt the replaced engine driven pump since the engine driven pump developed problems imediately after. Not the cheap fix but that would be my guess.

Lots of good ideas. since the plane has been sent to the woodshed for the year, I'll start tinkering with it to find the culprit.

Throwing this out there - one of my chemical accounts did a -550 conversion on their AgTrucks, couldn't get it to run well in the air and horrible in the turn. Everything on the ground looked ok.

This is very second hand info - is there a fuel fitting/line secured to the firewall or passes through? They said they couldn't find the problem until they realized that g force in a turn had enough flex or distortion on the firewall that the fuel fitting was just at the right tightness that it would separate in G force letting fuel out and loss of fuel pressure. On the ground no G or stress so no fuel was coming out with boost pump on and they thought the fitting was tight enough.


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Interesting. the alt belt has worn the hose the goes thru the firewall and there is a fuel stain near the bottom of the firewall I never could find the source of, so Ill check that area good.

SuperCub MD said:

I would think that when the boost pump failed it sent something down stream that hurt the replaced engine driven pump since the engine driven pump developed problems imediately after. Not the cheap fix but that would be my guess.

Click to expand...

This may have been the culprit. Went thru everything including a leaking return hose that went thru the firewall. Runup resulted in same scenerio. Replaced engine driven fuel pump and it seems to run normally. Test flight to follow.

Longwinglover said:

Those turns at the end of the field wouldn't happen to be down wind, would they....?

John Scott

Click to expand...

Now that is pretty funny!