About 40 minutes into a cross-country trip (with full fuel) at 6500 ft MSL, I experienced a series of power surges. When the surges first began, I immediately pulled carb heat (which had no effect) and adjusted the mixture both rich and lean (which made no improvement). The severity of the power surges increased rapidly, becoming so bad that the engine was effectively just windmilling. Switching from “both” (where I always have the fuel selector) to either right or left made the power loss worse. Because the placard instructions are to switch to a single tank above 5500 feet (the need to follow this instruction is negated in section C of the AD by the fact that I have the vented fuel caps) I ultimately switched to the left tank while trying to glide to a landing strip. About a minute later (by now down to 5000 MSL and zero power from the engine) I switched back to “both” and the engine power returned in full vigor. I proceeded to a nearby paved strip, and landed with no problems.
After landing, I inspected all fuel vents (main vent tube and 2 vent caps) by first removing vent cap on left tank and blowing into main vent tube below wing. Main vent tube was highly resistant to initial attempts to blow into it but subsequently yielded, providing only normal check valve resistance. Both vented caps were removed for inspection. The rubber check valve flap was slightly adhered to the mating surface but was easily dislodged. Suction was applied to both vent caps and they were found to flow freely.
I am not convinced it was just a fuel venting problem, as it would appear that for this to have been just a fuel venting problem, all three vents would have had to become blocked. The main vent was determined to be pretty well stuck until I dislodged it but the cap vents were only slightly adhered. One mechanic pointed out that the wing caps are in an area of low pressure so any adhesion at all might have been more than the limited pressure differential could overcome.
After landing, I pulled over 2 quarts of 100LL from the gascolator sump and found it to be pristine, with no evidence of moisture and only 5 small particles (less than 1 mm ea.) of debris. I plan to have the entire fuel system and carburetor inspected and the main vent check-valve replaced.
I am interested in any relevant comments. I want to ensure that I don’t have this experience again and am concerned that the venting issue may not fully explain things.
I had a 1959 Cessna 172 and this happened to me three times in the 22 years that I owned the airplane. I think Cessna put out a service kit in the 70's that modified the fuel system in an attempt to alleviate this problem. I had a copy of the service letter but gave it to the new owner when I sold the airplane. SK-172 comes to mind but you may have to contact Cessna customer service for assistance in locating the SL.
I have owned the same 172G for the last 14 years & it was always very reliable until a couple of years ago when I suffered the same fate as yourself. The only difference was that I was only 1000ft AGL & was committed to a forced landing. After trimming for 65 knots, I selected carb heat, switched tanks, magnetos etc., but it made no difference.
Remarkably, the surge in engine power was very smooth in that there was no vibration whatsoever. Fortunately, I was surrounded by flat terrain & got down without a scratch (flying under a powerline was a little uncomfortable though). I found that after the little 0300D cooled down, it started effortlessly as usual & was able to get back to the airport as though nothing had happened. My mechanic (with heart in mouth) checked everything as you did -: fuel quality, contamination, etc.etc., ignition spark etc., etc. I too, theorized that it was the fuel breather, fuel cap seals but they too were normal.
I, like you, was not comfortable with not knowing the cause of this & after the second engine failure decided to get the carburetor & magnetos fully overhauled & made some interesting discoveries -:
1. the carburetor did not have the proper fuel nozzle (mandatory AD), even though the log books indicated so. Also, a washer was missing that, I believe, held this nozzle slightly lower (remember, we are relying on the proper venturi effect for proper fuel atomization). The proper venturi was fitted before I purchased this machine!
2. the magnetos,even though well within the mandatory 500 inspection limits revealed what I believe caused both emergencies.
Both contact points were fully worn & showed evidence of some form of 'crystalization', but this I don't consider the main culprit.
Both coils showed evidence of leaking some 'gooey' substance, which, I understand keep both coils cool. In other words, once the engine gets to a certain temperature, the magnetos heat up, which cooks the coils & the 'gooey' substance gets overwhelmed by this heat & oozes out & won't alow the spark to continue to the leads.
Since being overhauled (both magnetos), the engine runs better due to a stronger spark & is less susceptible to plug fowling.
It is also interesting to note that since I have installed a new oil temperature guage, the engine temperature is always in the top arc of the 'green' range. In other words, the original guage never indicated this, so I am lead to understand that these engines always run hot (I am considering getting an oil cooler fitted to aleviate this problem).
As yourself, I didn't really consider that the problem was with the spark & only looked at blaming the fuel (delivery of) or some sort of air blockage.
This happened a couple of years ago & have not had any problem since.
I hope this helps & good luck.
I had the same thing happen to me in NOV. ('65 172F) I was about 30 mins into flight over mountains @ 10.5K MSL, when the power surges began. Cycles between full power...idle...full power...idle. Power lasted a few seconds, and idle was about twice that.
I was already on "left" tank at time, and was leaned to altitude. I immediately whent to full carb heat, tried switching to "right" switching to "both", various mixture/throttle combinations, etc. All to no avail.
Established best glide and declared emergency as I was losing alt over mountains. got down to about 6.5K alt (mountains about 6K in that area! yikes), when suddenly everything was fine and engine ran normally.
landed normally at point of origin (with full carb heat the entire time) and full rich from wence it started running normally again.
i had full carb heat on the whole time (probably 8-10 mins) elapsed time. everything checked out normally on landing. nothing could be found to be wrong. normal sumping, normal venting, normal carb, etc., etc. similar to above stories. Seems to be a mystery and one that could have had much worse consequences. but for the fact that i was high at the time (instead of canyon running) things worked out for me.
the a/c HAS had the AD for the vented caps, so the vent issue shouldn't be a factor, but it sure seemed like fuel starvation vs. carb ice. Question: how hard must one blow into the wing vent tube to get thru. the mechanic said everything was normal, but i blew on my next preflight and it didn't seem to want to budge. this doesn't seem right to me (unless you're supposed to apply suction rather than blowing in). Don
If you get completely stumped and cant find the problem check the venturi in the carb and make sure it is seated tightly. To be honest i can not remember what the symptoms are when that comes loose but i do know they are not good. I believe the engine will quit. Just something to check and try good luck
