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help diagnosing intermittent engine roughness


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Yesterday afternoon, I was in cruise flight in my M20F. Full throttle, 2500 RPM, leaned to 100 degrees ROP. My feet were resting on the rudder pedals, and I suddenly started to feel a slight vibration that was not there before. Then, I could hear the engine start to sound a little slower. I was able to make it sound more normal by enriching the mixture a little. Not full rich, but richer than normal. Pulling the mixture back much at all from that point made it really rough.

 

I diverted and did some checks on the ground. During a run-up, the engine sounded great. At first it seemed fine at idle, but then I started to notice some roughness at idle. A slight throttle up for taxi after a couple of minutes at idle led to a little more roughness. I think it was worse with just the right mag selected.

 

This morning, the mechanic where I left it says it is running great. I know there is something not quite right. Any suggestions for things to have the mechanic look for?

 

--John

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Do you have Champion spark plugs?

I had a bad resistor in one. The mag checks were okay, but the engine did just what you describe.

Fine wires solved the problem, but I just needed to replace one plug.

http://mooneyspace.com/topic/11666-ahhhhhh-relief-fine-wires/

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I swear this is the third post (my own included) in the past couple months I've seen like this. My M20F has the selectable 4 cylinder EGT. My issue ended up being an obstructed injector. The issue was inconsistent. In flight, it felt as if the engine was surging when I opened the ram air. Adding richness fixed the issue. 

 

Then, one day I went to take off and the engine stuttered a little as the engine passed through 2400-2500 RPM. I opted to not fly until I found a smoking gun as the cause. I recommend that you do the same. While it's easy to dismiss issues as intermittent or an indication error, we should always assume the worst until we've diagnosed the cause. It is in our nature to want to trivialize issues and say it's the old instrument or a spot of bad gas, it's best to treat it as if the symptom is a heavy-handed letter from the IRS as leaving it unanswered may likely burn you. 

 

With the clogged injector and what I presume to be the original 4 cylinder EGT gauge with the selector knob, here is what I observed on the ground. At 1900 RPM, one cylinder EGT was much higher than the rest, then at 2400 RPM (still on the ground), the EGT for that cylinder dropped to nearly zero with the engine misfiring. A clogged injector will do more than just make one cylinder run lean, it'll make the others excessively rich at the same time since they'll get the fuel intended for the cylinder with the clogged injector. On my engine, it was the #3 cylinder. On the EGT selector, that was the second click from the left, so I assume left to right on the knob is 4-3-2-1. 

 

The other issues suggested were fuel servo issues and spark plugs, which are easy enough to diagnose (the cylinder with one plug not working will have a sky high EGT while the others are normal -- also a way to detect a failing magneto). 

 

I also wanted to add that a rough running engine that clears up after running for a bit may be the sign of a failing valve. A valve failure will ruin your day. How much time (years/hours) is on your engine SMOH?

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There are many monitors going up for sale lately that can tell CHT and EGT for all four cylinders. The new color monitors can include fuel flow and all the primary engine instruments.

I suspect the troubleshooting you are about to encounter may pay for a used engine monitor.

You will really enjoy having one. No Mooney pilot would prefer to be without data collection on all four cylinders. Even if they have to spin the selector knob themselves and write down the numbers they see.

Spark plugs get fowled or have construction issues.

Fuel injectors get plugged.

Mags wear out.

And of course, exhaust valves get stuck. Not as common as the above issues.

Good luck,

-a-

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I suspect the troubleshooting you are about to encounter may pay for a used engine monitor.

 

 

This is very likely going to be the case.  FWIW, I have an EDM-700 with all probes and fuel flow for sale, but I haven't made a detailed listing yet.  Everyone plane should have a monitor as it makes cases like this a nuisance vs. hours of frustrating troubleshooting.

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In the absence of a multi probe EGT you can apply a big area of blue chalk on the exhaust stacks. Start and run the engine, the first one to turn white is the hottest cylinder. Start with that one for trouble shooting.

Clarence

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Here's my two weeks in "hell" with three very similar problems.   Took off from Arkansas headed for Eastern Ohio.  About fifty minutes into the flight.  Engine ran rough for about 3 seconds, then smoothed out.  I was looking at the EGT and it spiked to max.  I pushed in the mixture and it came back down.  About three minutes later it started running rough again and I put it down in Popular Bluff, Mo.  Off the runway, I did a run up and it was the left mag stumbling.  Three hours later the mechanics discover a blown intake gasket on #3.  Repaired and on my way.  Flew great rest of the trip--10 hours +.

 

A week later, wife and I are heading for Georgia.  Leave Arkansas and land two hours later at Monroe County in Mississippi.  Top off with gas and take off.  About 800 feet, engine starts running rough.  Turn for immediate downwind and do some trouble shooting.  Switch to left mag and engine dies, quick back to both.  Switch to right mag and engine continues to run.  Back to left and silence again, back to right and land.  Mechanic here finds left mag has lost three teeth off the plastic gear.  Overnight repair.

 

Next day, off to Valdosta, Georgia.  No problems with the two and a half hour flight.  Next day, take son-in-law up for a flight.  Now I'm a little gun shy and think the engine is missing but not real sure.  After landing, do a run up just to check things out.  No problems.  Next day, take my daughter up for her flight. Run up is well within limits.   About 500 feet off the deck, engine has started to have a definite miss.  Downwind again, check mags again, and both are missing like crazy.  Land again and do another run up,  out of limits mag drop on both.  Mechanic this time finds my fuel was contaminated at the last stop and the fuel strainer is plugged up.  Fixed and do two test flights.  All is well as was the 5 hour flight back home.

 

Point is---------three identical initial indications with three separate completely different fixes.  You never know what you might find when you start trouble shooting. 

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I made a trip back to KS this weekend being my longest flight to date (5.5 hours). We landed in Arkansas and put about 20 gallons in and took a little break. I checked for water after fueling. About 1,500-2,000 feet on climb the engine missed or hiccuped. We both were a little scared the rest of the flight. I'm guessing maybe a little water?? I really have no idea?? I have no additional monitors for diagnostics. Love to hear from more experienced pilots. Thanks. Troy

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Lycoming "misses" or "hiccups" are most often ignition problems, or sticky exhaust valves. Sticky exhaust valves are diagnosed by changing the mixture to a richer setting. If the problem goes away, then you have an exhaust valve with a tight stem that sticks when too warm.

 

Ignition problems usually stay the same with mixture changes, unless you are still running Champion spark plugs. Their infamous internal resistance causes problems that are worse when running lean of peak because weak ignition cannot reliably light off a lean mixture. 

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Unfortunately I don't have an engine monitor, or even a selectable EGT.

 

In my case with a clogged injector, I didn't know which cylinder the corresponding selection indicated, so I ended up, under the supervision of a qualified A&P, pulling each injector and looking through it. The clogged injector was obvious. 

 

Have you been able to notice anything unusual when switching mags? Fouled plugs or a bad mag should show up right quick. 

 

Another thing to check is leaking o-rings in your fuel servo (throttle body). Open your ram air, set your mixture to idle cut off, throttle wide open (so you can look inside), turn the electric fuel pump on and look at the four little tubes in the opening. See if any of them have fuel running out. 

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