M20F Engine Stumbling and Popping at Idle

[N201MKTurbo]

Probably the difference between the cylinders on the different mags has to do with air flow and the fact that each mag fires two top plugs and two bottom plugs.

Even though the valves are symmetric about the spark plugs, the intake charge has upward inertia because it comes from the bottom. Therefore there will be slightly more fuel charge at the top of the cylinder than the bottom. This means the cylinders firing the top plugs will be firing a slightly richer mixture than the cylinders firing the bottom plugs.

[carusoam]

Interesting observation Rich...

I am just extra focused on why two out of eight plugs are alone in their behavior...

Ever see the good plug manufacturer, have bad heavy electrode(?) (not fine wire) plugs....?

Best regards,

-a-

[Flyman2456]

I'm thinking right now I will first inspect and clean the plugs. If that doesn't fix it, move to injectors/intake, and mags/timing.

Edited September 18, 2019 by Flyman2456

[jetdriven]

2 minutes ago, Flyman2456 said:

Is the "Sniffle Valve" another name for the Fuel Drain Valve Adapter Assy? If so, I don't believe my engine (A1A) has one (see attached pages from Lycoming parts catalog).

Pages from Lycoming Parts Manaul.pdf 739.46 kB · 2 downloads

That sniffle  valve is actually a Mooney airframe part. So the bottom of the manifold may have a pipe plug in it but this is an airframe item. 

[Flyman2456]

4 minutes ago, jetdriven said:

That sniffle  valve is actually a Mooney airframe part. So the bottom of the manifold may have a pipe plug in it but this is an airframe item. 

Okay, I'll have to check that out as well...thanks!

I'll still post the times for the data when I get a free hour or so. 

[Flyman2456]

Also curious, is it normal to have different GAMI spreads at different Manifold pressures (i.e. I had a spread of 0.1GPH at WOT, and 0.4GPH at 17MP)??

[jetdriven]

That’s indicative of An induction leak. The higher  vacuum of part-throttle operation causes the vacuum leak to admit more air than at WOT. Which can change the spread 

[Flyman2456]

5 minutes ago, jetdriven said:

That’s indicative of An induction leak. The higher  vacuum of part-throttle operation causes the vacuum leak to admit more air than at WOT. Which can change the spread 

@Hector That's what we were thinking about as a possibilty too

Edited September 18, 2019 by Flyman2456

[PT20J]

1 hour ago, Flyman2456 said:

Is the "Sniffle Valve" another name for the Fuel Drain Valve Adapter Assy? If so, I don't believe my engine (A1A) has one (see attached pages from Lycoming parts catalog).

Pages from Lycoming Parts Manaul.pdf 739.46 kB · 3 downloads

The Lycoming part interferes with the muffler location. Mooney makes the right angle sniffle by modifying an AN fitting to insert a plastic ball and hold it in place with a small pin. If you suspect the sniffle valve is stuck open causing an induction leak, you can test it by temporarily plugging the drain line to see if that makes a difference.

Skip

[jetdriven]

Iirc airflow performance also makes a valve. 

[Hector]

[mention=10018]Hector[/mention] That's what we were thinking about as a possibilty too

That was my thought as well but was hoping for a bigger spread between WOT GAMI spread and the latter at 17 inches. If I had seen .1 GPH AND 1 GPH it would have been a pretty clear indication. But .1 and .4 are not that far apart, I think, so not positive if that was abnormal. Was hoping the experts here could let us know if a GAMI spread can be a little different at different power settings (I suspect probably so) and how much variation would be considered abnormal.


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[carusoam]

Expect something to happen under partial throttle...

So 17” at altitude, WOT your Gami number is going to be the best possible.... but you are above O2 alts and fewer pilots hang out there... and fuel molecules are getting pretty far spread from each other...

17” at a lower altitude with the throttle partially closed... is going to alter airflow/pressures in each intake... and probably cause undue problems with your Gami number...

 

The advantage of balanced airflow and balanced fuel flow is all cylinders producing power in a similar way.

having unbalanced fuel flow is pretty well discussed... hence the Gami fuel injectors...

having unbalanced air flow is lesser known... there isn’t a Gami intake... but they sure could sell one if they wanted to...

Anyone with nice curvy air intake runners in their engine, including cars, knows the importance of the hardware...

Bravo owners run into this often... LOP is more challenging with the log runner intake... all because of less balanced air flow that it delivers...

 

So...

It is pretty easy to expect the Gami spread to change / degrade with partial throttle...

Adding the randomness of the throttle plate blocking flow to the system, is not often beneficial...

PP thoughts only, not an engine intake engineer...

Best regards,

-a-

[jetdriven]

GAMi has a test just for this, they call it a high-low test.  I did that, and it was inconclusive. Then we pressurized the intake with the exhaust of a shop vac. Again no leaking Intake gaskets found. So I preemptively change them anyway, the simple cheap paper gaskets. But I coated them with permatex aviation sealer. Anyway after we did all that work the airplane wouldn’t start so I cracked the throttle a good bit  and then It would start but is soon as you pull the  power to idle it would shut it off cleanly. It took about three turns on the idle stop screw to get it to idle smoothly at low RPM. That’s how big of a vacuum leak we fixed. And this was on a 200 hour factory engine that hadn’t been apart. So I’m a big believer in coating  those gaskets now for sure. 

[Cruiser]

sounds more like worn cam lobes.

[Browncbr1]

Paper intake riser gaskets seem to last about 5 years.   I keep them on the shelf as an expected replacement item from time to time.  I have the white permatex Aviation sealant Byron is talking about, which I’ll try on the gaskets next time and maybe get better life out of them.  

The OP describes textbook induction leak symptoms, which I’ve had a few times.  

[jetdriven]

Permatex aviation sealer is a very dark brown, almost black. 

it’s not about getting  more Life from them, it’s about the gasket sealing when installed  

https://www.amazon.com/dp/B000HBM3QQ/ref=cm_sw_r_sms_c_api_i_9cTGDbYQ8MM1F

Edited September 19, 2019 by jetdriven

[Browncbr1]

16 hours ago, jetdriven said:

Permatex aviation sealer is a very dark brown, almost black. 

it’s not about getting  more Life from them, it’s about the gasket sealing when installed  

https://www.amazon.com/dp/B000HBM3QQ/ref=cm_sw_r_sms_c_api_i_9cTGDbYQ8MM1F

Yea, you’re right.  White bottle, black goop.  I was thinking about the permatex aviation thread sealant I also have. 

[jkarch]

How is the GAMI spread?  Unfortunately, one of the side effects of a bad intake cam lobe is a stumble at idle and #1 and #2 cylinders peaking >0.9 GPH  later than #3 and #4.  But an induction leak can cause this though it tends to cause a higher idle. Is your MAP around 13" at idle or lower?

 

https://www.savvyanalysis.com/articles/in-flight-diagnostics

"

For the high-MP test, start with a relatively high power settingi - wide-open throttle for normally aspirated engines, or MP equal to outside ambient pressure for turbocharged engines - and full-rich mixture. Write down the EGT for each cylinder.

For the low-MP test, reduce MP by about 10 inches and again write down the EGT for each cylinder.

Disregard the absolute EGT values. Instead, calculate the change in EGT ("delta") for each cylinder between the high-MP and low-MP tests. Ideally, the amount of EGT change should be roughly the same for all cylinders. If one cylinder (or two adjacent cylinders) exhibit(s) significantly less change than the others, suspect an induction system leak affecting that cylinder (or those adjacent cylinders)."

-J

[Flyman2456]

Update: Cleaned and inspected all spark plugs. Two of the champion plugs that had greater than 5k resistance where replaced with new tempest plugs. An induction leak test with soap and water was performed around all intake seals and gaskets with no leak indications.

Taxied out and did a runup and everything was perfect, RPM was stable, no stumbling. Then after taxiing a little more it started again, stumbling and popping. 

My next step is going to be to just replace all gaskets and seals on the intake. I'm thinking maybe when the engine heats up a small leak is introduced in the intake.

Will keep posting updates. Thanks. 

Evan

[Flyman2456]

On 9/17/2019 at 10:43 PM, PT20J said:

The Lycoming part interferes with the muffler location. Mooney makes the right angle sniffle by modifying an AN fitting to insert a plastic ball and hold it in place with a small pin. If you suspect the sniffle valve is stuck open causing an induction leak, you can test it by temporarily plugging the drain line to see if that makes a difference.

Skip

Is this the right angle fitting on the bottom of the sump? It has a hose connected going out the bottom of the cowling. This would be an easy test that I could perform but doesn't necessarily answer the question about why it only stumbles when the engine is hot..

[carusoam]

The sniffle valve is a modified angle fitting with a ball inside and a pin retainer that should be visible from the outside...

PP thoughts only...

Best regards,

-a-

[PT20J]

13 minutes ago, Flyman2456 said:

Is this the right angle fitting on the bottom of the sump? It has a hose connected going out the bottom of the cowling. This would be an easy test that I could perform but doesn't necessarily answer the question about why it only stumbles when the engine is hot..

Yes. 

As to stumbling when hot, one possibility is that there is an induction leak that gets worse as things heat up and expand. Gaskets where heads mate with risers seem to be problematic as well as the seals where risers mate with sump if not installed correctly.

[carusoam]

Hmmmm...

seems as if something may be changing with the temp of the engine...

Is it possible to perform the same leak tests as before, only while the engine is still hot..?

PP thinking out loud, not a mechanic...

Best regards,

-a-

[Flyman2456]

37 minutes ago, PT20J said:

Yes. 

As to stumbling when hot, one possibility is that there is an induction leak that gets worse as things heat up and expand. Gaskets where heads mate with risers seem to be problematic as well as the seals where risers mate with sump if not installed correctly.

Exactly what I was thinking. 

32 minutes ago, carusoam said:

Hmmmm...

seems as if something may be changing with the temp of the engine...

Is it possible to perform the same leak tests as before, only while the engine is still hot..?

PP thinking out loud, not a mechanic...

Best regards,

-a-

I think at this point I'm just going to replace all gaskets and seals. 

[carusoam]

Give them each a good look...

It is a real comforting feeling when you find something wrong, before replacing it...

It really helps if you find a seal is crumbling and no longer working when you try to remove it...

Or part of a seal is missing...

That kind of thing...

Best regards,

-a-