Scudrunners

Hi,
Today I noticed, after climb out that my fuel pressure was indicating 0. The engine was running fine. I initally suspected a faulty fuel pressure gauge, but pumping the manual pump, showed an expected increase in fuel pressure. So the gauge seemed to be operating normally.
Tank was more than half full.
Outside air temperature between 0 and -5 celcius. First flight of the season in freezing conditions. So ice somewhere would be a normal suspect. No vents seemed to be blocked, fuel drained normally. Loads of +4G, -1G didn't make any difference. Below 2400 RPM the fuel pressure was building up again, in the green, and then bled of again slowly when I increased throttle to full. Full throttle + manual pump does bring the fuel pressure back in the green, and then it bleeds of again.
Even with 0 PSI fuel pressure, the engine was running normally.

No visible fuel leaks in the cowling either.

Plane is a pitts with O360 with Ellison throttle body.
Does anybody have any ideas? Since it only happens at full power, it is very hard to reproduce on the ground, as I can't do a static run up at that power...

Thank you,digits

What type of fuel gauge is it? analog/digital?


If its one of those dual analog fuel pressure , manifold pressure gauges they sometimes have little 'snubbers' right in the inlet ports of the gauge. The snubbers are designed to eliminate the pulses from the pumps and smooth out the needle movements. They are usually little set screws that open and close against a small brass seat to restrict the amount of fuel or air that can enter. They can get gummed up, or can move. Certainly thats something that happens with my aircraft occasionally but usually on the manifold pressure side. Of course that does not explain why the wobble was able to raise the pressure.


Does the fuel pressure climb when you wobble to to prime? That's the normal starting procedure at least with mine. Throttle open, mixture rich, wobble until you see the pressure climbing etc.


Anyway I'd start with the gauge because the engine won't run long without proper pressure.

It is a small analog fuel pressure only instrument.

I have a separate primer, but if I wobble before start, the fuel pressure rises as well and then slowly bleeds off.

Looks like the problem is fixed after I cleaned the fuel filter, which didn't exist according to my mechanic  ::) Who knows how many years of gunk was in there...

I have often wondered about the real value of a fuel pressure gauge,
because the engine will tell you immediately if you don't have enough.

Whenever I have lost fuel pressure before, the engine gets remarkably quiet.

Like single-engine, over the Gulf of Mexico, when you run the front seat
tank dry.

digits wrote: ↑

Fri Aug 21, 2020 3:04 pm

Looks like the problem is fixed after I cleaned the fuel filter, which didn't exist according to my mechanic  ::) Who knows how many years of gunk was in there...

You have a Christen wobble pump? If so a good idea to get it overhauled by the OEM guy while he is still doing them and stock up on the little filters as I am not sure they are easy to get anymore. I bought 20 or so a few years ago as they were getting hard to find.

On the pressure problem... there is usually a screen in the throttle body and while I don't know about the Ellison the Bendix has a spring on the end of the screen which allows it to enter a bypass mode if too much pressure builds up if its gummed up. I guess that's a last ditch safety feature to keep the engine running even if it will fill with junk. Bottom line is both the wobble pump filter and the screen in the throttle body need to be cleaned.

I'd also look at the venting carefully.

I don't like weird behaviour .. its a signal something is not quite right and needs to be investigated.

cgzro wrote: ↑

Sat Aug 22, 2020 1:34 am

digits wrote: ↑

Fri Aug 21, 2020 3:04 pm

Looks like the problem is fixed after I cleaned the fuel filter, which didn't exist according to my mechanic  ::) Who knows how many years of gunk was in there...

You have a Christen wobble pump? If so a good idea to get it overhauled by the OEM guy while he is still doing them and stock up on the little filters as I am not sure they are easy to get anymore. I bought 20 or so a few years ago as they were getting hard to find.

On the pressure problem... there is usually a screen in the throttle body and while I don't know about the Ellison the Bendix has a spring on the end of the screen which allows it to enter a bypass mode if too much pressure builds up if its gummed up. I guess that's a last ditch safety feature to keep the engine running even if it will fill with junk. Bottom line is both the wobble pump filter and the screen in the throttle body need to be cleaned.

I'd also look at the venting carefully.

I don't like weird behaviour .. its a signal something is not quite right and needs to be investigated.

It's been a while and I have had no problems since, but jeesh, that's a lot of filters in the system!
Yes, I do have that wobble pump.

The fuel filter just above the fuel strainer was the culprit. It looked like it hadn't been changed in a while. We checked the other filters we could find (a big inline one in the engine compartment), and that one looked brand new. I'll look into the throttle body as well.

Thanks!

I've had the "finger filter" in the throttle body plug up, too. I think it's
meant as a last line of defense before the spider and the injectors.

Easy to pull and clean.