Intermittent stumble at low rpm when engine hot

[Chocks]

Got a weird one:

'64 E. IO360a1a. SMOH 150
Had this low rpm stumble before and after an engine overhaul. Worst occurrence in video link at bottom. Stumble creates RPM (100-200) drop and vacuum drop. Fuel pressure / CHT / EGT ok. Totally intermittent, but only happens when engine is hot after a medium-long range flight. Stumble only happens between idle and about 1200rpm. Completely subsides with power add. Only items out of norm are my #2 cyl runs about 40 degrees hotter than the other 3 - with balanced EGT, but recently on landing I'm noticing that #1 and #3 egt seem to drop off at landing idle. All runups, mag checks, and ignition checks ok.

Troubleshooting completed:
Engine overhaul in March 2022  about 150 hour in now.
Overhauled: Fuel Servo, Flow Divider, Slick Mags
New: Spark Plugs (gapped correctly), Plug wires, Boost Pump, Intake Boot, Sniffle Valve and tube
Existing equipment: Gami Injectors/EDM 700 (no Fuel flow)

Testing:
Run boost pump during stumble
Check mags for change per mag
Change mixture settings during
Open and close Ram air
Cover Ram air port completely
Checked intake gaskets and tubes
Checked sniffle valve and line for leaks/loose connections
Checked all metered fuel flows
Checked spark
Checked timing
Adjusted idle cutoff
Adjusted fuel flow
Cleaned air filter
Mag test after landing
Ignition test after landing



If I think of anything else, I'll edit this. Thanks for any help.

Video of worst occurrence:
https://vimeo.com/748591623

Edited September 11, 2022 by "Chocks"

[47U]

27 minutes ago, "Chocks" said:

Had this low rpm stumble before and after an engine overhaul.

Perhaps something that was not changed with the engine overhaul?  Your fuel pressure fluctuates a bit.  Is it sucking air into the fuel system, somewhere between the tanks and engine driven fuel pump?  Were fuel hoses changed at overhaul?  They can suck air and not leak externally.  Are the seals good in the tank selector valve?  Does it make a difference which tank you’re running on?

Just brainstorming here… I’m not really IO-360 knowledgable.  Someone smart will be along soon.

[takair]

I was thinking it was the typical popping I often hear on Lycomings at idle…., but the video shows something different.  Your list also covers everything I would have suggested.  If I had to guess, leaning toward fuel Vapor type issue affecting flow.  47U has some suggestions, pulling air.  How is your fuel pump cooling air set up?  I just have scat pointed at the pump and seems to work fine.  I don’t think it’s a valve, but might want to scope the valves to be sure.

[PT20J]

Idle roughness is often due to an air leak. When the engine was overhauled, did you check the sniffle valve to make sure it seals relatively well? They can get gummed up over time so that the ball doesn’t seat. Some of your Testing list isn’t clear to me. I don’t understand adjusting idle cutoff or fuel flow. The only adjustments on the RSA injector are idle speed and idle mixture. Did you set those according to the Service Manual? Did you do a soapy water pressure test on the intake tubes? How about the connection between the servo and the sump? 

The vibrating fuel pressure gauge may not mean anything. Many analog gauges have some needle vibration at lower rpm because they are accurately indicating the action of the mechanical pump which actually pulses. At higher speeds, the meter mechanism cannot respond fast enough and the effect is to average the pulses and indicate a steady flow.

The attached RSA documents have troubleshooting info.

Skip

15-338e.pdf

RSA Troubleshooting.pdf

[Sixstring2k]

17 hours ago, 47U said:

Perhaps something that was not changed with the engine overhaul?  Your fuel pressure fluctuates a bit.  Is it sucking air into the fuel system, somewhere between the tanks and engine driven fuel pump?  Were fuel hoses changed at overhaul?  They can suck air and not leak externally.  Are the seals good in the tank selector valve?  Does it make a difference which tank you’re running on?

Just brainstorming here… I’m not really IO-360 knowledgable.  Someone smart will be along soon.

If this was my plane and the fuel lines were not changed this is where I would look first, check all the fuel connections.

 

[carusoam]

Hey Chocks!
 

Any signs of fuel leaks…

Blues stains on the outside, often turn into air leaks on the inside…

Check hours on the fuel injection system… it may not have been OH’d when the engine was… it may not be behaving well…

Why is the vac gauge keeping time with the rpm…? Does that settle out at a particular rpm?

Usually, the vac pump draws excess vac, the regulator controls what the gauge sees… so it is usually pretty steady with the regulator working…

PP guessing only…

Best regards,

-a-

[Chocks]

I like all of these thoughts so far. I'll start checking fuel lines, connections and such for anything out of order. No leaks, drips or staining anywhere that I can see from the gascolater up to and around the engine. But I will check the lines from the senders into the belly if I get the guts up to pull interior panels. :\ Thanks @47Uand @Sixstring2k et all. 

@PT20JThe sniffle valve and the lines are all new and clamped tighter than a ducks butt. The fuel pressure gauge was changed out recently. The new one seems to vibrate the same as the old. So I just chalked it up to the "check it later" category. Looks like I need to revisit. A note on that though, my fuel level gauges do a very intermittent bounce sometimes, depending on the tank selected. I'll have to ponder that part. Thank you.

@takairI never even thought of that cooling tube. I have a piece of scat tube that comes out of the back of the doghouse and just kind of points in that general direction. That would be interesting to see if there is a cooling issue with that engine driven pump...

Hey @carusoam, yes the vac time does settle out above the 1200 rpm. That is something I or my A&P haven't worked through yet. I'm wondering if there is a crossover leak somewhere that would cause that inappropriate sync at low rpm, but haven't started asking yet.

[carusoam]

speaking of cooling…
 

The top back of the dog house has two cooling tubes that point at the mags…

The old generators had a tube of air coming from the front cowl… with a direct connection to the generator…

The fuel pump has a shroud that gets connected somewhere… for cooling…(fuzzy memory only)

Hot days, hot engine, getting fuel to boil is pretty easy…

This is what makes hot starts extra challenging…

PP thoughts only,

 -a-

[Chocks]

9 hours ago, carusoam said:

speaking of cooling…
 

The top back of the dog house has two cooling tubes that point at the mags…

The old generators had a tube of air coming from the front cowl… with a direct connection to the generator…

The fuel pump has a shroud that gets connected somewhere… for cooling…(fuzzy memory only)

Hot days, hot engine, getting fuel to boil is pretty easy…

This is what makes hot starts extra challenging…

PP thoughts only,

 -a-

Yes, I have the two metal tubes that come out of the back. Then I have an additional hole that takes about a 2" scat tube and it runs about a 14" length just open ended down to the fuel pump area. This is a really good area of investigation for me now. Just have to figure out a good process for testing.

[Browncbr1]

that's the kind of engine surging that seems like a clogged fuel servo screen, but it doesn't make sense because you're getting enough FF to fly it.. .. it might be worth the simple effort to clean that anyway...  

i don't think that kind of surging could be caused by the sniffle valve or intake gasket.. 

I see you had the flow divider rebuilt...  hmmmm ... I see you are checking into the mechanical fuel pump cooling shroud... that's important

 

[Shadrach]

Plug the sniffle valve outlet (elbow coming off the bottom of the airbox) and see if things improve.  Could be the combination of several induction leaks. A leaky sniffle can cause similar symptoms to those you're experiencing. 

[Browncbr1]

If you’re going to mess with the sniffle, remove it completely and soak it in 100LL or hopps.    Blow it out. Make sure the little ball inside toggles freely.     Simply tightening hose clamps on the elbow does nothing.    

[Shadrach]

12 minutes ago, Browncbr1 said:

If you’re going to mess with the sniffle, remove it completely and soak it in 100LL or hopps.    Blow it out. Make sure the little ball inside toggles freely.     Simply tightening hose clamps on the elbow does nothing.    

Not very difficult to block the sniffle drain tube to verify that it is not allowing unmetered air into the intake. If the engine runs better, then it's time to pull and soak the valve.

[Chocks]

On 9/13/2022 at 1:54 PM, Shadrach said:

Not very difficult to block the sniffle drain tube to verify that it is not allowing unmetered air into the intake. If the engine runs better, then it's time to pull and soak the valve.

That's right. It actually took longer to get the clamp off of the valve than it was to test. hehe. I actually replaced the valve, the fittings and the tube due to age, and to make sure it wasn't that. 
Good thinking!

[carusoam]

Nice pirep for Ross! @Shadrach

Go MS!

Best regards,

-a-

[Chocks]

Just as an update, I also found a post from 2018 that was dealing with a very similar issue on a J. I believe that there is a malformed face on the intake tube on cylinder 2. That would explain the level EGT's but the cylinder running hotter and leaner than all the other cylinders even with a balanced fuel flow. 
The soapy water / air hose up the exhaust test showed no leaks. But that was with the engine cold. Also, I'm unsure the fluid test can be done on a hot engine as well. So I'm going to pull the intake for that cylinder and put one of the new oversized gaskets in it, and see what the result is. Will report back with updates.

Link to earlier thread that helped with my issue: 

 

[PT20J]

1 hour ago, "Chocks" said:

Just as an update, I also found a post from 2018 that was dealing with a very similar issue on a J. I believe that there is a malformed face on the intake tube on cylinder 2. That would explain the level EGT's but the cylinder running hotter and leaner than all the other cylinders even with a balanced fuel flow. 
The soapy water / air hose up the exhaust test showed no leaks. But that was with the engine cold. Also, I'm unsure the fluid test can be done on a hot engine as well. So I'm going to pull the intake for that cylinder and put one of the new oversized gaskets in it, and see what the result is. Will report back with updates.

Link to earlier thread that helped with my issue: 

 

You said you pressurized the exhaust. Did you mean intake? Best way is to use a clean shop vac with a fresh filter. The IO-360 intake tubes are sealed to the sump with o-rings. There are no stops, and if you insert the tubes too far, the o-ring goes through the hole and can catch on a sharp lip when you pull the tube back out and be damaged. Sometimes the gasket at the cylinder head end gets installed incorrectly below the tube flange. It should be between the flange and the head.

[EricJ]

15 hours ago, PT20J said:

You said you pressurized the exhaust. Did you mean intake? Best way is to use a clean shop vac with a fresh filter. 

It's easier to do through the exhaust with the throttle closed.   Turn the prop a little if needed so that one cylinder has both the exhaust and intake valve open and the intake becomes pressurized.

[Chocks]

Still no resolution yet. No intake leaks found after the pressure test. Intake tube donut gaskets are fresh and supple. Replaced the collar gaskets with new just because they tore during tube removal. Issue is resolved when keeping the vacuum pump spooled up to anything above 4 during taxi/idle ops. I received some input from other drivers with the same engine and they all claim that is just a "Mooney Lycoming thing". I'm inclined to believe that, as I have flown that engine more than 200 hours in the last year, and there is no change or progression in the symptoms.

Next will be to make sure I'm not sticking any valves when the engine is hot, double check the rebuilt items (fuel servo and mags) for any failures and probably change from massive plugs to fine wires and see how that goes.

The search continues...

Edited October 1, 2022 by "Chocks"

[takair]

3 hours ago, "Chocks" said:

Still no resolution yet. No intake leaks found after the pressure test. Intake tube donut gaskets are fresh and supple. Replaced the collar gaskets with new just because they tore during tube removal. Issue is resolved when keeping the vacuum pump spooled up to anything above 4 during taxi/idle ops. I received some input from other drivers with the same engine and they all claim that is just a "Mooney Lycoming thing". I'm inclined to believe that, as I have flown that engine more than 200 hours in the last year, and there is no change or progression in the symptoms.

Next will be to make sure I'm not sticking any valves when the engine is hot, double check the rebuilt items (fuel servo and mags) for any failures and probably change from massive plugs to fine wires and see how that goes.

The search continues...

For what it’s worth, have been flying mine for over 25 years, 2 engines, and have not seen what you showed in the video.  I think you are right to keep investigating.

[takair]

It’s kind of hard to see the MAP in the video, but seems to be moving as well….I have seen the MaP lines chafe through or crack on the back side, hard to see, but they can introduce a leak as well…easy thing to check….

Also, be sure your fuel vents are clear….although it seems that would be worse with high power.

[Chocks]

On 10/1/2022 at 7:28 PM, takair said:

It’s kind of hard to see the MAP in the video, but seems to be moving as well….I have seen the MaP lines chafe through or crack on the back side, hard to see, but they can introduce a leak as well…easy thing to check….

Also, be sure your fuel vents are clear….although it seems that would be worse with high power.

That is a good place to look. You're talking about the manifold pressure line through the firewall? If so, I'll have to find where the calibrated hole is and make sure I don't mess with it.

The manifold pressure stays very constant, unless I lean the mixture, then I get the rpm rise and mp drop. 

[takair]

9 minutes ago, "Chocks" said:

That is a good place to look. You're talking about the manifold pressure line through the firewall? If so, I'll have to find where the calibrated hole is and make sure I don't mess with it.

The manifold pressure stays very constant, unless I lean the mixture, then I get the rpm rise and mp drop. 

The line typically starts as a hard aluminum line at the #3 cylinder, converts to copper, through the firewall and converts to flex.  They often chafe or crack near the cylinder…especially where it rounds the bend.  The bleed hole is usually on the bottom of that line near the tap.  In the short piece of video where I see MaP, it seems to move a little in rhythm with the surging.  Question is, what comes first??? 

[Kelpro999]

I had the same running condition and found it was slightly bent intake tube clamps at each cylinder intake. The clamps are aluminum and bow around the tube flange & gasket when the fasteners are tightened. The curve is in the machined recess for the tube and easy to miss. Mine was diagnosed at  300hr since lyc factory rebuild.

 

[carusoam]

See if @“chocks” is coming by….

hey… the quotes interrupt the search for “chocks”…

Some additional input has come in…

Best regards,

-a-

On 10/3/2022 at 12:51 PM, "Chocks" said:

That is a good place to look. You're talking about the manifold pressure line through the firewall? If so, I'll have to find where the calibrated hole is and make sure I don't mess with it.

The manifold pressure stays very constant, unless I lean the mixture, then I get the rpm rise and mp drop.