Fuel Pressure/Flow issue on new plane

I have a 180hp Superior XP-360, with a Bendix fuel injection system. I have the standard AFP High Pressure boost pump system. My first flight went without problems, I climbed to 6000', did slow flight, stalls, cruised around at 4000', had no problems, landed. Second flight, shortly after turning off the boost pump after takeoff, the engine monitor showed the fuel pressure dropping. It dropped all the way down until the engine was starting to starve, when I turned the boost pump back on. Pressure was instantly back to 28psi and I felt a power surge as the engine got what it wanted. I cruised around the field for a few minutes thinking this through, then with the boost pump on, I switched tanks then turned the boost pump off. Pressure immediatly started dropping. Boost pump on. I then landed to diagnose. Immediately after turning off the runway, I turned off the boost pump. Fuel pressure was at 18psi with boost pump off. I looked everything over, confirmed I had freeflow of air through both vents, etc. I then unhooked the fuel line to the fuel pressure transducer, ran the boost pump and reconnected. I did the same with the main fuel supply line to the servo. Another test flight had pressures in the 17psi range with the boost pump off. When I went to full power to climb from 4000 to 6000, the pressure dropped again and I had to turn the boost pump on again. Pressure immediately went to 28. I'm stumped.

Rather than tear the whole system apart (not even knowing what I'm looking for), I'd like some ideas for a logical solution. Here's what's on my list for now:

Disassemble fuel filter in the AFP unit and examine/clean
Flow and pressure test each hose
Disconnect inlet line to engine pump, submerge in gas can and confirm pressure and flow of engine driven pump

And then??

If anyone has ideas, I'm all ears.

Thanks,

Bob

Valves

Debris/contamination of valve(s) in mechanical fuel pump???

John

New mechanical fuel pump? Might have a small piece of debris stuck in the diaphram. It sure sounds like a mechanical fuel pump problem to me, but....I'm not an engine expert either.

A friend of mine had exactly the same problem. It turned out to be a loose fitting at the gascolator, between the boost pump and engine driven pump.
Decowl and have someone turn on the boost pump (master off) while you watch for leaks. Be ready to turn off the pump immediately.
A leak like this will suck air from the engine driven pump, causing a very lean condition/low pressure. But, it will squirt fuel with the boost pump on. With the boost pump on, the engine will probably run fine, but your GPH would suffer depending on the size of the leak.

I'm no expert but here's another suggestion: obstruction upstream of the engine-driven pump may cause it to cavitate, but the boost pump will force fuel past the obstruction. Does the AFP pump have a bypass valve? If partially blocked it too could cavitate the engine-driven pump.

Conor McCarthy
RV-9A 90990 fuselage
Queensland, Australia

sf3543 said:

A friend of mine had exactly the same problem. It turned out to be a loose fitting at the gascolator, between the boost pump and engine driven pump.
Decowl and have someone turn on the boost pump (master off) while you watch for leaks. Be ready to turn off the pump immediately.
A leak like this will suck air from the engine driven pump, causing a very lean condition/low pressure. But, it will squirt fuel with the boost pump on. With the boost pump on, the engine will probably run fine, but your GPH would suffer depending on the size of the leak.

Click to expand...

"Decowl and have someone turn on the boost pump (master off) while you watch for leaks."
Obviously not Master off, but Mags off.
Sorry about that.

Fuel Pressure Issues

Assume this is an experimental Silverhawk system? If so, contact Precision Airmotive for guidance. (360) 651-8282

I would say it is the diapragm/engine pump at fault.
Also note that the engine pump will NOT deliver as much pressure as the electric one. Not even close. In a steep climb at high power settings the electric pump is often needed.
This becomes more obvious at higher altitude and temperatures.
The engine could starve out slightly with the engine pump only in the above scenario.
That is why they invented the Romec/vane engine pump. It delivers a much better pressure and will not struggle to keep up.

Just for interest, I have heard of many people replacing the diapragm pump quite frequently.[300-500hours] as the pump starts loosing capacity.
Also doesnt hurt to have a blast tube on the pump.

rv72004 said:

I would say it is the diapragm/engine pump at fault.
Also note that the engine pump will NOT deliver as much pressure as the electric one. Not even close.

Click to expand...

I don't agree with this statement - the engine driven fuel pump should deliver about 22 to 25 psi, and the AFP pump should be only a few psi higher.

I would second the theory that the engine pump or the lines between it and the AFP pump could be the culprit.

As mentioned, another thing to examine is the bypass check valve on the AFP pump. It is through this that the engine pump has to pull fuel.

Vapor lock should not be ruled out - was the OAT or fuel temperature any different on the first (and non problematic) flight vs the problematic ones?

Another confounding issue with pressure indications is that they typically drop in a rapid climb due to the back side of the transducer not being vented (nothing you can do about that). Your problem is obviously more than that due to engine symptoms also.

An ideal test would be to get another electric pump and connect it where the engine pump is, and see what sort of flow you can get there (time how long for a quart, for example).

Good luck!

Bad fuel pump

Bob,
I had the same problem. I removed the Lycoming fuel pump 154733199 and replaced it with a "new" Tempest AF15473 fuel pump. No problems since I did that. You might look at Lycoming Service Bulletin #565, it did not apply to the pump I had but I'm thinking mine should have been included.

Good luck!!

So far all good suggestions. Since I am travelling on business now, I will have to wait for the weekend to do further diagnosis. To clarify the situation, this is a fuel injected setup, not carbureted. There is no gascolator in the line and all parts are new. OAT during the last test flight was 7C. I have no good reason to believe this except for a hunch (yes, I actually said that), but I don't believe this is a vapor lock issue...I think it's either cavitation occurring somewhere in the lines (perhaps caused by hose or fitting radius issues somewhere) or I think it's a problem with the engine driven pump.

Thanks for the continuing suggestions, I'll keep you updated as I try to logic my way through this.

pressure

Hmmmm... The following suggestion is probably not the culprit but I hope it educates the readers who read this on a possible cause. I was working on an Aztec with low fuel pressure..... every line was tested and spiders were swapped to no avail.... We removed all the injectors and found one of them had a cone missing... causing a blue stain on the cylinder pushrod tubes and a low fuel pressure reading. I hope it's this simple....
Thanks
Brian

Problem solved!

After reading everyone's suggestions, I too came to the conclusion that the problem was probably the engine driven pump. I ordered a new one from Chief, then...I removed the pump on my engine and took the bottom cover off, where I found 3 relatively large (1/4") pieces of helix shaped metal, two were aluminum and one was STEEL. All three were lodged under the intake valve on the pump. I carefully removed them, then examined the valves with a hand lens. Judging everything to be OK, I reassembled the pump and re-installed it. The test flight this evening yielded completely normal fuel pressures on the engine pump.
Where the metal came from, I don't know since I ran 5 gallons of fuel through each tank from a hose attached to the bulkhead fitting (the same one that feeds the intake to the engine driven pump). These pieces were way too big to have gone through the filter on the AFP boost pump assembly, they either had to be in the lines or in the pump. The steel piece really threw me...I was not expecting that.
Anyway, thanks for everyone's help!
Bob

Glad you found your problem. Doesn't it feel good to track down a complex problem and actually solve it?

I've found that it is easy to get a little sliver of aluminum inside the fuel lines when completing the flare. I have one of those expensive flaring tools and it manages to leave a little piece dangling inside the tube every time. I have to be careful to file smooth and blow out the debris every time. Maybe that's a source?