Cessna Fuel Vent

[Merf]

I own a 1876 C172M-Airplains 180 conversion. I have never had so much as a hiccup with this engine. I flew to Llano from my home base 45r (about 2 hours each way) on Sat. On the return flight, about 1.5 hours into it the engine hiccuped. There was a definite very momentary loss of engine power. I was a 5500 ft, and was leaned out -- It was too short an episode to see any RPM loss, but the engine definitely missed for a second or two. It immediately caught back and I saw no more problems. There was some turbelence at the time. When I got back home, I went to check fuel, and I believe I "felt" a vacuum.

This morning I ran the engine for about 15 minutes and ran on each tank with no problems. I gently ran safety wire up thru the fuel vent -- I am not sure there was any retriction there or not. There was no vacuum when I shut down.

How long would the engine have to run to generate a vacuum if the fuel vent is plugged? It was suggested to blow low pressure air thru the vent to make sure it is not blocked. Is this a good idea? Thanks for your thoughts.

 

[Let'sgoflying!]

Merf. From East Tx? Haven't heard from you for years. Over in the red board. I guess I missed your posts here.
Anyway, IF the "hiccup" is very infrequent, and very brief, my suggestion is that it is usually the electrical system and not a valve or fuel, including water in the fuel.
Dirty plug, wire insulation breaking down, something in the mags.
Tell us the ages of each and we'll go from there.

 

[webdaddy]

I have seen fuel leaving the vent after I fueled the plane before and also have experienced vacuum in the tanks when I take off the cap. Anybody know what the deal is here because it's not supposed to do that. It was a very pronounced vacuum and made a sound like a milk carton with vacuum in it when I removed the gas caps. I thought the purpose of the overflow was to let fuel/air in and out of the tanks as needed but I have seen vacuum in my left tank on more than once occasion.

 

[Let'sgoflying!]

Also Merf if you decide to pressurize the fuel system. Keep any pressure applied very low, it is not designed for pressure. I think I read <2psi somewhere, but when I did it (forget the reason), I kept it to <0.5psi and that was adequate for what you need. i have seen a crumpled tank that lost its vent...not pretty. Amazing what suction powers an engine has.

 

[Dan Thomas]

I own a 1876 C172M-Airplains 180 conversion. I have never had so much as a hiccup with this engine. I flew to Llano from my home base 45r (about 2 hours each way) on Sat. On the return flight, about 1.5 hours into it the engine hiccuped. There was a definite very momentary loss of engine power. I was a 5500 ft, and was leaned out -- It was too short an episode to see any RPM loss, but the engine definitely missed for a second or two. It immediately caught back and I saw no more problems. There was some turbelence at the time. When I got back home, I went to check fuel, and I believe I "felt" a vacuum.

This morning I ran the engine for about 15 minutes and ran on each tank with no problems. I gently ran safety wire up thru the fuel vent -- I am not sure there was any retriction there or not. There was no vacuum when I shut down.

How long would the engine have to run to generate a vacuum if the fuel vent is plugged? It was suggested to blow low pressure air thru the vent to make sure it is not blocked. Is this a good idea? Thanks for your thoughts.

An 1876 model? Are you sure you can trust this thing?:wink2:

It's not likely a fuel problem. If your tank couldn't get air, the engine would quit and stay quit. There's an AD that applies to all Cessnas with the small fuel caps; you should have a vented cap on at least the right tank, and might have them on both. If you have the old non-vented caps you're illegal. The vented cap allows air into the tanks if the underwing vent gets blocked. You won't get a wire into the tank; there's a check valve in the way, screwed into the tank itself. It reduces the amount of fuel running out of the vent. If you blow into this vent (one lungful is plenty, much less if the tank is nearly full) and then run around quick and undo the right tank's cap, you should hear a bit of air escape. If you're slow, the air will get back out of the underwing vent. Might need a chunk of 3/8" hose if you're short like me or have a stiff neck or don't fancy getting a few bugs on your lips. And don't use any more than lung pressure. Don't want to split those flimsy aluminum tanks.

The vented caps have the little red rubber check valve disc on the bottom. The non-vented cap has only the steel lock bar. The vents let air in but won't let air or fuel out. They can get rotten. The caps aren't too expensive. The cap gaskets get rotten and crack, too. That lets fuel out and rain in. Bad. Look for fuel stains on the wing around the cap.

Most likely you had a bit of carbon or lead residue break off inside the cylinder and get itself stuck under the exhaust valve for a couple of firings. The compression and combustion pressures get away and the engine will stumble briefly. Might be an idea to get the plugs checked and cleaned. If you're running 100LL lead fouling is always going to be a hassle with low-compression engines. Look into the sparkplug holes and you might see carbon and crud built up in the threads just beyond where the plug reaches. That stuff needs to be picked out of there. It will break loose and cause more heart attacks.

Or it might be a sticking valve. Look for a bent pushrod tube. If a valve is starting to stick, it will often do it when the engine is cold and then free up once it warms a bit. Not good to fly that.

Dan