M20J Strange engine behavior - question to the forum

[treyizy]

So my engine has recently developed a very concerning issue that two different mechanics (one a mooney service center) are having a hard time diagnosing.

 

I regularly fly between the Bay Area and Los Angeles.  On this flight my engine works perfectly for 2 hours and 14 minutes of a 2 hour and 15 minute flight.  The behavoir that is observed is: I can feel a subtle vibration that accompanies my #1 clyinder droping massively.  The issue resolves itself in under an minute and seems impossible to recreate on the ground.  This always happens in the final decent at low power with the gear down.

 

I had my passenger video my instrument panel for the final 4 minutes of the flight which you can see below. (issue starts at 1:19, continues until 2:15).  Ignore the voltage warning light (this is unrelated and I believe it is triggering errantly becuase the left fuel light was illuminated)

 

 

 

Configuration in which the issue appears:

Long Flight

Gear Down

Descent greater than 500 fpm (though not above 1000)

Manifold pressure < 15" (in the case on the video just under 14")

RPM ~ 2500

Altitude between 2500' & 1000'

Fuel pressure normal

Oil pressure normal

Temps in normal range

Ram Air closed

 

As the issue occurs the #1 Cylinder EGT drops over 600 degrees

I also notice in the video the manifold pressure increases over an inch as the cyclinder drops (unfortunately my passenger didn't keep the manifold pressure in frame much longer to see more coorelation)

The cylinder recovers and then this cylce repeats about 4-5 times over the next 45 seconds

After this the issue resolves itself

During this time I didn't adjust the throttle, mixture or prop.

 

 

Things that have been investigated:

Inspected the Mag and did a 500 hour overhaul

Cleaned the injectors and swapped injector nozzles with cylinder 3

Checked and flow tested the spider valve (passed)

Checked the induction system and replaced all the gaskets (nothing looked odd)

Replaced the spark plugs (even though they looked good)

Borescoped the cylinder (nothing looked off)

Performed a wobble test on the valve (it passed)

 

The issue has occured about 5 times, each time after performing one of the above listed maintenance procedures.  I'm not thrilled about flying with this outstanding issue so I'd welcome any suggestions anyone has about potential causes.  We are at a loss and about to replace the entire cylinder.  Obviously I'd rather not throw $2500 away if that isn't the problem but noone has come up with a reasonable explanation.

 

One other interesting note, my approach into Oakland is much shallower than my approach into Santa Monica and it seems to happen less frequently on that approach.

 

The engine performs wonderfully during run-up and for the entire rest of the flight.  That said I'm concerned about this leading to a total power loss.  Being that I'm at a relatively low altitude over a congested area, I'd rather not be looking for a nonexistant safe landing spot short of the field.

 

As you all have seen a whole host of things I thought maybe one of you had experienced or heard of something similar and might provide some ideas.  I'm happy to answer any additional questiosn.

 

A bit about me:

J model owner for 10 years

>1200 hour IFR pilot

 

Thanks for your thoughts,

Trey

 

 

[N601RX]

My thought would be possibly a valve sticking as the engine cools down.  Also lycoming doesn't recommend long descents with less than 16" of manifold pressure. 

[treyizy]

My thought would be possibly a valve sticking as the engine cools down.  Also lycoming doesn't recommend long descents with less than 16" of manifold pressure. 

We considered that as well, though from what I've heard a valve sticking will make for quite a rough ride and this is not noticible to a passenger.  Additionally we did the valve wobble test to check the valve tollerances and they were in range.  That said it may be some slight sticking that we aren't able to pinpoint.

 

Thanks for the thoughts.

T

[MB65E]

Might pull the valve covers and check and see if the lift and duration match an unaffected cylinder. Keep the power up and lean as much as practical. Increasing the prop rpm helps slow down too.

-Matt

[MB65E]

Just watched the video again. That's pretty wild. I've seen spikes, but never drops like that! Looks like the exhaust valve just slams closed...

-Matt

[ArtVandelay]

Did you try swapping the EGT sensors, see if problem moves with the sensor?

[Marauder]

Before I dug further into the cylinder, I would either replace or swap the EGT probe. You may be dealing with nothing more than a failing probe. The roughness you are feeling could be nothing more than the automatic roughness we all feel at times. I think of it as a heightened sensivity due to a potential problem.

If the engine monitor can be downloaded, I would send a copy over to the Savvy guys.

Sent from my iPad using Tapatalk

[treyizy]

Did you try swapping the EGT sensors, see if problem moves with the sensor?

I can feel the issue in the plane, there is a noticeable vibration.  Additionally you see the manifold pressure increase.  I'm certain its not a sensor error.

[Bob_Belville]

Trey, I don't see where you've verified that the EGT probe and the engine analyzer are not involved in the problem. ISTM that such a large drop in EGT would indicate that cyl was not firing at all in which case that 4 cyl engine should be complaining. More than a subtle vibration. 

[treyizy]

I can feel the issue in the plane, there is a noticeable vibration.  Additionally you see the manifold pressure increase.  I'm certain its not a sensor error.

Additionally you'll notice I point to the indicator before you notice the indicator drop as I can feel when it happens and the indicator lags by a sec or so.

[treyizy]

I wish this were an indicator problem.  I really do. The first few times it was the feeling of the airplane that caused me to look over at the EGT.

[treyizy]

I also suspect that at this phase of flight because the power is so low that the propeller may actually be pushing the engine rather than the other way around.

[Marauder]

I agree with Bob. If you lost that much temp on a cylinder, it would be very noticeable. The other thing that sticks out is that cyclic. Strange to say the least.

Sent from my iPad using Tapatalk

[treyizy]

I agree with Bob. If you lost that much temp on a cylinder, it would be very noticeable. The other thing that sticks out is that cyclic. Strange to say the least.

Sent from my iPad using Tapatalk

It is noticeable, I never miss it, the only reason I say subtle is that my passenger wouldn't have noticed.  It is not nearly as bad as a fouled mag test.

[N201MKTurbo]

I would also suspect a sticking valve. A valve cannot stick closed without bending a push rod. A valve stuck open would keep that cylinder from sucking air in and thus the manifold pressure would tend toward ambient which is higher. That cylinder would have no compression and would generate no heat so the CHT and EGT would drop a lot. At the low power setting the vibration would not be that much.

[ArtVandelay]

That cylinder would have no compression and would generate no heat so the CHT and EGT would drop a lot. At the low power setting the vibration would not be that much.

It was hard to tell from the video, but did the CHTs drop too...I guess downloading would allow a better look at the data.

[chrisk]

It's odd that the MP went up, as the EGT went down.  To me, this suggest the cylinder was not firing, but perhaps my logic is convoluted (i.e. the engine is sucking less air, so the throttle is less of a restriction. --well it works when the engine is off, and MP = 29.92 ).  

 

How repeatable is this?  Maybe it is a fuel pressure issue?  Have you tried with the electric fuel pump on?

[N201MKTurbo]

If the valve sticks open you will hear a tapping from the rocker arm. Kind of hard to hear in flight, but you might hear it if you pay attention to it. Take your earphones off.

[bonal]

im no expert on stuck valves in desent but I am an expert on sticking valves on departure and let me tell you every one in the area of the airport notices.

i think swaping the sensors is a good idea it cost nothing to do and will rule out the instrument if it still hits on number 1. if it jumps to 3 then problem solved. that would have been the first test i would have done after all the normal checks were completed.  you may have a lifter that might be collapsing as the oil pressure drops with your low power setting, good luck sir.

[treyizy]

If the valve sticks open you will hear a tapping from the rocker arm. Kind of hard to hear in flight, but you might hear it if you pay attention to it. Take your earphones off.

I'll definitely try to listen for the tap of the valve sticking open.  Thanks for the suggestion.

[chrisk]

  you may have a lifter that might be collapsing as the oil pressure drops with your low power setting, good luck sir.

This is an interesting theory, and it got me thinking.  What RPM are you at when you experience the problem?  And have you tried recreating this on the ground after a long flight with a heat soaked engine?  Maybe its a combination of low RPMs, a hot engine, and hot thin oil?

[treyizy]

This is an interesting theory, and it got me thinking.  What RPM are you at when you experience the problem?  And have you tried recreating this on the ground after a long flight with a heat soaked engine?  Maybe its a combination of low RPMs, a hot engine, and hot thin oil?

I'm at about 2500 rpm.  I haven't tried recreating it on the ground after a long flight.  Good suggestion.

[Piloto]

Trey it appears that you have the UBG-16 engine analyzer. I had exactly the same readings and found there is nothing wrong with the engine. The sudden drops was due to the EGT wire splice/connector on the probe end. Check this connection, undo and redo the connection using contact cleaner. Mine uses a small piece of tubing that pinches the wires together with an allen screw. EI uses thermocouples which are sensitive to ohmic rsistance. You can tell if the connection is sound by applying heat with a heat gun to the probe and wiggling the probe wires while looking at the numerical display. The most you will read is about 350F so the bar will not show.

 

José  

[takair]

I just watched your video again. Sure enough, the manifold pressure goes up rapidly, 1", just as it happens and you point at it. I was leaning toward indication until I saw that. My gut now says you have a valve issue. If you did the wobble test, the only other thing I can think of is to pull the valve. Did they look at it for discoloration? Could be a seating problem too.

[Bob - S50]

Just a question first.

 

Have you tried using a different power setting for the descent to see if that solves the problem?

 

I personally like 20" and 2200 RPM.  If that doesn't give you the descent rate you want, maybe try 18" and 2000 RPM.

 

See if the problem happens at those settings.  If it solves the problem, I'm wondering if it could be a broken landing on a piston that only causes a problem when you use high RPM and low MP.  I've read that can lead to ring flutter.

 

Bob