M20K (252) Engine Stumble

[Ragsf15e]

So yesterday I had a pretty good engine stumble.  It was during climb through about 10,000'.  All knobs were full forward and everything was looking good, then there was a sudden shake, loss of power, and then boom it was all running normal again.  I wasn't sure if it would show up on engine data, but it looks like #4 EGT suddenly dropped and then quickly came back up.  Nothing else I can see (although the G3X has way more data in it).  

If a single EGT drops completely off and comes back, that has to be a blocked injector that clears, right?  I mean a plug or mag or harness wouldn't drop off completely?  Anyway, I continued and it was fine for the next hour and a half and then I flew it home today another hour an a half all perfect.  Any thoughts?

Here's a screenshot and a link to the Savvy flight.  The purple (EGT 4) has that drop right where the engine stumbled.

https://apps.savvyaviation.com/flights/shared/flight/10483998/ff3a4466-5d69-47fa-85f7-b34d03d6348c

 

[Z W]

Was the low fuel boost pump on? I see a small drop in fuel flow right about the same time as the EGT drop. Maybe some vapor in the fuel lines?

Maybe a drop of water in the fuel system working its way through?

[anthonydesmet]

Did you just refuel? Sit in the rain or refuel while it was misting. I had similar years back after I was finishing refueling and it just started to lightly sprinkle before I got the gas cap back on. I always carry a bottle of HEAT and put half in the tank if I am in doubt. Looks like you had some water.

[Ragsf15e]

6 hours ago, Z W said:

Was the low fuel boost pump on? I see a small drop in fuel flow right about the same time as the EGT drop. Maybe some vapor in the fuel lines?

Maybe a drop of water in the fuel system working its way through?

 

3 hours ago, anthonydesmet said:

Did you just refuel? Sit in the rain or refuel while it was misting. I had similar years back after I was finishing refueling and it just started to lightly sprinkle before I got the gas cap back on. I always carry a bottle of HEAT and put half in the tank if I am in doubt. Looks like you had some water.

Water or vapor lock went through my mind too, but I doubt it.  First, plane hasn’t been in the rain for months and it wasn’t yesterday (although I did refuel).  I had flown for an hour the day prior with no issues.

As far as vapor lock, I was passing 10,000 on a cool day, although the fuel pump was off.  Doesn’t seem like a recipe for vapor.

Also, it was only 1 cylinder?  I would have thought either of those issues would affect more cylinders?

[Z W]

Something went temporarily unusual with either air, fuel, or spark. I'd suspect in just that one cylinder since the rest seem stable.

Air seems least likely to have been the problem, considering you were pushing a lot of it in through your turbocharger, and it wouldn't just affect one cylinder.

Fuel seems the most likely to me. Either a temporary drop in fuel pressure or a small contaminant, like water, in the fuel. You do have an altitude-compensating fuel pump that changes pressures as you climb. How many hours/years are on your mechanical fuel pump? 

As far as spark goes, it wouldn't hurt anything to pull the spark plugs out of cylinder 4 and take a look at them. I've heard of lead deposits causing a plug to short out and not fire correctly before they burn off and it goes back to normal. Never experienced that myself though.

[Ragsf15e]

1 hour ago, Z W said:

Something went temporarily unusual with either air, fuel, or spark. I'd suspect in just that one cylinder since the rest seem stable.

Air seems least likely to have been the problem, considering you were pushing a lot of it in through your turbocharger, and it wouldn't just affect one cylinder.

Fuel seems the most likely to me. Either a temporary drop in fuel pressure or a small contaminant, like water, in the fuel. You do have an altitude-compensating fuel pump that changes pressures as you climb. How many hours/years are on your mechanical fuel pump? 

As far as spark goes, it wouldn't hurt anything to pull the spark plugs out of cylinder 4 and take a look at them. I've heard of lead deposits causing a plug to short out and not fire correctly before they burn off and it goes back to normal. Never experienced that myself though.

Yeah, I tried to look at it that way too and also came up with some kind of quick fuel disruption.  I would think that a burp in the fuel pump would affect all the cylinders, but anything is possible I guess.  It's also possible that more of the cylinders were affected, but it happened so quick that the data didn't capture them.  Luckily, it goes in for annual next week, so we'll be tearing into it anyhow.  I plan to look at the fuel filter(s) to see if there's any "gunk" in there.  I had a tank patched last spring (maybe 85 hours ago), but I don't see any particulate matter come out of my sump drains.

The reason I don't think it has to do with "spark" is that a failure of one plug, mag, or harness wire still leaves the other plug in the cylinder firing.  Firing on one cylinder would make the EGT rise (as it does in a mag check), not fall.  So that's why I didn't think it was ignition related.

Thanks for the thoughts.  I agree it's probably fuel related, so we'll at least start there next week in the annual.

Drew

[PT20J]

It's hard to see on the scale of the plot, but it looks like the FF dropped after the EGT decreased. This would make sense on a Continental if the hiccup caused a momentary rpm drop.

The EGT only dropped about 80 deg F. and came right back.

My guess would be ignition.

 

[Ragsf15e]

6 minutes ago, PT20J said:

It's hard to see on the scale of the plot, but it looks like the FF dropped after the EGT decreased. This would make sense on a Continental if the hiccup caused a momentary rpm drop.

The EGT only dropped about 80 deg F. and came right back.

My guess would be ignition.

 

Interesting.  You're probably better at troubleshooting this then I am, but I'm curious, wouldn't an ignition issue cause a rise in EGT?

We'll obviously be cleaning and checking the plugs during the upcoming annual, so we'll see if they look ok.

Not that there's much data, but i'll let you guys know what Savvy thinks when they get back to me.

Drew

[Ragsf15e]

So looking at the raw excel data, the EGT drops from 1190 to 1100 and then comes back to 1190 within 7 seconds.  There is a small but perceptible drop in rpm and fuel flow at the same time.  RPM drops by about 30 and then pops right back up.  It was stable before.  FF drops from 26.5 to 25.8 and then goes back to the mid 26s.  It is a little bumpy, but that's the only time it hit 25 in the climb.

Drew

[PT20J]

 

1 hour ago, Ragsf15e said:

Interesting.  You're probably better at troubleshooting this then I am, but I'm curious, wouldn't an ignition issue cause a rise in EGT?

We'll obviously be cleaning and checking the plugs during the upcoming annual, so we'll see if they look ok.

Not that there's much data, but i'll let you guys know what Savvy thinks when they get back to me.

Drew

Probably, but for such a short episode I was thinking about the drop in power having a greater effect than the delayed combustion.

Looking at the plot again, it did take a few seconds to fully recover. Since it was confined to one cylinder it could also be an injector nozzle having and clearing a momentary clog. I had something similar happen in my M20J that I thought was a clog, but I now believe it was a valve sticking momentarily .

[Ragsf15e]

So savvy said it’s most likely a fuel related issue (water drop, vapor, or debris blocking an injector), however they don’t really get to figure these things out until they happen more often.  One thing I learned is that there aren’t any fuel filters, just 3 screens - gascolator, electric pump, and spider/distributor.  The gascolator is the only “easy” one to check.

[Will.iam]

I was thinking stuck exhaust valve as a possibility, or some crud that kept the exhaust valve from seating thus all that compression would be lost and fuel air mixture pumped out of the cylinder. Until what ever was keeping the exhaust valve from seating was removed or the valve itself got unstuck. 

[MikeOH]

1 hour ago, Will.iam said:

I was thinking stuck exhaust valve as a possibility, or some crud that kept the exhaust valve from seating thus all that compression would be lost and fuel air mixture pumped out of the cylinder. Until what ever was keeping the exhaust valve from seating was removed or the valve itself got unstuck. 

That seems inconsistent with the drop in fuel flow.  A clogged injector would be more probable, I think.

[PT20J]

7 minutes ago, MikeOH said:

That seems inconsistent with the drop in fuel flow.  A clogged injector would be more probable, I think.

This is a Continental engine, so fuel flow is controlled by the fuel pump and is directly proportional to rpm. So, a momentary drop in rpm will cause a momentary drop in fuel flow.

I don’t see how a clogged injector nozzle would affect fuel flow (except indirectly through a  drop in rpm) as it should just cause the flow divider to redirect the flow from the pump to the remaining cylinders. 

 

[EricJ]

27 minutes ago, PT20J said:

I don’t see how a clogged injector nozzle would affect fuel flow (except indirectly through a  drop in rpm) as it should just cause the flow divider to redirect the flow from the pump to the remaining cylinders. 

The instrumentation matters.   For systems where fuel pressure is used as a proxy for fuel flow, like the stock gauges in many Mooneys, "fuel flow" actually goes up due to a clog since the pressure goes up.    In this case I *think* the case in point has an actual fuel flow meter, so as you mention it should be either unaffected or perhaps go down slightly due to the higher back pressure.   Since it is digital it may be filtered or the sample rate may not be showing any momentary changes.    Fuel pressure is expected to go up a little when a clog happens, assuming it is measured post-metering.