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Might be a little bit of a tangent, but frequent 400 hr life on Walbro pumps sounds off-nominal. They're used in a lot of modded cars, and a quick interweb search for 'lifespan' showed guys talking about 8-10 years, in cars (probably 100k+ miles). 400 hrs in a car would be, at most, ~24,000 miles, and I suspect the 'tuners' would give up on the pump pretty fast if they were only getting 20-30K miles of life from them.
Charlie
Finn, Where is your fuel filter located? Mine in downstream of the pump, but before the fuel injectors. Fuel pressure gauge is just upstream of the filter for me, which I now realize is sub optimal but does provide good info about the state of the fuel filter. The pressure starts going up as the filter clogs.
I also have a fuel flow sensor on my return fuel line, which has turned out to be a great troubleshooting tool and reflects the overall health of the fuel system.
I am using the walbro pumps that Tracy recommended. I bought 10 of them once to have a supply on hand because I never again want to delay replacing them when I see the performance start to degrade. For me, the first sign would be decreasing return flow. It more or less seems to linearly decrease with the total time on the pump. For me, the pressure starts dropping as the return flow gets closer to zero. I have taken to replacing the pump earlier in the process, like 50% reduction in return flow. With pumps like the walbro, current demand to the pumps goes up as the flow goes down. At some point the current demand will blow the breaker and terminate the flight. I would guess that fuel pumps seem to last 400 hrs or so. This is an improvement since I installed a larger fuel filter to keep down the pressure differential.
If your pumps are similar and your fuel pressure is dropping at the injector, and the problem is not a clogging fuel filter, then I suggest you replace the pump ASAP. They seem fairly susceptible to damage from particulate contamination or being run while dry.
Cheers, Dave Leonard
Finn,
What the fuel pump should be flowing and what it is flowing may be two different things and needs to be evaluated before being "comfortable". That is why I would encourage you to measure the actual flow rate if you can do it safely. When I left MS71 in May
of 2017 following Charlie's rotary fly-in, my 13B burned 18 gal/hr on take-off and initial climb at 5800 RPM and initial OAT of 80 degrees F. Your set-up likely would require
this much or more. This was the indication from a Flowscan turbine type sensor which has been demonstrated to be accurate. The flow measurement would just be a next step in the investigation. If the flow rate is not what is what it should be, then determining
why would follow. The replacement pump should be helpful in this regard.
If you recover the fuel being bypassed by the regulator, bubbles in that fuel will not necessarily indicate air leaking into the fuel drawn into the pump. The process of dropping the pressure when the returned fuel passes through the regulator, causes dissolved
air in the fuel to separate. Bubbles in this flow will be obvious. If fuel is drawn from a small reservoir (< a gallon or so) and returned to that same reservoir where the bubbles are allowed to escape to the surface, the bubbles will eventually disappear
as the fuel is degassed. Degassing all the fuel from the wing tank is unlikely to occur due to the large volume of fuel and the large surface area of the fuel in the tank. This is why returning bypassed fuel directly to the fuel pump intake or to a small
unvented header tank (where the separated air is allowed to accumulate) may not be the best practice.
FWIW
Steve Boese
Finn,
It seems to me that the problem you’re seeing is consistent with the capacity of the pump not being sufficient for the max flow required by the engine and the pump flow capacity is decreasing with use. You appear to have checked everything but
the flow capacity of the pump under normal system pressure which is what you really need to know. If you can divert the returning fuel flow from the pressure regulator to a measuring vessel and and collect that fuel for an accurately measured time, the pump
flow rate can be calculated. This flow rate should be greater than the maximum required by the engine by a comfortable amount.
My apologies in advance if this is obvious.
Steve Boese
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Hi Finn
Thanks for the reminder of my pump issue.
Yes, you chose more wisely the pump capacity. Mine were way overkill.
So I’m thinking options are:
1. Fuel Pressure transducer fault - No - as this was confirmed by the mixture monitor and tune going out.
2. Power supply to pump via fault in termination or switch current path - Unlikely, given the slow reduction - I would imagine a faulty connection would create a less uniform symptom.
3. Alternator Battery voltage/current reduction over time - No - You would see this in the log.
4. Significant Fuel leak - No - Would think this would have become obvious on inspection and not intermittent.
5. Fuel Regulator - They are pretty simple but unsure if they can fail with this symptom?
6. Fuel filter blockage - Certainly this would have to be an obvious, but I take it you swapped this out.
7. Pump internal failure -
Electrical - ?
Mechanical breakage - ?
Flow Path - blockage - unlikely if filtered effectively
Where is your pressure transducer in relation to Reg/Any post pump filtering/Flow Transducer/Rail?
If fuel regulator and filters can be discounted, I’m struggling to trust the pump. But your test had it sounding in good shape and to spec.
So frustrating.
Cheers
Steve
Thanks Steve.
>From your March/April 2017 posts it was a GSL392 that failed (high current draw/low pressure, even after you removed the insect you found in the inlet).
So this afternoon I ran the pump for about an hour. Amp draw from 3.04 to 286 depending on voltage. Pressure between 43 and 40 psi (also depending on voltage). No noticeable degradation. Of course the difference with this test is that no fuel went to engine
-- all recirculated through pressure regulator and back to tank. But no signs of a failing pump in terms of higher current draw and less pressure.
I guess I should put a temp probe on the pump and see what it reads during flight to see if that could be a factor.
Finn
On 5/21/2022 9:01 PM, Stephen Izett
stephen.izett@gmail.com wrote:
Hi Finn
I had from memory a 393 fail. Can’t recall the symptoms sorry.
I test before takeoff and then run both pumps below 2ooo ft incase of failure.
I plan to instal a comparator relay board in fuel pressure circuit to automatically bring on backup pump and failure indicator.
So testing before flight becomes - switch off and back on main engine pump. This will save amps and wear of backup pump.
When I switch off a pump at WOT the engine dies pretty much instantaneously and don't want to be fiddling with a restart in the Glasair which glides like a brick with the gear out in the breeze.
Regards
Steve Izett
Recently I noticed my fuel pressure being low. Noticed because engine did not come up in power when advancing to full throttle. Noticed mixture real lean. Checking fuel pressure as low as 28 psi (normally 35 and up to 40 at full throttle).
Going over my engine logs since first flight I now see that fuel pressure has been trending lower through each flight -- more pronounced on longer flights (40+ minutes). More and more pronounced over the months and became really noticeable over
last month. Could explain why mixture tuning appeared to have changed. Nice to have engine logs from practically each flight.
But why? Pump getting weaker?
My primary fuel pump is a GSL414. I figured it was wasted energy to push way more fuel than needed through the pressure regulator back to the tank.
My secondary (backup) pump is a GSL393. When turning that on, fuel pressure comes back up to where it needs to be.
Anyone have experience with failing or weak EFI pumps, particularly the GSL414?
Finn
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