C182 Fuel Flow Decrease on L or R?

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keatre

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keatre
My C182G is having an "odd" issue that can't be recreated every flight. On x/c flights I do the old "right tank, then both, then right tank" to manage the uneven C182 burn that the plane is known for. However on 1 out of 5 flights, it appears that the fuel flow will gradually decrease while the selector is on R. I have a JPI 450 and in level flight, cruising on both, the flow will be ~15 GPH. But going to R it will stay ~15 until it will slowing start dropping... At 12 GPH, the engine will start getting cranky. Going back to BOTH, the fuel flow will spike, you can hear the engine recover, then settle back down to ~15. In testing, I also tried flipping to L tank and encountered the same issue; fuel flow slowing started decreasing. Back to both, and it stabilizes - no issue.

Any idea what could be the cause? Perhaps the o-rings are going bad, and causing a block on each selector? I've read a good test would be to idle the engine at 1,000 RPM and set the fuel selector to OFF and see what happens.

Tried doing a search, but it seems like most everything for C182 is around uneven fuel burn.

Thanks for any input.
 
No idea re: fuel flow, but you can have the vents adjusted to even out the left and right tanks
 
I was going to say that the crossover vent may be blocked, but if it does it on the L tank also, maybe the lone tank vent has a full or partial blockage. When on both, perhaps both the fuel cap vents are compensating enough for the blockage to keep it barely going. <EDIT: I didn’t make any sense...there’d have to be a crossover vent pluggage also for that to happen.>

I am not an airplane mechanic. I’d say it may be something very simple but with serious repercussions. Time for an A&P. Also, join one of the Cessna forums. I’m on Cessna Pilots Society and there’s some experts there for sure.
 
455 Bravo Uniform said:
I was going to say that the crossover vent may be blocked, but if it does it on the L tank also, maybe the lone tank vent has a full or partial blockage. When on both, perhaps both the fuel cap vents are compensating enough for the blockage to keep it barely going. <EDIT: I didn’t make any sense...there’d have to be a crossover vent pluggage also for that to happen.>

I am not an airplane mechanic. I’d say it may be something very simple but with serious repercussions. Time for an A&P. Also, join one of the Cessna forums. I’m on Cessna Pilots Society and there’s some experts there for sure.
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Replacing both caps may be a good place to start. Cheap and easy
 
I do have monarch caps that don't leak, as far as I can tell, with relatively new seals. I imagine it has something to do with the pass-through vent or the o-rings on those ball bearings, but agreed - need to have an A&P check it out. Was just trying to come up with some ideas before passing it over.
 
Try flying for a while on one tank then land and open it and listen for a whoosh of air, then do the same for the other side.
 
ktup-flyer said:
No idea re: fuel flow, but you can have the vents adjusted to even out the left and right tanks
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No you can't. There's is only one underwing vent, and it feeds the left tank. There is a crossover vent tube that runs from the top front of the left tank over to the right tank, keeping the pressures the same, as long as the right cap's vent check or cap gasket isn't leaking. If that cap has any leak, the airflow out of it (due to the low pressure atop the wing) causes uneven fuel flow due to the drag inside the vent crossover line, and the right tank gets less pressure so it flows more slowly. And leaky vent check valve and gaskets are way too common. This stuff does get old, folks. If those caps are the old flush caps, I can almost guarantee that the sealing O-rings are shot. They should be replaced yearly. Bad seals or gaskets or vent check valves also let rainwater and snowmelt into the tanks.

I would look first at that cap gasket. The vent arrangement in the cap (which is there to provide venting if the underwing vent gets blocked) mostly provides ambient air by design, similar to a static port, so that fuel head pressure will still feed the engine. Then I would check the entire run of fuel line from the tank down to the selector, looking for dents or kinks. Cessna wants those lines inspected every 100 hours; most of these are getting old, and chafed and corroded lines are common enough. Yet I've opened up interiors that haven't been opened since the airplane was built, and see some stuff that should have been addressed decades ago.

The underwing vent line typically gets plugged up by mud wasps with their nests. Vent system inspections are another 100-hour item.

There might be some crud in the right tank, partially blocking the finger strainer.

Then the selector. There's a drain plug in the bottom of it that Cessna wants out every 100 hours, too, and they never get checked. Debris and sludge can pile up in the selector and back up into the lines.

Maintenance. It's all about regular, thorough maintenance.
 
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