How to troubleshoot your Lycoming Engine...

[DakotaHawk]

Today's flight was billed as an opportunity to do some close work with the local formation flight group. E-mails were sent out, times and locations ironed out, and six planes were set to tear up the skies over Arlington. We met at the airport restaurant for a bite to eat and a pre-flight brief. After ironing out the details of the flight, we sauntered out to our aircraft like the cool pilots that we are, strapped ourselves into the planes, and waited for the START signal.

When Lead raised his hand and twirled his finger, six starters engaged and almost instantly we had aver 1000 hp purring away! Gauges checked - all in the green - and Lead taxied out to the run-up area. I'm #2, so I position my plane offset from Lead on the taxiway, and everyone else falls in place behind us. It's about 1/2 mile from the restaurant tie-downs to the run up area, so I have plenty of time to make sure that "all systems are go". A scan of the gauges shows no problems, the engine is purring, I wiggle the stick and rudder to make sure I have full control throws, radio is set to primary and backup frequencies... I'm ready to fly!

We taxi into our run up position, cocked at a 45* angle, all eyes on Lead, Waiting for the signal to do our pre-flight run up. Once again, the hand is raised, the finger twirls, and six sets of eyes swivel from Lead down into the cockpit. Brakes set, throttle up to 1800 rpm, cycle the prop, check the Mag, check the Electronic Ignition...

WAIT! As soon as I turned off the Electronic Ign, my Dynon shows EGT on cylinder #3 goes cold! Hmmmm... This could be a fouled plug... could be a bad magneto... could be a bad ignition harness wire... Well, let's do some quick troubleshooting while everyone else is still doing their run up... Mag and EI both on... Engine is running smooth as a baby's butt. 1800 rpm. Lean hard and see if I can clean the plug. Turn off the EI again, and the Dynon shows EGT on cylinder #3 is cold.

I call Lead, "Two's out - got a cold cylinder." Lead quickly rearranges the flight and I taxi back to my hangar to begin troubleshooting.


to be continued...

 

[DakotaHawk]

the story continues...

As we left the story in Part I, I had just taxied back to my hangar after getting indications of a cold EGT on cylinder #3.

Before shutting down, I ran up to 1800 rpm again, but I was still showing CHT all around 280F, and EGT ~1100, 1100, 375, 1100F. I tried leaning the mixture hard, but couldn't get the #3 EGT to do anything significant. CHT on all four cylinders were tracking normal, but EGT just wasn't behaving!

After shutting down and de-cowling, I pulled the bottom plug (magneto on the bottom, Elect Ign on the top). There were a couple of lead cruddies in the plug, so out comes the dental pick. Five minutes later, the plug was cleaned and looking fine. A little anti-seize on the threads, and the plug was reinstalled and torqued, and the harness back in place. Engine start was easy (well, there was a little vapor lock, but that was to be expected in the warm weather), CHT on all four cylinders was still just fine, but EGT is still only around 375F on #3 cylinder.

Around this time, my next door hangar neighbor wanders over. I explain the situation. He flies an RV-4 / 0-320 / wood prop with basic instrumentation.

He asks me "When you close your eyes, how does the engine run?"

"Well, ... Fine..."

"Then go fly!"

I'm sure that I have a confused look on my face, but he continues by saying that the engine sounds fine, so just go fly it. But I believe my instruments. They tell me that something is wrong, and I'm not about to get up in the air when I don't know what's going on up front!

It's time to grab my son and head down to the local burger joint for some brain food.

to be continued...

 

[RV10inOz]

The EGT probe is not telling lies if its about right on both. The electronic ignition on your engine may well be giving a good enough sparke on one plug not to notice. Strange its not roughish on the mag only though.

My bet is the plug is firing, but just poorly.

Tell me this, do you taxi leaned to the point the engine is only just running? That is leaned till it dies and then half a turn in? If not, now is a good time to start. The ONLY time my mixture is full rich on the ground is for priming (IO540) and as I commence the takeoff roll. I do not taxi onto the runway rich.

Next thing folks, when doing your runups, run the throttle forward in this highly leaned state and the engine will cough a little, then slowly advance the mixture a fraction. Keeping the runup done only at bare minimum mixture. Then test for EGT drops.

Also to clear a fouled plug, do not do that agressively leaned as there is not enough energy to clear the plug. Do not use full rich either as that defeats the purpose. Use a best power mixture or there abouts.

I have never had to clear a fouled plug.....why because I follow the above.

Now here is the one thing I bet you did not check, and the most likely cause if the plug is working but not well.............. How many hours since you gapped the plugs to 0.016" - 0.018" and cleaned them?

If the answer is more than 60 hours ago......start there and do all of them. If you are using auto plugs and EI use the minimum gap setting recomended for that setup. On the Massive electrodes on the mag, 16-18 thou.

As for your mates friendly advice.....he might be right.....but I would NEVER do that.

Let me know how you go.

BTW I love Arlington WA and that region

 

[DakotaHawk]

and the stunning conclusion...

After spending some quality time with my son, choking down Jack's finest cuisine, we made it back to the airport to continue troubleshooting. First step on my list of ideas was to test my neighbor's theory that the EGT probe may have failed. After removing the EGT probe from #3 exhaust, I fired up the propane torch and held it up to the probe. My son confirmed that #3 temp indicated about 1100F. So the probe couldn't have failed.

Next, pulled the #3 lower plug again, and swapped it out for a new plug. Lucky that I happened to have a spare plug buried somewhere amongst all of the junk in my toolbox. The plug was installed, torque checked, and it was time to fire up the engine again. It's had about an hour to cool, so no vapor lock issues this time, and it fires right up! Purring along, and the EGT on #3 is where it's supposed to be! Whoo Hoo! It must have been a bad plug! I shut down the engine and we reinstall the cowling. We still have time to get a flight in today!

Back in the cockpit with seat belts tight, my son and I taxi out to the run up area and do a thorough check - all good. Take off is smooth, and we point the nose south for distant destinations. As we climb through 3500' and find smooth air, I pull back on the throttle and begin to lean it out. And then it happens...

EGT on #3 drops off to about 375F again. I point it out to my son, and we make a 180 turn back to the airport. No more flying until this is solved! Back on the ground with the cowling removed again, I think that I may have a fuel delivery problem. I removed the stainless tube from the spider to the injector and blow it out. All clear! Next I pulled the injector body and checked it. It's in great shape - no problems there!.

Finally, I call my engine guru. "Here's what's happening! What's wrong with my engine?" And his answer was "Probably nothing. It's running smooth on both mag and EI, and smooth when mag or EI is shut off. You're suffering from data overload."

But just to make me feel better, he has me perform a test. Start the engine, run it up to 1800 rpm, and then slooowwwwlllyyy lean it. Really slowly! If there's a fuel delivery problem, the engine will shake and scare the heck out of you as the #3 cylinder finally starves. If there's good fuel delivery, it will be able to run lean down to about 1300 rpm before the engine quits. Sure enough, my engine quit about about 1250 rpm.

Finally, I did what I should have done hours ago. I pulled the #1 EGT probe and plugged it into the #3 exhaust. Engine started fine, and EGT on #1 (in the #3 exhaust) came right up to 1100F!

DANG! It was the probe all along! I had always heard that EGT probes fail to the high end of the spectrum - like at 1600F. So when my EGT showed 375F, I didn't even think that the probe had failed. If I had listened to my RV-4 buddy ("How does the engine sound when you close your eyes?"), or gave heed to my engine guru ("Probably nothing, you're suffering from data overload."), I would have realized that my engine WAS running smooth the whole time. I still had to find the problem, but a Lycoming doesn't have to have EGT probes to fly safely.

After finding the failed probe, we buttoned her back up, did a thorough pre-flight and run up (ignoring the EGT on the Dynon), and then tore up the skies for some late afternoon fun.

Why did the EGT probe work for a short while after swapping out the plug? Maybe because the engine had cooled for more than an hour... But it only worked for long enough to throw a little confusion into the whole process of troubleshooting.

BTW - one new EGT probe is on order from Aircraft Spruce. Cost $36.

 

[Sig600]

I learned a trick a few years ago for troubleshooting this (identifying the bad cyl or if it's an EGT probe fail)...

When you go to the mag with the bad plug (identified by the significant drop in RPM and rough running), let it run for a little bit. Shut down, then jump out and touch each pipe with a crayon (or just the suspect pipe if you think it may be an EGT probe fail). What happens with the crayon will tell you whats going on, whether it melts.

Not the ideal solution, but got me out of a bind one night.

 

[RV10inOz]

Well there ya go......I missed it, and a tricky probe had you and me fooled.

By the way have you checked all your plug gaps?

 

[PCHunt]

Disagree

............. Tell me this, do you taxi leaned to the point the engine is only just running? That is leaned till it dies and then half a turn in? If not, now is a good time to start. The ONLY time my mixture is full rich on the ground is for priming (IO540) and as I commence the takeoff roll. I do not taxi onto the runway rich.

Next thing folks, when doing your runups, run the throttle forward in this highly leaned state and the engine will cough a little, then slowly advance the mixture a fraction. Keeping the runup done only at bare minimum mixture. Then test for EGT drops.

I respectfully disagree with your run-up technique. I think the engine should be run up at full rich, or if at a high DA, leaned for best power. Reason: I want to run my engine up at the same mixture setting that I will use for takeoff.

Otherwise, a surprise might await you as you push the mixture up at the very last second.

My two pesos. YMMV, etc.

 

[pierre smith]

Nope.

......... Shut down, then jump out and touch each pipe with a crayon (or just the suspect pipe if you think it may be an EGT probe fail). What happens with the crayon will tell you whats going on, whether it melts.

Not the ideal solution, but got me out of a bind one night.

Spit is always available!

Best,

 

[Phlyan Pan]

An infrared thermometer weighs mere ounces and is less than $50. I find them invaluable for diagnosing engine problems related to only 1 cylinder.

 

[apkp777]

Hey Scott,

I had a similar indication with my Dynon probe. Except it went cold....then hot...then cold. A whole lot easier to troubleshoot that. I now have a CHT probe doing the same thing. Funny, I flew a 414 the other day, that had a single EGT probe on one of 12 cylinders. How did people ever fly like that

I am having my doubts about the quality of the Dynon probes?

 

[Ron Lee]

Your friends were right. Had a cylinder not been firing correctly, you would have known it without instrumentation.

I had the same thing happen as a passenger in a RV-6A. His engine monitoring system was showing a really low EGT on one cylinder. The engine was running fine. We were flying. I told him that many airplanes have only one or no EGT probe so with his CHTs fine and the engine running right, it was most likely a sensor issue.

What were your CHTs during this episode? I don't see any mention of that parameter.

Good learning experience. Can't blame you for taking the cautious route.

 

[David-aviator]

Your friends were right. Had a cylinder not been firing correctly, you would have known it without instrumentation.

I had the same thing happen as a passenger in a RV-6A. His engine monitoring system was showing a really low EGT on one cylinder. The engine was running fine. We were flying. I told him that many airplanes have only one or no EGT probe so with his CHTs fine and the engine running right, it was most likely a sensor issue.

What were your CHTs during this episode? I don't see any mention of that parameter.

Good learning experience. Can't blame you for taking the cautious route.

I agree on the caution note but truth be known, I do not look at EGT on the ground. If RPM is normal checking one mag against the other, my brain sees a green light and I go. If rpm is 2180-2200 at the start of take off, that too is another green light. EGT's are not in the equation at this point in the flight.

 

[Ron Lee]

Good point David. RPM info is prime along with sound/feel of the engine. While I have four probe CHT/EGT, I have to switch a rotary knob to check all four and I never do that during the run-up. Oil temp minimum 100 degrees F is the only temp I really go by although I casually glance at CHT and EGT for ONE cylinder.

I have a Rocky Mountain engine monitor which is fine but does not have all the info you see at once on modern engine monitor systems.

I will add that taking the cautious approach shows the signs of the kind of pilot I would fly with. Too many accidents are caused by ignoring one sign after another of impending doom. Scott detected an apparent problem and took a safe route to determine the cause. Thumbs up for good judgment.

 

[Jamie]

The question to ask yourself is...how could the EGT be cold and the engine still run smoothly and CHT still be in the green?

The question was asked about the durability of the Dynon-supplied EGT probes. Here was my experience.

 

[az_gila]

Hey Scott,

I had a similar indication with my Dynon probe. Except it went cold....then hot...then cold. A whole lot easier to troubleshoot that. I now have a CHT probe doing the same thing. ....

That's what I would expect probe failures to act like, either short circuit or open circuit - which would make your instrument peg either high or low.

The physics to make a thermocouple give a wrong output voltage must be quite peculiar...

 

[vlittle]

That's what I would expect probe failures to act like, either short circuit or open circuit - which would make your instrument peg either high or low.

The physics to make a thermocouple give a wrong output voltage must be quite peculiar...

These probes have a stainless sheath over fiberglass insulation. Sometimes a shard of the sheath will penetrate the insulation and short one side of the probe to ground, giving a much lower than normal temperature indication.

This can occur at any point along the lenght of the cable, but most likely near any connectors.

V

 

[DakotaHawk]

The question to ask yourself is...how could the EGT be cold and the engine still run smoothly and CHT still be in the green?

Jamie hit on the core issue of my missive! The engine was running "smooth as a baby's butt" during this whole event! CHTs were all right where they were supposed to be - in the 250F range. This whole farce was based on ONE sensor - the #3 EGT probe!

 

[Russ McCutcheon]

Scott did the right thing checking it out before flight, even though we know it was not the engine but an indication problem. I have had a few low EGT readings where the engine was running fine but the plane was not safe to fly, on one occasion the clip that is held under the hose clamp had broken off and the probe had come out of the exhaust pipe, this resulted in a steady reading in the 200F range and the exhaust leak and possible flame coming from the hole may have been able to cause damage to it?s surroundings, in a different case I had at one time some exhaust flange gaskets on my plane that had metal on each side with a fiber material in-between, the fiber burnt out and released the tension on the exhaust nuts holding the flange on, the nuts fell off and the exhaust fell down about 2? off the cylinder, this happened in flight and the EGT was reading about 700F steady, you could not hear or detect any problem with the engine, luckily I chose to turn back and land.

If the temp is jumping around from 1200F to say 200F like one of mine is doing right now then maybe it?s ok and just a probe/connection but if the temp is way low and steady wile the engine is running fine and CHT is normal you best check it out promptly.

 

[RV10inOz]

I respectfully disagree with your run-up technique. I think the engine should be run up at full rich, or if at a high DA, leaned for best power. Reason: I want to run my engine up at the same mixture setting that I will use for takeoff.

Otherwise, a surprise might await you as you push the mixture up at the very last second.

Fair enough, but lets examine this statement a little closer. Lets deal with the last comment first. The possible surprise you might get? What the mixture does not deliver fuel? In which case the aircraft will not perform, you will roll down the runway and abort, taxi back and find the problem. Of course you do a full power / airspeed check as you begin your takeoff roll, don't you.

At full rich and low power settings you are not going to get the runup check environment to test mags/EI's and plugs. To be quite frank the only way you will get a good mag check is to be in flight cruise power, and LOP. This will then test thoroughly the plugs, the leads and the ignition source. On the ground will only ever show up a completely failed (or stolen) device.

leaned for best power. Reason: I want to run my engine up at the same mixture setting that I will use for takeoff.

Are you sure that is what you really meant Sure a best power runup is fine, way way better than a full rich runup. But at best power you may not pick up degraded performance as effectively as being LOP or agressively leaned.

Now surely you do not use a best power mixture setting for takeoff

The correct mixture setting for takeoff is full rich. At an elevated DA you should use a mixture that equates to that exact same engine state. A TARGET EGT is the way to do that. If you know that a particular set of numbers is what you always get at sea level, thats what you want at any DA.

Hope that clears that up.

Happy to debate further if you wish.....its all good viewing for the rest of the gang I hope.

DB

 

[PCHunt]

Well, all I know is that at high DA, let's say 9,000' as an example, I would lean until I found best static RPM (FP prop), then not touch the mixture, and then take off. I call that best power mixture, maybe I am mis-using the the term.

And, yes, I would, and do, use best power mixture, as described above, for high DA takeoffs. Best power is not peak EGT, BTW.

 

[RV10inOz]

Pete,

Best power is slightly ROP, and yes it is BEST POWER, but that is not a lot of detonation margin during your take off.

I would suggest an APS class or online class, and learn why the target EGT method would be a much safer bet.

You might then understand what I am trying to say. Seems like you are getting 98% of it.

 

[Bob Gordon]

One more data point

Although all four of my EGT probes are healthy after 550 hours, I had one bayonet-style CHT probe go out at around 500 hours. Purchased the system from RMI back in the day. Considering all the thermal cycles these things see, I am still impressed.

 

[airguy]

I respectfully disagree with your run-up technique. I think the engine should be run up at full rich, or if at a high DA, leaned for best power. Reason: I want to run my engine up at the same mixture setting that I will use for takeoff.

Otherwise, a surprise might await you as you push the mixture up at the very last second.

My two pesos. YMMV, etc.

Agreed. On my runups at high DA, I put the mixture to where it looks "about right", advance the throttle, then tweak the mixture to make best RPM, and check the mags. After 500' AGL I'll play with the mixture to keep cylinders cool.