Cessna 182 Uneven Fuel Burn - Is It The Vent Or Is It....

[luvflyin]

it's been awhile since this flight so details are getting sketchy. She was PIC in the left seat. I was just getting in some flight control time in the right seat. And unlike all these crazy instructors, I find it really hard to see the AI and turn coordinator from the right seat. Plus I hadn't done a lot of cross countries myself to learn about these long periods of flight with a wing low. In other words I wouldn't have noticed if she hadn't said something. Now I'm much more aware of it. But I get to blame her since she was PIC Hell, maybe she needs a little spanking for that tomorrow night!

Ah Ha. It was a setup. Next time tell her if she wants a spankin, just ask and quit wasting gas.

 

[bnt83]

Just crush that 182 and buy a 1971 Cardinal RG. Besides getting a wider cabin, you get the on/off fuel selector from the C-150.

So when you get uneven fuel flow you can either go nuts with the rudder to try and clear the vent (and then have to clean up passenger puke), or do what the cool kids do and just live with. The fuel will sort itself out sooner or later.

I think all 182s have a "left", "both", "right", and "off" selector. Dad's 182L does.

182 can burn mogas,
no DXXXX series magnetos, not cheap to get rid of them
30 more horsepower --3 bladed props seem a bit silly on 200 horse engines to me, which is very popular on 177RG fleet. Some folks have sunk mega bucks in the IO390 which could have bought a bigger plane.
182 has a huge STC market for them. Almost always going be one of the first models to get new autopilot products and such.

If I was going to be stuck buying avgas and wanted a retract I'd go 182RG or most likely a 64-66ish 210.

 

[OkieFlyer]

I can tell you this, if you're tanks aren't brim full, the tanks will cross feed pretty quickly on uneven ground. I rarely fill my 84 gallon tanks to the top, so there is always room for cross feed. It only takes a couple minutes sitting on uneven ground for quite a bit of fuel to transfer to the low side. I'm not saying this is your problem, but it happens. I always keep the selector on left or right when I'm on the ground to avoid any cross feed before taking off. I switch to both just before takeoff and leave it there until I'm back on the ground. My tanks stay pretty even now, which makes me think that my previous uneven tank problem happened before I ever left the ground.

 

[JimNtexas]

I think all 182s have a "left", "both", "right", and "off" selector. Dad's 182L does.

182 can burn mogas,
no DXXXX series magnetos, not cheap to get rid of them
...

I've never seen a 182 with an on/off fuel selector either. All 177RG's after 1971 have on/off/both/l/r also. The 1971RG has two real mags, starting sometime later they switched to the notorious 'D' (for dangerous) fake dual mags.

I agree, three bladed props on a 200hp engine is silly at best. A three bladed prop on a Cardinal RG or an Arrow shows that the airplane was landed gearup at some point, because for some reason three blades are often cheaper than two.

The Cardinal does have the STC for the IO-390, which is pretty cool.

 

[denverpilot]

182 can burn mogas,

Until the O-470U in the non-restart era aircraft. Then no. And no restarts after the 182Q with that engine have the STC available either.

Just a point of order. Not all can.

 

[neilw2]

After this weekend, now I have a question. As I said in my previous post I have always had very uneven burn in my 182Q.

I flew the other day and set the fuel to pull from the right tank as that is the one it seems to never pull from. I flew like that for a uneventful 45 minutes before, all of the sudden, I got a rpm decrease- which was very unpleasant as it woke me up from my traditional cruise flight nap .

Anyways I switched the fuel back to both and immediately got back by lost rpms. The rest of the flight was fine with not as much as a hiccup. On the way back I pulled from the right again for another 45 mins without any issues. After landing I looked at the tanks and was pretty even, both about 15 gals lower than the full they were are the start of the day.

So I ask the group- what happened and would you be concerned at all?

 

[jsstevens]

After this weekend, now I have a question. As I said in my previous post I have always had very uneven burn in my 182Q.

I flew the other day and set the fuel to pull from the right tank as that is the one it seems to never pull from. I flew like that for a uneventful 45 minutes before, all of the sudden, I got a rpm decrease- which was very unpleasant as it woke me up from my traditional cruise flight nap .

Anyways I switched the fuel back to both and immediately got back by lost rpms. The rest of the flight was fine with not as much as a hiccup. On the way back I pulled from the right again for another 45 mins without any issues. After landing I looked at the tanks and was pretty even, both about 15 gals lower than the full they were are the start of the day.

So I ask the group- what happened and would you be concerned at all?

The loss of RPM after that time would make me wonder if the right tank doesn't vent.

 

[neilw2]

The loss of RPM after that time would make me wonder if the right tank doesn't vent.

That's what I was thinking. However, wouldn't that always cause problems instead of just occasionally?

 

[jsstevens]

That's what I was thinking. However, wouldn't that always cause problems instead of just occasionally?

I am not an A&P. It seems like a fully clogged vent on the right might cause what you're seeing. Slow consumption on both, eventual starvation on the right only. But once the fule level is down some, the both setting allows some venting. I'm guessing 45 minutes a cruise is ~9 gallons? Out of a 27.5 gallon bladder that was still at take off elevation pressure (if completely clogged). So about 1/3 capacity before it starved the engine. I'm making a guess based on what I know of physics and engines. Did I mention I'm not an A&P?

John

 

[Sinistar]

Until the O-470U in the non-restart era aircraft. Then no. And no restarts after the 182Q with that engine have the STC available either.

Just a point of order. Not all can.

We bought a 182P from a owner who had used MOGAS and then stopped. He said his bladders started leaking after the switch away from 100LL - could have just been coincidence as they ultimately leak anyway. The previous owner's mechanic at the time of the sale swore off car gas too. He said along with those issues, the cabin will just plain stink...and we think it still smells of car gas. And the leaks were ugly, I am still cleaning off ugly caked chunks of brown car gas which probably happened after the bladders leaked or tanks were overfilled or last drips from the sumps flying off and sticking to the rear side of the door, etc.

The price of car gas is appealing. That is about it though. All mechanics we spoke to say don't. And its ugly when it leaks. There are some stains on the belly that are brownish, probably also from car gas. Even with gojo I can't get them out - sucks.

 

[denverpilot]

After this weekend, now I have a question. As I said in my previous post I have always had very uneven burn in my 182Q.

I flew the other day and set the fuel to pull from the right tank as that is the one it seems to never pull from. I flew like that for a uneventful 45 minutes before, all of the sudden, I got a rpm decrease- which was very unpleasant as it woke me up from my traditional cruise flight nap .

Anyways I switched the fuel back to both and immediately got back by lost rpms. The rest of the flight was fine with not as much as a hiccup. On the way back I pulled from the right again for another 45 mins without any issues. After landing I looked at the tanks and was pretty even, both about 15 gals lower than the full they were are the start of the day.

So I ask the group- what happened and would you be concerned at all?

First, I assume the Q is the same as our P. No vent on the right tank. Vent is on the left. Crossover tube in the top of the left and right tanks.

When that eight plugs up or is full of fuel when the tanks are full, and the right tank is being “suctioned” with no way to get air, the fuel cap has a rubber air valve in it that is supposed to pull down toward the tank and open up.

Under normal pressure (ram air into the vent on the left side that crosses over to the right via the upper vent line) it is lightly pressed upward into a closed condition.

I bet you need a right side new gas cap with new soft/pliable relief vent rubber. (Probably both are hard.) Or someone put a non-vented cap on the right side. Both sides should have the emergency vent. It pulls open if your ram air vent is clogged with anything, including ice/snow. (Which is why the diagram for proper placement of the ram air vent shows it slightly behind the strut but not completely, in the service manual.)

Usually by the time it’s that bad, the rubber cap seal on that cap is also cracking and worn too. Take a look.

You could also have a more dangerous condition, there could be a problem in your fuel selector. More rare, but definitely if you replace the right side cap and can still starve the engine for fuel, have the system checked.

By the way, if you look at the System diagram, you can’t truly isolate the left tank when they’re full. That’s why these systems are always uneven. Even in “Right” mode, when the tanks are full, the left tank is pressurized by the vent and fuel will push across the upper vent line into the right tank until the level falls below the level of the vent line. Then air pressure will equalize across both once it drains, but the left tank will fall for a while with the right tank.

If you always fly with the tanks fairly full, switching to Right isn’t doing anything useful other than adding wear to the fuel selector. Ha.

But check that emergency vent in the gas cap.

 

[neilw2]

First, I assume the Q is the same as our P. No vent on the right tank. Vent is on the left. Crossover tube in the top of the left and right tanks.

When that eight plugs up or is full of fuel when the tanks are full, and the right tank is being “suctioned” with no way to get air, the fuel cap has a rubber air valve in it that is supposed to pull down toward the tank and open up.

Under normal pressure (ram air into the vent on the left side that crosses over to the right via the upper vent line) it is lightly pressed upward into a closed condition.

I bet you need a right side new gas cap with new soft/pliable relief vent rubber. (Probably both are hard.) Or someone put a non-vented cap on the right side. Both sides should have the emergency vent. It pulls open if your ram air vent is clogged with anything, including ice/snow. (Which is why the diagram for proper placement of the ram air vent shows it slightly behind the strut but not completely, in the service manual.)

Usually by the time it’s that bad, the rubber cap seal on that cap is also cracking and worn too. Take a look.

You could also have a more dangerous condition, there could be a problem in your fuel selector. More rare, but definitely if you replace the right side cap and can still starve the engine for fuel, have the system checked.

By the way, if you look at the System diagram, you can’t truly isolate the left tank when they’re full. That’s why these systems are always uneven. Even in “Right” mode, when the tanks are full, the left tank is pressurized by the vent and fuel will push across the upper vent line into the right tank until the level falls below the level of the vent line. Then air pressure will equalize across both once it drains, but the left tank will fall for a while with the right tank.

If you always fly with the tanks fairly full, switching to Right isn’t doing anything useful other than adding wear to the fuel selector. Ha.

But check that emergency vent in the gas cap.

Wow, really helpful, thanks!

I'm going to replace the gas cap and report back.

 

[Sinistar]

I thought I heard someplace that these caps are quite expensive? Or maybe it was the entire conversion kit that was expensive? Hell, if the caps are cheap why not just replace them once every 3-5yrs to be safe.

And if they are expensive or you just want to know, is there a easy way to test this relief valve? To bad there's not a small tank with the same opening where you could screw the cap on and try pull a vacuum.

 

[denverpilot]

I thought I heard someplace that these caps are quite expensive? Or maybe it was the entire conversion kit that was expensive? Hell, if the caps are cheap why not just replace them once every 3-5yrs to be safe.

And if they are expensive or you just want to know, is there a easy way to test this relief valve? To bad there's not a small tank with the same opening where you could screw the cap on and try pull a vacuum.

I forget what they cost, but they’re not $5 car gas caps. LOL.

The rubber doesn’t go bad THAT fast. But the vent does need to work.

 

[Sinistar]

I hope I am wrong but I thought this guy down the row from me with a 206 and 182 said the new caps were like > $100. Maybe that is for the entire new kit?

 

[neilw2]

Hey @denverpilot - quick question. The 1979 182Q has a vent on both sides- not just the left as the P. Do you still think in that case the gas caps are the likely issue?

 

[denverpilot]

Hey @denverpilot - quick question. The 1979 182Q has a vent on both sides- not just the left as the P. Do you still think in that case the gas caps are the likely issue?

Doubtful. But the system is so screwy for venting and cross over vents anyway it’s always hard to find what’s making a slight pressure difference.

I just treat ours like one big tank for the most part and leave it on Both. If it gets way out of whack I’ll fill that side more at next fill up and half of it will migrate to the other tank before takeoff anyway. LOL.

 

[denverpilot]

You reminded me to ask my co-owner to put “Check gas cap seals” on the list for the mechanic at annual next week though @neilw haha. I think one of ours has a tiny little crack forming.