Induction leaks

Ed Haywood

Ed Haywood

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Big Ed
My engine has started skipping sometimes. Does this at or near idle when taxiing after a long flight. By skipping, I mean it audibly sounds like it is missing a cylinder every 5 seconds or so. No evidence of doing so when throttle opened more than a crack.

A&P says this is a symptom of an induction leak. We had this happen when we installed the new engine 100 hours ago, though it was more pronounced and happened on descents. Was resolved by applying sealant to the cylinder head fitting for the manifold pressure gauge line.

1. How do I go about troubleshooting this issue?

2. How urgent is the need to do so?
 
See this thread:
www.pilotsofamerica.com

Very uneven Fuel flow NA O-540

So my #3 cylinder EGT is generally just around 1300 in cruise and the #4 runs at peak 1460. If I continue to lean #4 EGT goes down and #3 comes up. Many times there is a 200 degree spread. CHTs are normal. Should I be concerned? Is an issue causing such a large spread? …in descent #3 comes up...
www.pilotsofamerica.com www.pilotsofamerica.com
 
It is certainly not a 'perfect' operational situation. You may be able to find clues in 'reading' the sparkplugs looking for a lean condition. Perhaps borescope the cylinders looking at the exhaust valves.

I remember an AD or service bulletin (not necessarily your airplane/engine) but it involved sealing off the exhaust (plumber plug(s) or duct tape) and the crankcase vent, then setup to apply (low) air pressure to the induction through the carb or servo or even with carb/servo removed. Can be regulated compressed air(20 psi or less) or even the exhaust air of a shop vac (through cheese cloth filter?) then soapy water in a squeeze spray bottle looking for bubbles,,,

I am now subscribed and waiting to see how this unfolds for you.
Please let us know how it turns out, what you find, how you find it, and what it takes to correct it.
Cheers
 
Yep, sounds like an air leak. Easiest thing to check first are the intake gaskets for rips or cracks and the intake clamps for proper tightness. Is it flight critical? Not really, but I’d want it addressed as soon as possible.
 
Think of it as unfiltered air sneaking in. As it gets worse, it may cause a lean condition. If the leak is a loose connection in the intake, the loose parts can chafe from vibration and now you potentially have debris in the intake, valves, rings etc.
 
RyanB said:
Yep, sounds like an air leak. Easiest thing to check first are the intake gaskets for rips or cracks and the intake clamps for proper tightness. Is it flight critical? Not really, but I’d want it addressed as soon as possible.
Click to expand...

Would a leak in the intake tube from the air filter to the airbox cause this? If so, that is probably the issue. The Decathlon and Citabria have a short section of large diameter tube clamped to the cowl on the backside of the air filter. It slip fits over a flange on the airbox, with no clamp. There is plenty of play in the mounting of the cowl to the firewall. It is easy to misalign the tube so that it bows and does not seal well. Would be easy to check this by switching to alt air. Easy to fix by exercising more care in aligning parts when remounting cowl.

intake.jpg
 
It would need to be leaking downstream from the carburetor. The cylinder vacuum pulls uncarbureted unfiltered air, resulting in a lean condition and a misfire. If you have a leak upstream of the carb, the air is still mixed with fuel, it just doesnt get filtered first and you probably wouldnt notice any difference in how the engine runs.
 
Good point^^. Could be a magneto issue but you only NOTICE it at idle.
 
moparrob66 said:
It would need to be leaking downstream from the carburetor. The cylinder vacuum pulls uncarbureted unfiltered air, resulting in a lean condition and a misfire. If you have a leak upstream of the carb, the air is still mixed with fuel, it just doesnt get filtered first and you probably wouldnt notice any difference in how the engine runs.
Click to expand...
Engine is fuel injected.
 
I used to look for automotive vacuum leaks (induction related) by spraying carb cleaner at hoses and fittings while engine was running. A change in RPM identifies a leak.

To rule out ignition, turn one mag off while idling and taxiing (after a long flight when you say it occurs). See if the skip goes away. If not, try the other mag off and see if it goes away. If not, you’ve ruled out mags, wires, and plugs.

Last would be a dirty injector nozzle that can’t function with a fine spray pattern at low fuel pressures. Would cause a misfire.

Sounds like heat (after a flight) is what triggers it.
 
Since it was previously found to be the MP gauge line I'd suggest that would be the first thing to check
 
Several times I’ve mentioned that the Big Box stores are selling

Slick mags with outstanding Service Bulletins. Sometimes 3 on 1 mag!

New might not be better if it has a defect.
 
If it was just one plug, the other would still fire the mixture at idle, at least, and we wouldn't get that total occasional misfire. At runup speeds and pressures such a plug should make itself known instantly.

Lycomings use a thin gasket at the intake flange at the head. The flange ring forces the flanged intake tube against the gasket. There is very little useful contact area. If the pilot is careless (students, usually) and accidentally switches the mags to to Off when doing the runup mag check, then switches it back on, the flame as the mix afterfires in the exhaust system can travel back through whichever cylinder is at the top of its exhaust stroke, where the valve overlap has both valves open, and this sets off the intake mixture as well. The blast from that can split that little intake gasket and push it out. I have found whole chunks of gasket missing there. Big induction leak.

Another factor involving that gasket is that it compresses and takes a set, and a few hours after overhaul one can find the torques on those flange bolts quite low, allowing leakage and easier blowout.

SDS makes an uncertified flange for that job. It uses an O-ring instead of the gasket. It's blowout-proof, and since there is no gasket to squish, the retaining flange goes solidly against the head and the bolt torques should not change.

1723419632970.png

And of course, make sure the induction tube connection hoses are in proper position and the clamps are snug.
 
Follow up on this: more care and attention to lower cowl positioning made the issue mostly disappear.
 
To make sure I'm understanding; the engine was missing or 'skipping', and adjusting the position of the cowling (how?) made this problem go away?
 
Ed Haywood said:
Follow up on this: more care and attention to lower cowl positioning made the issue mostly disappear.
Click to expand...
"Mostly?" You still have a problem. And it isn't likely related to the cowl positioning.
 
Let'sgoflying! said:
To make sure I'm understanding; the engine was missing or 'skipping', and adjusting the position of the cowling (how?) made this problem go away?
Click to expand...

The left picture is the air intake tube from the air filter in the cowl. The right picture is the air intake box on the bottom of the fuel servo.
intake (1).jpg airbox.jpg


The intake tube slips over the oval flange on the front of the air box. It is a loose fit, with no clamp. Suction pulls the hose inward so it seals on the bead rolled in the flange.

There is a lot of play in the cowl until the fasteners are tightened. If the cowl is not aligned with the air box and at the correct depth, the hose kinks and will not make contact with the bead around the entire perimeter of the flange. With the air filter removed, it is easy to see.
 
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