Guys, I'm having a tough time with a weird and intermittent issue. I can't be sure, but it seems to only happen right after fueling with full tanks. I've never experienced it with partial fills and it seems to bounce back after the fuel levels have dropped.

With full fuel, usually within the first 15-20 minutes of flight, my EGT's will climb, the engine will start to run rough, and just generally act as if it's starving of fuel. It lasts for a couple of nerve racking minutes and then clears up. Fuel burn is lopsided, with only the left tank indication fuel consumption and then after an hour or so of flight the fuel levels even out.

My IA has gone through the vents, and properly positioned the single underwing vent, but it's still doing it. He did find that the vent tube over the head liner was sagging, he fixed that, and it helped with really lopsided fuel burn that was going on all the time, but the starvation still happens and does the lopsided burn early in a flight.

I have 30 gallon fiberglass fuel tanks. The only thing I can think of is maybe somehow I'm overfilling? Everyone I ask gives me a different opinion of what "full" is. There is a slash cut tube on the fuel neck, I fill to the bottom of the tube.

Any ideas?

A number of years ago I recovered a C180 that landed in a paddock when the engine quit. The owner said the motor stopped, he landed, it started straight up, off he went again just to have it stop again, and did this several times. For some reason he never changed the tank selector position

Anyway, on further investigation, in one of the tanks, I found several rags, a complete roll of tank tape that had unraveled when the fuel dissolved the adhesive and two lengths of timber. The tape was restricting the tank outlet, but then allowing it to start again. Was an interesting find!

I would suggest investigating the check valve in the vent system, if it has one

The vent location placement was to balance between ice formation and avoiding suction.

I dropped the vent by a tad and the uneven feed literally went away. Being in the tight zone that Cessna dictated created problems. The extra pressure head the new position creates really shouldn't be an issue. The max pressure at 200 mph is only about .6 psi, or about 1200 feet of altitude, minus what was there in the first place.

The check valve is the other obvious place, and not too bad by the 3rd or 4th time you have to do this with bladders. It sucks the first and second times for sure.

-0-

dogpilot wrote: ...If you fill them to the brim and fuel comes out the vent, then your check valve is no good. They look like this....

Note the arrow on the check valve!!!

Check valves have a small bleeder hole in them. If you blow in them in the stop direction, there is still leakage (on purpose), and a plane parked on an incline with full tanks will generally usually still drop because of this unless a person puts the gooseneck in the vent line.

I thought I had it fixed. I flew two three hour legs on Friday and had zero trouble. The last leg on saturday I had it happen again right at 3/4 fuel indicated in both tanks. Was cruising at 11,500 MSL in smooth air. EGT's started to climb, I noticed right away because my EGT gauge is on my regular instrument scan because of all these problems, rocked the wings and boom the EGT's dropped back down. By rocking the wings I either move something away from a choke point or cleared fuel from the vent line, I can't think of anything else?

I thought I had it fixed because I found the flapper on the right tank cap was crusted shut. There are multiple issues though, because once the crusty flapper was fixed my fuel burn was now perfectly even.... but the weird leaning issue still exists.

There is a known defect with my fuel selector valve. It leaks by when "off," I was waiting for the mcfarlane valves to be approved, now that they are I'm going to buy one. I'm not sure if this defect could contribute to the issue or not.

I don't believe the check valve has been inspected, I'll ask my IA. I'm at the point where I'm going to just ask my mechanic to replace all the vents and hoses when we do the fuel valve and basically shotgun the issue. When I took off on the first leg I filled fuel to within 3/4" of the brim and no fuel came out the vent. In fact, on the first two legs when I had no issue that tanks were more full then the usually are. On the third leg when I had the issue, I had filled only to the bottom of that filler neck tube.

CParker wrote:….. By rocking the wings I either move something away from a choke point or cleared fuel from the vent line, I can't think of anything else?.....

I'd stick a borescope down into that tank & look for flotsam.

Yeah I'll pull it all apart when I get back and check it out. But if one pickup tube was being blocked, would I not then just get fuel from the other tank?

-0-

Just throwing this out there because it’s hard to really experience everything you are going through. For instance, to what degree is the fuel imbalance really a consistent problem or not? I think my fuel system is working normally and I tend to have what someone might perceive as uneven tank draw when the imbalance is simply due to the cross-feed occurring if I’m not flying with the ball exactly in the center over time. Could this be a distractor?

Rising EGTs and a rough engine can also be an ignition problem. Could you potentially have an intermittent problem somewhere in the ignition system that takes a mag offline, and thus raising all of your egts? I realize that it doesn’t explain why it only happens at some fuel levels and not at others and that it seems to fix itself by wobbling the wings, but could this be one of those ‘true - true - unrelated’ situations? I recently had an ignition switch go bad on me that showed up as an intermittent mag problem.

An inflight mag check during the problem could help to rule this out. Or a fancy monitor or tach can tell you that as well. But if you decide to mag check it and one of your mags is out and the engine quits, be sure to check on the proper restart procedure. I always plan to restart the engine, not by flipping the mags back to both, but by moving the mixture to full lean first followed by the mags to both and then bringing the mixture in. This is for sure important in an injected engine with an engine driven fuel pump and I would think might apply in a carbureted engine as well. Don’t I love sitting in bed dropping a penny’s worth of info into cyberspace?

Good luck

Guys, just wanted to check back in and let you know how it's going.

I haven't had a lick of trouble since replacing both fuel caps. Whatever is going on is at least now being masked by the secondary venting from the caps.

I also found that the small aluminum vent tube that crosses the top of the tank was bowed down in the right tank, I'm wondering if this was essentially creating a liquid trap that was preventing air from venting properly. As far as I can tell, the vent air from the single vent on the left wing has to vent through that tube to the right tank. If that's an accurate assessment, I think that was the problem all along.

Either way, equal fuel burn and no issues for a few months now.

I have been seeing things like this in other aircraft and in another aircraft community and rarely in Supercubs. In your first post you say a fuel imbalance is common when this happens. You also say that fixing your fuel cap vents fixed the problem.

I have a suspicion.....But my suspicion is predicated that you had BOTH tanks selected rather than running on the L or R tank.

If a fuel imbalance develops with BOTH selected after starting out with full tanks, then evidence tells us that a pressure differential between the L and R tank exists. In your case, you found a suspicious cap vent that may have been the cause. The tank with more fuel must have a lower pressure....lets call it a suction, while the low tank is allowing fuel to flow and lets say has atmospheric pressure in the tank.

Now, if something happens to where the suction is relieved for some reason in the tank with more fuel, then fuel has the ability to flow from the high tank to the low tank thru the fuel selector. And I think that this flow across the TEE from the high tank to the low tank might be enough to disrupt the flow out the fuel selector to adequately feed the engine.

I stress that this is my theory, and my take away is that when running in BOTH, if a noticable imbalance develops, then do not run in BOTH. I think I would select the high tank with suspicion if I was at a high altitude in cruise flight. I think watching the EGT like you did is a great idea because a higher EGT will give us an indicator of a leaner mixture. Maybe the suction gets so bad, the engine dies due to fuel stoppage. Does the mixture change with fuel tank selection? That seems like a good way to help trouble shoot the issue.

Thank you for posting the conclusion to your issue. Which vented fuel caps did you go with?

Based on a recent Cessna 140 article, it is my understanding that some fully vented caps can create a vacuum in the tank if the cap is located within a low pressure area on top of the wing. The solution in this article was to use a ram-facing vent tube attached to the caps.

https://mcusercontent.com/faf0d688533d7 ... letter.pdf

This is on a 206, correct? If so, there should be no "both" position on the fuel selector. Or is this a 205? Not familiar with the fuel selector there.

Fuel imbalance in flight is pretty normal in virtually every Cessna I've flown. Some worse than others.

I totally agree that positioning the under wing fuel vent tube precisely as Cessna suggests doesn't work. I've had to fiddle with positioning that to more into the air flow to get it to work right. They're likely all a bit different.

I'd be VERY suspicious of the vent check valve, frankly.

Also, I picked up a brand new C-185 once. EGTs would increase, check things out, gascolator full of a white fibrous material....fly it, repeat. Got a mirror and light into the fuel tanks, couldn't see anything but gas. This went on for about 200 hours.....gascolator full of white stuff every 50 hours or less.

Then it stopped. Never happened again. Years later, I spoke with a gent who built those airplanes in Wichita. I told him that story, and he smiled. He said someone on the line had left a cotton rag in the fuel tank. This airplane had wet wings, and Cessna used a black gooo to seal them inside. That goo was spread throughout the tank with cotton rags, which were SUPPOSED to be counted in and out. He said those rags became soaked in the black stuff, and you'd probably never see one spread out in the bottom of a tank. He figured someone left a rag in there and signed off on being one rag short.

Point is, there COULD be something in your tanks that is fouling the outlets......frankly, I suspect your issue is far more likely to be vent related, however, and that crossover tube is certainly suspect.

Good luck, and good for you for paying attention to the engine indications......good piloting!

MTV

Thanks guys,

To be clear - this post was recently resurrected from a few years ago. This was on a 182E

VikingBorn wrote:Thank you for posting the conclusion to your issue. Which vented fuel caps did you go with?

Based on a recent Cessna 140 article, it is my understanding that some fully vented caps can create a vacuum in the tank if the cap is located within a low pressure area on top of the wing. The solution in this article was to use a ram-facing vent tube attached to the caps.

https://mcusercontent.com/faf0d688533d7 ... letter.pdf

I had word for word the authors experience last summer in my 182. never was able to find a definitive answer but my gut always told me it was a vacuum issue. always noticed strange pressure differentials in the tanks. cessna's venting caps seem to be very anti-venting. great article, thank you for sharing

Resurrecting an old thread.

I don't know that the question brought up late in the discussion was answered regarding operating on BOTH or if the aircraft in question only had L and R positions, but, the early Cessna 172 has an AD against it regarding fuel system venting that requires a POH addition and a placard on the fuel selector to not operate on BOTH in cruise above 5,000 ft as they had the engines quitting in certain conditions. If you forgot to switch to L or R above 5,000 ft in cruise and the engine experiences a power interruption the procedure was to switch from BOTH to R or L, wait 30 seconds, then switch to the opposite side and the engine is supposed to then start getting fuel again.

It's AD number 72-07-02 covering certain serial numbers of the 172 through 172K.

I'm not aware of anything similar on other Cessna models, but the symptoms sound somewhat similar.