The International Cessna^® 170 Association

Going for a short joyride flight I started the engine. Engine was running very rough, did a quick mag check with equal drop on both mags then shut it down. Feeling the cylinders all were warm except #2 & 4 (Connie O300C engine ECI cylinders 350 SMOH). Pulled cowling, checked plugs and they were good. Pulled valve covers found #2 exhaust stuck open. Touched it with my finger and it popped closed. Due for annual scheduled a week later I fired it up again taxied about 100 yards shut down and A/P did compression check. #2&4 still cold all compressions 76 or higher, #2&4 were 78. Pulled all plugs they tested good, pulled exhaust and intake, checked good. Borescope showed everything looked good. With plugs out checked movement of the valve train, looks good. Checked mags and got a spark at all plugs.

I'm feeling something in the valve train maybe springs or possibly lifters.

Any ideas?

What you are describing sounds like an intake leak, whether from the stuck valve or something else. I just had the same symptoms on a new Continental factory overhauled engine. My engine had two small intake leaks at clamped connections that knocked two cylinders off-line at idle up to about 1700rpm. Have you started the engine again since all the troubleshooting? It may have just been the stuck valve.

If the problem persists, another troubleshooting idea would be to check for intake leaks. It doesn't take much of an intake leak to kill a cylinder or two, especially at idle. An easy way to do that is to turn ON carb heat, remove the carb heat hose from the muffler and connect it to a leaf blower or exhaust from a shop vac. Using duct tape to seal the connection is good enough. Turn on the blower and spray a soap-water solution onto all of the intake manifold connections whether bolted or hose connections. Any intake leak will show up with bubbles.

Good luck!

Could be springs; they are fairly easy to check. The specs are in the OH manual.

I used to have problems like this, plus low compression due to stuck rings, and it always turned out to be carbon fouling. My experience seems to differ from most, but a steady diet of 100LL and ALWAYS leaning on the ground solved the problem. Not even a hint of a stuck valve in 20 years.

N2625U wrote:Pulled valve covers found #2 exhaust stuck open

You needed to go no further. No exhaust valve should be stuck open. Even only stuck enough that it releases with just a touch of your finger.

Drop the exhaust valves in #2 and #4 and ream the carbon out off the valve guide. Pull the valve stem out a plug whole and clean off the stem. While your at it check the valve springs for proper tension.

If #2 exhaust is stuck likely also #4 and if it where me I'd assume all the rest are close and I'd clean them all. If the guides where adjusted tight 350 hours is just right for them to start sticking.

Bruce, I could swear the initial post said the intake valve was stuck but maybe I was seeing things.

Did the rope trick on it this morning on #2 cylinder. A/P wasn't too happy about getting it clean enough so pulled the cylinder. Cleaned it up, put it back together and fired it up. Runs fine now. What we are scratching our heads about is why #4 cylinder was affected...we feel that somehow the burned gasses from #2 made their way to #4 diluting its mixture.

#4 could have been a fluke.

Here is what has always bothered me. Assuming all the valve guides are reamed to the same size, if one coked up, why wouldn't all the rest be just about ready to do so?

This is why when I've had one stuck valve, ALL the rest also get dropped and cleaned. But I never pull a cylinder to do so. It is just not necessary.

Heck 2 months ago just for piece of mind, I pulled the valve springs off all 4 cylinders on my Cub to wiggle the valve and see how tight they might be getting. All 4 were in good shape. Didn't have to drop any valve and clean and this is a Cub with little baffle to contend with. I did the entire job in 2 hours start to all cleaned up and hands washed. I've done a bank of three on a C-145 in about the same time.

Here's my thought on it:... No 2 ex valve stuck open creates a LEAN condition for adjacent cylinders on the same intake-side because when No 4 piston came DOWN during intake cycle...it actually grabbled a bunch of burned gasses from No 2, which had entered the intake manifold due to No 2 intake-cycle sucking exhaust gases back into the intake manifold. This led to No 4 only taking-in the exhaust gases from No 2 which had back-flowed.

George, you just explained the fluke I spoke of.

Bruce Fenstermacher wrote:George, you just explained the fluke I spoke of.

Ha!

This phenomena is also sometimes experienced when a fractured exhaust valve in one cylinder.... can actually cause the failure of an adjacent cylinder. The failed valve part will be sucked into the intake manifold and passed-along to the adjacent cylinder on a subsequent intake stroke. (This actually occurred to me one night in a D-50 Twin Bonanza. The resultant vibration made the entire instrument panel unreadable and sent my adrenalin-rush up past the redline. Fortunately I was entering the pattern at the destination. Whew!)

gahorn wrote:

Bruce Fenstermacher wrote:George, you just explained the fluke I spoke of.

Ha!

This phenomena is also sometimes experienced when a fractured exhaust valve in one cylinder.... can actually cause the failure of an adjacent cylinder. The failed valve part will be sucked into the intake manifold and passed-along to the adjacent cylinder on a subsequent intake stroke. (This actually occurred to me one night in a D-50 Twin Bonanza. The resultant vibration made the entire instrument panel unreadable and sent my adrenalin-rush up past the redline. Fortunately I was entering the pattern at the destination. Whew!)

I had something similar occur when I had the exhaust valve failure on the way to the Dearborn 170 convention in 2005. When the head of the #5 exhaust valve quit holding hands with the stem, it beat the #%^* out of the piston before getting jammed sideways in its seat. Some of the debris from the #5 piston made its way into #1 cylinder and fouled both spark plugs. (An O-300 will still turn 2000 rpm in cruise on 4 cylinders, but there is nothing smooth about it...)