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Engine suddenly running very rough

JackinMichigan

Well Known Member
Had me a bit of a scare today.

Took my RV-10 up for the first time after it's annual inspection. I was about to turn on the downwind leg of the pattern when suddenly the engine started running very rough. My first thought was that something was off balance as the whole plane began shaking (not severely, but enough to have my undivided attention).

I immediately throttled down, turned around as fast as I could, and landed on the opposide end of the runway I had just taken off from. Once safely on the ground I throttled up as fast as I could with the brakes on (about 2400rpm) and the engine was running as smooth as could be. No sign of that shaking.

I called an A&P friend and he said there was likely condensation in the tanks and the engine had sucked up a bit of water. I had borrowed a heated hangar to do my annual and the temp outside was about 25F, and the tanks were about half-full.

This seemed like a completely plausable explaination, so before I tear the cowl off I'd like to know if anyone else has other things (or other theories) I can inspect before I take her up again.
 
Stuck exhaust valve?
Do you have EFIS engine data that you can reference?

Not very likely on an engine with around 75 hours. Op sig says he did 50 hour oil change in august of last year.

Larry
 
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water is certainly possible. Is this a carb or FI. In 650 hours on my 6, I have never had a noticeable amount of water in my tanks and I am in similar weather patterns. I almost never top off the tanks before putting the plane in the hanger also. Further, cold air doesn't hold much moisture, so water from condensation would be much more likely or more severe in the summer. We haven't seen north of 40 in several weeks. If the air can't hold a lot of moisture, it can't come out in condensation.

It does sound like a fuel thing though. If you have efis logs, go back and look at your EGTs during the event and post the results. It can sometimes help narrow thinkgs down some. We want to know if it was system wide or limited to one or several cylinders. When it feels like the engine is shaking apart, that points to one or two cylinders going cold. Water in the fuel would behave like typical choking and sputtering, like when running a fuel tank dry; no serious shaking.

Assuming FI, If one EGT dropped to 0 during the event, pull the restrictor from the injector in that cylinder and look for debris and clean it out. A large chunk can fall into the hole, blocking fuel and then bounce around, leaving the hole open again, especially if you went to idle after the problem.

Larry
 
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Annual inspection ... maybe have a look at your spark plug to wire end connections.

High probability of a human hands induced problem.
Check the B nuts or any joint in the fuel system that was inspected or worked on.
 
Engine Data

Well, yes, but I didn't pay attention to it. From start to finish the whole episode lasted about 10 seconds.

Recommend downloading your engine data from the flight in question, upload to Savvy Analysis (free). Take a look at fuel flow, fuel pressure, EGTs, and CHTs at the time of the event. That way you can see if one cylinder was impacted, or all cylinders impacted.
 
I wonder if you have the ?bad vintage? of Champion spark plugs? I had six of twelve plugs fail in the first 300 hours - all high internal resistance.

Pull the plugs and verify center electrode is between 1000 and 4000 ohms. The six bad ones I had (new engine from Lycoming) were all in the 12,000 - 14,000 range.

High resistance plugs not only run bad, but they are very hard on your magnetos (or EI coils).

While your at it, check the spark plug lead resistance.

I replaced all the Champion plugs with Tempest - problem gone and while I no longer fly with aircraft plugs, I only recommend Tempest for those who do.

Do you have an EMS system that collects data? If so, reviewing EGT variations durning the rough engine period may tell a story.

Carl
 
Annual inspection ... maybe have a look at your spark plug to wire end connections.

High probability of a human hands induced problem.
Check the B nuts or any joint in the fuel system that was inspected or worked on.

+1 for a look at your plugs and wires. Check the torque. I know someone who had a similar problem that turned out to be ignition related caused by the A&P during the conditional inspection.
 
I had this happen one time, declared an emergency, and then fixed it before turning Base.

Turns out I pulled the mixture at the same time I reduced the throttle. Not enough to make the engine stop but enough that it was running really rough. Duh!
 
+1 for a look at your plugs and wires. Check the torque. I know someone who had a similar problem that turned out to be ignition related caused by the A&P during the conditional inspection.

We haven't seen data yet, but severe shaking/imbalance usually points to one cylinder dropping off (failing to combust). If that was the case, ignition is improbable, as both plugs would have to fail at the same time for the cyl to drop out. One plug failing on one cylinder won't even be noticed by the average pilot without looking at the EGT gauge. Wiping out one whole ignition system, like a mag, also results in only slght changes to engine sound/feel; Though most will usually notice it as it occurs but may not notice if they missed the occurrence. Definitely no shaking / vibration issues.

Not saying ignition was not the culprit. Just not enough data to determine how likely. Never a bad idea to validate ignition health.

Larry
 
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One other thought as reading the OP reminds me of a similar incident. Back in the day working to get a ticket flying a rented spam can, this same scenario happened. The engine ran strong on take off, then when throttling back to stay in the pattern the whole plane started to shake. Put the throttle back in an the engine was smooth. Throttled back and again got the vibration.

I was over the airport so pulled the throttle out idle and landed.

Short inspection of the engine showed an intake tube with one bolt missing, the other bolt almost all the way out. It seems at full power there was enough suction to pull the intake tube tight on the cylinder. Other than full power it dropped out - as this was a carb plane that cylinder went cold. I assume with an injected engine you would still have fuel flow but not enough to burn with the excess air coming into that one cylinder.

Carl
 
I just ran down a similar issue. It turned out that I had an intake leak that would only rear it?s ugly head at certain power/mixture settings.

It took a while for me to believe I had a leak, because my engine was very new. But the age of the rubbers and gaskets were more of a contributor than the hours on the engine. It?s now on my list of things to replace every 3rd annual and it only cost about $75 and 4 hours of work to replace them.

Running great now.

Check out Vic?s article in the latest (Feb 2020) Sport Aviation magazine. It?s well written. It might wing a bell.
 
It seems at full power there was enough suction to pull the intake tube tight on the cylinder. Other than full power it dropped out - as this was a carb plane that cylinder went cold.

Carl

At WOT/full power, there is very little vacuum in the intake tube. Therefore, the leakage area doesn't introduce a lot of additional air. Air is somewhat like electricity in this regard, taking the easy path, which is the big fat intake tube. The leakage will create some leaning, but propotional to the difference in size between the tube and leak opening. Lyc's are typically quite fat at full rich that this extra air wouldn't make a significant impact. At lower power settings and especially idle, vacuum is very high and that will draw in a much large proportion of air with no fuel mixed in through the leakage area. Vacuum is caused by a restriction and the vacuum will happily pull disproportionately more air from the leak air to overcome the restriction.

Larry
 
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At WOT/full power, there is very little vacuum in the intake tube. Therefore, the leakage area doesn't introduce a lot of additional air. Air is somewhat like electricity in this regard, taking the easy path, which is the big fat intake tube. The leakage will create some leaning, but propotional to the difference in size between the tube and leak opening. Lyc's are typically quite fat at full rich that this extra air wouldn't make a significant impact. At lower power settings and especially idle, vacuum is very high and that will draw in a much large proportion of air with no fuel mixed in through the leakage area. Vacuum is caused by a restriction and the vacuum will happily pull disproportionately more air from the leak air to overcome the restriction.

Larry

+1. This is the correct explanation. A simple test for small induction leaks is to run full rich, wide open throttle, observe EGTs. Smoothly retard throttle to idle. If any cylinder shows an egt rise significantly different than the others, look for an induction leak on that cylinder’s plumbing.
 
Another data point is idle MAP, which should be around 12" or less. If higher (15"), suspect an induction leak.
 
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