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Engine stumble and smoke. Advice needed

Simon Hitchen

Well Known Member
Hi everyone, we recently flew the RV down to Florida from Toronto and back, 2500 miles, 16 IFR sectors and the aircraftperformed beautifully, with one exception. Climbing out of KMQI the engine stumbled momentarily. Looking at the trace it appears EGT 3 and 4 dropped causing a drop in RPM. The YouTube video link attached clearly shows the smoke associated with the event.

Any thoughts on what may cause it would be greatly appreciated.

Many thanks,

Simon

https://youtu.be/G3IW6RPtWFo

1-F86-F120-2-D70-4-C3-A-A726-4-E11-B9-E659-BF.jpg
 
Any chance you sucked in a washer or such? I’ve seen where a loose fastener ended up under an intake valve for a few rotations. The momentary loss of power under heavy load and some exhaust smoke was just as you discribe.

Carl
 
response

i hope it wasn't a foreign object injestion, there had been no work done on the engine and i always keep the plugs in overnight.

The aircraft had sat out on the cold ramp on a clear night and refuelled in the morning a few minutes before this take off. I did drain all three sumps during the walk around, no water found. But i guess any water in the new fuel supply may not have had time to sink to the drains.

Would water in the fuel cause smoke?
 
Lost rivets from the FAB air cleaner housing cost me 2 cylinder replacements many years ago…
Same symptoms as you described
 
i hope it wasn't a foreign object injestion, there had been no work done on the engine and i always keep the plugs in overnight.

The aircraft had sat out on the cold ramp on a clear night and refuelled in the morning a few minutes before this take off. I did drain all three sumps during the walk around, no water found. But i guess any water in the new fuel supply may not have had time to sink to the drains.

Would water in the fuel cause smoke?

Yes. It wouldn't be steam but could easily cause incomplete combustion -> smoke.

It is always advisable to let fuel settle for a while (time length debatable) to let gravity do its job on any suspended water after fueling. Also advisable to give the wings a good rocking afterward to help any bulk water get past the tank rib and related drain holes.

A good data point might be to do the aforementioned and sump again before you add any fuel. This could let you know that there's as issue with the tank condensate drain paths.

It could be any number of things that caused this but it's a great example of when to apply Occam's razor; at least initially. Let us know what you find.
 
Lost rivets from the FAB air cleaner housing cost me 2 cylinder replacements many years ago…
Same symptoms as you described

Yep - pull the cowl and get an extra set of eyes to help look for missing hardware on the intake side of the engine. Follow up with a borescope of the cylinders.

Considering the single event I’d guess whatever it was is rattling around in an intake pipe (gets sucked up when under heavy load) or went into the cylinder and back out.

Carl
 
Water

An empty fuel tank and a cold night in a coastal area.
I would be very surprised if this was not water.
I guess the engine is carburated.
Water does not evaporate in the intake manifold, therefore I think it´s more
likely it will end up in cylinder 3 and 4.
I´m a former floatplane pilot.


Good luck
 
fuel contamination makes more sense to me based on the limited info available

Firing order is 1-3-2-4. EGT 3+4 drop same time as the RPM. EGT 4 has a more pronounced drop.
I would think that fuel contamination would affect, if at all, 2 cylinders in the firing sequence.
FOD on 2 separate cylinders is not common, but not impossible. I'd quickly give it a borescope check.
 
Classic short term lack of combustion. The raw fuel dumped into the exhaust creates the white smoke. Can't tell you why 3&4 stopped firing for a couple of seconds, but quite likely to be benign. could be slug of water, large air bubble in fuel supply, fouled plugs, etc. I wouldn't worry about it untill it happens again.
 
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i hope it wasn't a foreign object injestion, there had been no work done on the engine and i always keep the plugs in overnight.

The aircraft had sat out on the cold ramp on a clear night and refuelled in the morning a few minutes before this take off. I did drain all three sumps during the walk around, no water found. But i guess any water in the new fuel supply may not have had time to sink to the drains.

Would water in the fuel cause smoke?

Ingestion of foreign objects into two separate cylinders at the exact same time is so small of a possibility that it is not worth considering. Something that affects multiple cylinders, such as fuel or ignition are WAY more likely

The water prevents the firing, but there is still some gas pumping through until it gets rich enough to ignite. That raw fuel produces a white smoke out of the tailpipe. Just a puff, as quickly the mixture gets rich enough to light and the event is over. OR the spark stopped igniting for a few rotations with the same visual effect. Note that 3 rebounds in 5 seconds and 4 takes 20 seconds. This irregularity points to water as it is affecting the cylinders in a different way. 4 stayed lean for a bit until all the water worked out. Just guesses here, but all things point to intermittent lack of ignition or an incombustible air fuel ratio (somewhat narrow range of air fuel ratios that can be ignited).
 
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water

I must admit I'm leaning towards water. Yes, Dare county is on the outer banks next to kitty hawk and the plane was covered in a heavy dew the next morning, it did sit with half tanks all night. I tend to agree the probability of FOD causing issues in 2 cylinders at exactly the same time is incredibly low. I will have a look with the boroscope though. The engine ran perfectly for the next 15 hours.

I was thinking of the firing order and the 1-3-2-4 puzzled me too.

FWIW it is fuel injected with matched injectors. GAMI split 0.1 all the way home.

appreciate the thoughts guys.
 
?

Straight up... my opinion is the least of these.

I have two thoughts.

First: Alarming for sure. Any misfire will cause a loss of that confidence acquired after many smooth running hours. Happened to me once in a 152. Took several weeks to return to that "feel good" feeling again.

Second: I'm sorry, I can't sign on to the foreign object thinking although I'd check that out as well. Like someone else mentioned what are the odds of it affecting two separate cylinders. And... Most importantly this, It seems this most likely is a fuel/water/bubble??? supply issue. It resolved as soon as it began. Good thought that maybe some water in the new fuel hadn't made its way to a sump port yet. Maybe two seperate "blobs" of water might explain the skipping of one cylinder. And as someone has already mentioned... I don't feel you have a problem.
 
You said: "The engine ran perfectly for the next 15 hours."

That would be way good enough for me..............
 
Sure looks like a little bit of water. Your engine monitor picked up two but you can't be sure that the EGT sample of the other cylinders was taken at the same time or not, or perhaps smoothed by the software. In any case, EGT probes typically do not react instantaneously.

Also, if there was a slug of water in the system, it could have been flashed off on some cylinders during a different part of the ICE cycle than those that showed the EGT drop or the puff of what looks like water vapor.

Of course, you could tear down the whole engine, just in case! :D
 
Is there any clue in the fact that #4 EGT never recovered back (in the timeframe shown) to the extrapolated "trajectory" it was on before the event? #3 did so after a few seconds. However, without seeing the scale on these we do not know how many degrees below the extrapolated curve #4 was running.
 
Is there any clue in the fact that #4 EGT never recovered back (in the timeframe shown) to the extrapolated "trajectory" it was on before the event? #3 did so after a few seconds. However, without seeing the scale on these we do not know how many degrees below the extrapolated curve #4 was running.

I had the same question. Here's my estimation of the differences, simply using the arrow lengths in PPT.

EGT Deltas VAF.png
 
Is there any clue in the fact that #4 EGT never recovered back (in the timeframe shown) to the extrapolated "trajectory" it was on before the event? #3 did so after a few seconds. However, without seeing the scale on these we do not know how many degrees below the extrapolated curve #4 was running.

clearly things were different between the cylinders and how they reacted and this would kind of be expected with water, as it would not necessarily be evenly distributed with FI. However, we only see around 45 seconds after the event. Clearly it mostly recovered and being 10* less than before the event is not a strong conclusion that something is wrong. Very possible that it came back inline several seconds later. Not enough data to conclude something is wrong with that cylinder

You can see all the cylinders get blips in the EGT about 15 seconds before the big event. IMHO, this is when the first signs of water appeared and created mini spikes from rising EGT due to the leanness created from the small dilution of fuel. Can even see a couple blips in the RPM. Same blips are occurring for 20 seconds after the event on 3 & 4 as the water works its way out.
 
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Lower cylinder?

Water is heavier than fuel,
this is a FI plane (I assume with a divider mounted atop the engine case)
water is pumped up into the divider, and exits the lowest ports, which in the case of the climb attitude is cylinders 3 & 4.

A thought...
 
I wonder if there could have been a small accumulation of water in the bowl of the snorkel in front of the servo. I have a drain hole in my snorkel to address this.
 
Water is heavier than fuel,
this is a FI plane (I assume with a divider mounted atop the engine case)
water is pumped up into the divider, and exits the lowest ports, which in the case of the climb attitude is cylinders 3 & 4.

A thought...

Best explanation of why 3&4 got the water. I was trying to visualize this . . .

Thanks
 
Not sure why we went with florigen object so fast and ruminated on that. The washer theory? Two washers went into each cylinder simultaneously. Unlikely.

Pre all Cyl EGT and CHT EFIS recorded data and GoPro Camera footage, Lycs for 80 years have momentarily stumbled on occasions. No one cared or worried. Sometimes engines hiccup.

Of course if there is doubt, pull the plugs, compression check, borescope if you have one. Change in oil consumption?
 
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I had the same issue a couple years in a row, many months apart. It turned out to be sticking valves.

The guides needed reaming with only 700 hours on new cylinders. Two were so tight that I had to drive the valves into the cylinders to ream the guides.

The final issue was that the cam had failed a lobe and there was metal in the case. I overhauled it and am hanging it as we type.

Please do a wobble check on your valves, it may save you a lot of pain.
 
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If I had a dollar for every time my engine stumbled... KIDDING. But I will say that in 2600 hrs flying, I've had "anomalous stubles" twice in Cessnas and twice in my RV-6 (which I've been flying for 22 yrs and / 1600 hrs). I don't know what to take from that, just a data point - or more precisely, 4 of them. In any case, all were essentially one-time events, so I chalked them up to water or carb ice. Note that everything I've ever flown has been carbureted, including my -6.

It would be interesting to know (a poll?) how many folks have had an anomalous stumble in how many hours of flying.
 
I had multiple stumbles the first 20 hours. Had carb rebuilt and still an issue. Realized that though the carb was correct for a 320 on a Cessna 172, not optimal for a RV6. Upgraded my carb and solved my issues. However now when flying over very hostile terrain, my imagination runs wild with new sounds
 
I’ve Injestion?

What were the weather conditions before and during your flight? How was the plane stored prior to the flight Is there a chance that you could have had some icing somewhere in the induction system that broke free and was ingested by the rear cylinders? I’m thinking that if some moisture found it’s way in and froze, then during your climb-out engine heating warmed things enough to cause it to partially thaw and pieces were shed and ingested.

Skylor

RV-8
 
Thanks

Thanks for all the replies guys. Having read all the replies I’m leaning towards what I suspected In the first place, water in the system making its way to the rear cylinders during the climb. It’s funny, if I were flying an old Cessna 172 and it did this I’d probably just shrug and say, no biggy, probably just a bit of water considering where The aircraft was and the conditions overnight. I normally fly two very reliable RR turbines, spoils you…Sometimes having all this diagnostic information at hand leads to over analysis.

I have reamed the exhaust valve guides previously on this engine. If this event had been one cylinder I may well have considered doing that again, but not two cylinders at the same time.

I will creat a ticket with SavvyAnalysis and report back.

Thanks all.
 
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