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M20F Engine Stumbling and Popping at Idle


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Hey Guys,

So recently I have been having an issue where the IO-360-A1A starts very nicely, but then the engine stumbles every few seconds at idle with occasional popping/quiet backfire. Tried lean, rich, fuel pump...no adjustments made with the fuel seemed to help. The issue goes away when adding power (>~1800RPM) but when I bring it back down, the stumbling/backfiring starts happening again. All EGTs and CHTs looked normal throughout. I checked all the bolts on the intake to make sure they were tight, which they were. 

I downloaded the data from the JPI and couldn't spot any issues. Before I go pulling/inspecting fuel injectors and spark plugs, I was hoping someone might be able to spot the issue in the data... 
 
NOTE: I noticed the issue on 9/8/19 so all the data from that date forward had the issue. For the U190915A.JPI file, I changed the sampling rate of the JPI down to 1 sec. 

Thanks in advance!

U190915.JPI U190915A.JPI

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3 minutes ago, carusoam said:

Please send the JPI data to Savvy... then copy the link here...

i clicked on the JPI files and my iPad’s eyes crossed...

Best regards,

-a-

https://www.savvyanalysis.com/flight/3461969/c1763104-2558-4ee7-84aa-d1a984f7534f

https://www.savvyanalysis.com/flight/3461967/0f532371-5415-4a7e-90cd-030e8fdfbe4b

https://www.savvyanalysis.com/flight/3461965/4c99944c-2c58-4632-8785-c5452df8cd1e

These all had the issue. Thank you. 

-Evan

Edited by Flyman2456
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The first file....

EGT#2 looks like it stands out....

It is showing what looks like a sticking valve.... saw tooth pattern of the graph line.

The kind of valve stick, where the valve doesn’t rotate...

 

The second file...

EGT#2 looks a bit hotter than the others as if it FI may be slightly blocked...?

 

Got any files where you have done a run-up slow enough to allow the EGTs to rise, stabilize, then fall...?

 

See if Paul is around... @kortopates

 

PP thoughts only, not a mechanic...

Best regards,

-a-

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The sawtooth was interesting but I was wondering if that's just because I switched the sampling rate from 6 seconds to 1 second (compared to the previous flights). You'll notice in the first file that other EGTs exhibit the sawtooth later on in the data as well albeit not quite as much as EGT #2.

You thinking intake or exhaust valve?

 

Edited by Flyman2456
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See if you can find the explanation for the saw tooth pattern on the Savvy website....

Expect that exhaust valves see more stress than intake valves...

And I have no explanation for the saw tooth pattern on the CHTs... but they are doing it uniformly... so it is more instrument than actual...

 

a well orchestrated run-up captures a lot of data than telling if the mags are both working...

PP thoughts only...

Best regards,

-a-

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A note on stumbling and popping...

If the spark doesn’t ignite the fuel... it causes a stumble...

The unburned fuel then goes on to the exhaust where it encounters something hot and ignites there...

There are a few reasons for fuel not getting lit... most often spark plug related or mag/ electrical issue... or as Mr. Brown indicated a subtle change in air/fuel ratio on one cylinder.....

check your logs to see what plugs you have... if they are Champion... get ready to measure their resistance values... but first... identify what cylinder and plug is misbehaving...

The good clean run-up is usually pretty good at identifying these...

Best regards,

-a-

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I had these symptoms once before and it turned out the be an induction leak.  Replaced intake riser gaskets and problem went away.  
I’d make sure my plugs aren’t lead fouled first though.  


Worth trying but if one of the plugs was bad it would fail the mag check on run up (the cylinder would go cold when you select that plug)? Is there a failure mode for a plug that would not show up on the JPI data and would result in low idle stumbling? If a fuel injector was partially blocked I would think he would have an issue at any power setting not just idle but I guess the problem could be aggravated by the already low fuel flow at idle? Induction leak due to a bad/worn gasket still a possibility but wouldn’t that show up on the JPI data as well?


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Evan,

Read up on the flight test profiles on Savvy... it gives some good ideas on standardizing the test data... making it a bit easier to perform and get more out of...

 

Otherwise, to follow the ground runs and flights without knowing what you were doing each minute or five... is a bit tough.

Your JPI 900 has a lot of data in it...

But, i’m a bit Surprised you are having stumbling and popping and we are not able to put a finger on it immediately...

 

Those are signature issues and shouldn’t be too hard to find with the JPI data...

The hour long flight around Herlong wasn’t showing any maladies that I could see...

 

The saw tooth pattern I thought I was looking at was not part of a cruise flight where the sawtooth really holds meaning...

The first file looks like 25 minutes of ground running with a few mag tests done in pairs... but.... so little EGT data was collected each time, there isn’t much to go on...

With a good run-up... you will see all eight spark plugs performing... EGT vs time... for both Lmag and Rmag.... with enough time like 20-30 seconds or more on each key position...

The data I am seeing looks like the key may have been flipped for a second or two to see a reaction and then flipped back... the only thing that changed was rpm briefly...

If you have you data set for one second... you will find the engine doesn’t react that fast... (a good thing) allow the engine to react and stabilize...

 

No need to fly, until some long run-ups can be performed... spend extra time on each, single mag... watch the display for the EGTs to rise and stabilize, then fall and stabilize each time...

Then post that data...

In the event you have done this already... select that flight’s data and describe the graph as each step was taken...

1)start-up

2) or taxi

3) taxi

4) run-up

5) T/O

6)  Climb

7) Cruise

8) Leaning In Cruise...

9) Cruise mag check.

 

That kind of thing...

Is any of this making sense?

Best regards,

-a-

 

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1 hour ago, Browncbr1 said:

I had these symptoms once before and it turned out the be an induction leak.  Replaced intake riser gaskets and problem went away.  

I’d make sure my plugs aren’t lead fouled first though.  

I had a similar issue with the induction seals failing.  Same symptoms as described.

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Nobody asked yet, so I will, what has been your normal practice with the mixture after starting the engine and all the way until the runup?  What about after landing, where is the mixture while you’re taxiing to park?

It needs to be pulled so lean that the engine almost stumbles.  In fact, if you add power to taxi, you may have to give it a half turn rich just to keep it running.  You can richen it up for runup if you desire.  Obviously for takeoff, full rich density altitude dependent.

Edited by Ragsf15e
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@carusoam

35 minutes ago, carusoam said:

Evan,

Read up on the flight test profiles on Savvy... it gives some good ideas on standardizing the test data... making it a bit easier to perform and get more out of...

 

Otherwise, to follow the ground runs and flights without knowing what you were doing each minute or five... is a bit tough.

Your JPI 900 has a lot of data in it...

But, i’m a bit Surprised you are having stumbling and popping and we are not able to put a finger on it immediately...

 

Those are signature issues and shouldn’t be too hard to find with the JPI data...

The hour long flight around Herlong wasn’t showing any maladies that I could see...

 

The saw tooth pattern I thought I was looking at was not part of a cruise flight where the sawtooth really holds meaning...

The first file looks like 25 minutes of ground running with a few mag tests done in pairs... but.... so little EGT data was collected each time, there isn’t much to go on...

With a good run-up... you will see all eight spark plugs performing... EGT vs time... for both Lmag and Rmag.... with enough time like 20-30 seconds or more on each key position...

The data I am seeing looks like the key may have been flipped for a second or two to see a reaction and then flipped back... the only thing that changed was rpm briefly...

If you have you data set for one second... you will find the engine doesn’t react that fast... (a good thing) allow the engine to react and stabilize...

 

No need to fly, until some long run-ups can be performed... spend extra time on each, single mag... watch the display for the EGTs to rise and stabilize, then fall and stabilize each time...

Then post that data...

In the event you have done this already... select that flight’s data and describe the graph as each step was taken...

1)start-up

2) or taxi

3) taxi

4) run-up

5) T/O

6)  Climb

7) Cruise

8) Leaning In Cruise...

9) Cruise mag check.

 

That kind of thing...

Is any of this making sense?

Best regards,

-a-

 

Thank you for your input. Correct, the saw tooth observed was only during the ground runup. I did not experience any stumbling or popping during flight. I can spend more time during the ground run collecting data and doing the mag checks if that would be helpful. I will upload my results. Thanks!

@Ragsf15e I have been leaning very aggressively during taxi. Like you said, just slightly rich of the point it runs rough. 

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An IO-360 should idle well at full rich at sea level if idle mixture is set correctly. Ground leaning increases combustion temps to aid lead scavenging and reduce plug fouling. 

If the roughness is typical Lycoming morning sickness caused by exhaust valve sticking it should only happen after a cold start. When the engine is warm, the valve will usually free up. 

There are backfires and afterfires - both can make a pop. Backfires are due to too lean a mixture which burns slowly so there is still fuel burning when an intake valve opens causing ignition of the mixture in the intake manifold. This can be caused by an induction air leak. Less common afterfires occur with a rich mixture that allows unburned fuel to accumulate in the exhaust and is ignited when an exhaust valve opens. This is what happens if you shut off the ignition at cruise power for a few seconds and turn it back on (don’t do that - the overpressure can damage the exhaust components). 

Skip

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We are getting closer....

Another thing to look for... is fuel flow at idle... (full rich)

Idle should be set at about 700rpm...

 

There is a test for this as well... during shut-down slowly leaning towards idle cut-off... the rpm should increase as the mixture leans...

If the FF isn’t set properly or air is leaking in (possibly) the rpm won’t rise before the engine stops...

Are you familiar with this?

 

If there is an intake gasket leak... it is usually one intake affected... guess where to look to find out which cylinder is affected...?

JPI.

:)

Best regards,

-a-

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9 minutes ago, carusoam said:

We are getting closer....

Another thing to look for... is fuel flow at idle... (full rich)

Idle should be set at about 700rpm...

 

There is a test for this as well... during shut-down slowly leaning towards idle cut-off... the rpm should increase as the mixture leans...

If the FF isn’t set properly or air is leaking in (possibly) the rpm won’t rise before the engine stops...

Are you familiar with this?

 

If there is an intake gasket leak... it is usually one intake affected... guess where to look to find out which cylinder is affected...?

JPI.

:)

Best regards,

-a-

Don't know about idle FF, but pulling Throttle to ICO at 1000 RPM should result in about 50 RPM rise.

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25 minutes ago, carusoam said:

We are getting closer....

Another thing to look for... is fuel flow at idle... (full rich)

Idle should be set at about 700rpm...

 

There is a test for this as well... during shut-down slowly leaning towards idle cut-off... the rpm should increase as the mixture leans...

If the FF isn’t set properly or air is leaking in (possibly) the rpm won’t rise before the engine stops...

Are you familiar with this?

 

If there is an intake gasket leak... it is usually one intake affected... guess where to look to find out which cylinder is affected...?

JPI.

:)

Best regards,

-a-

Do you recommend going full rich for the run up?

Also how would you determine what cylinder has an intake leak? 

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How about doing a GAMI spread test at high and low power? At higher power an induction leak may not have that much of an effect on the GAMI spread( MP is already near ambient pressure) but at low power and induction leak would present itself in a really bad GAMI spread.


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1 hour ago, PT20J said:

An IO-360 should idle well at full rich at sea level if idle mixture is set correctly. Ground leaning increases combustion temps to aid lead scavenging and reduce plug fouling. 

If the roughness is typical Lycoming morning sickness caused by exhaust valve sticking it should only happen after a cold start. When the engine is warm, the valve will usually free up. 

There are backfires and afterfires - both can make a pop. Backfires are due to too lean a mixture which burns slowly so there is still fuel burning when an intake valve opens causing ignition of the mixture in the intake manifold. This can be caused by an induction air leak. Less common afterfires occur with a rich mixture that allows unburned fuel to accumulate in the exhaust and is ignited when an exhaust valve opens. This is what happens if you shut off the ignition at cruise power for a few seconds and turn it back on (don’t do that - the overpressure can damage the exhaust components). 

Skip

Agreed.  I was mostly wondering if he was running too rich on the ground and fouled a plug.  Probably not, but relatively easy to diagnose, confirm, and fix, so thought it was worth asking.

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8 hours ago, Flyman2456 said:

Do you recommend going full rich for the run up?

Also how would you determine what cylinder has an intake leak? 

1) full rich for the run-up May depend on DA... most often I use full rich for the run-up mostly in prep for T/O...

2) Expect an air leak in the intake system to cause a lean situation because the air/fuel ratio is getting more air... when one cylinder is more lean than the next, EGTs will be hotter on the lean cylinders.., they will peak first... when LOP,  they will be leaner and colder....

3) use caution when leaning on the ground... memory challenges and loose checklist procedures can leave the mixture out during full power T/O runs...

PP thoughts only, not a CFI or memory expert...

Best regards,

-a-

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So...

we have two strong contenders in the discussion...

1) a dead/dying plug...

2) an induction leak...

 

Find the dead plug using proper run-up technique. It can’t hide from the engine monitor...

Check the logs to see what plugs are in there... if Champions, read up on resistance measurements... how many hours do they have?  How football shaped is the electrode?

The other brand isn’t error free... but, its challenge is slightly different...

Often, lower spark plugs collect things like lead balls and burned oil drips... find the dead plug, pull it, take pics... clean it and take pics... measure its resistance, report back...

What plugs do you have?

PP thoughts only, not a mechanic...

Best regards,

-a-

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