Induction Leak Test Question

[Llewtrah381]

I’ve been searching the interweb a bit to try and learn more about how one would locate an induction leak. This video keeps popping up and seems to be the only/best one I can find on the topic and is referenced by several other aviation sites:

https://youtu.be/CfG4QGv-r64

While talking through the procedure, around 2:15 they talk about starting the prop at top dead center for the cylinder being “charged” (they say it’s #1 but it sure looks like #3 or maybe #4 to me, unless Continentals are numbered very strangely). They then show him rotating the prop forward a bit to “charge” the cylinder and then going back, past the starting point, when you can hear it “chuff” and put air into the induction system.

That all makes sense BUT shouldn’t that be happening at BOTTOM dead center, between the intake and compression strokes? Seems like if it’s actually at TDC on the compression stroke he’d be rolling back and forth between the compression and power strokes - with no valves opening for a good while in either direction.

I haven’t seen anyone on other sites comment on this but it seems incorrect to me. Am I missing something?

The rest of the procedure seems pretty straightforward. I imagine it can be done with any cylinder and you don’t need to repeat it on any others: doing it in any one would charge the whole induction system for testing. I suppose, debatably, doing it on a cylinder from both sides would increase the “yield” by ensuring a leak on the opposite side gets a good amount of air flow but I’m not sure.

 

[455 Bravo Uniform]

You’re always going to have at least one valve open at BDC. The piston will be relatively motionless while the crank is rotating. Power stroke is ending, gotta start getting gasses out well before the upstroke is started and resisted (the combustion pressure doesn’t do much at that point). Intake stroke is ending, but you want some more air/fuel momentum to ram into the cylinder before shutting the door.

 

[Magman]

If you plug a clean shop vac or leaf blower into the Carb Heat duct you can pressurize that way. Plugs IN, Throttle OPEN and Carb Heat ON for best results.

Other methods might be better but I’ve found quite a few leaks with this system.

i think you’ll hear from others.

 

[Pinecone]

FYI Continentals number from the back, nearest to the firewall. Opposite Lycoming.

 

[Timbeck2]

Use Magman's method, or even simpler, get a strong light and a mirror and look for tell-tale blue stains on or around your induction tubing.

 

[Dan Thomas]

Use Magman's method, or even simpler, get a strong light and a mirror and look for tell-tale blue stains on or around your induction tubing.

If it's carbed.

 

[mondtster]

I haven’t needed to try it on an aircraft engine but you might search for “smoke test” and try that. Commonly used on late model cars to find air leaks.

 

[William Pete Hodges]

>>> I haven’t seen anyone on other sites comment on this but it seems incorrect to me. Am I missing something? >>>

Starting at TDC with both valves closed, charge the cylinder with air under pressure, then rotate backwards until the intake valve lifts off the seat. The result is a high volume pulse of air that blows back against the closed throttle plate pressurizing the intake runner for a short period of time. It is the same principle a Cheetah Tank uses to seat the bead on a truck tire.

https://www.google.com/url?sa=t&sou...xa8BegQIChAB&usg=AOvVaw1-MPuTrggzpc41tqGr6zGp

 

[Llewtrah381]

>>> I haven’t seen anyone on other sites comment on this but it seems incorrect to me. Am I missing something? >>>

Starting at TDC with both valves closed, charge the cylinder with air under pressure, then rotate backwards until the intake valve lifts off the seat. The result is a high volume pulse of air that blows back against the closed throttle plate pressurizing the intake runner for a short period of time. It is the same principle a Cheetah Tank uses to seat the bead on a truck tire.

https://www.google.com/url?sa=t&source=web&rct=j&url=https://www.youtube.com/watch?v=SkDTS1uaXb0&ved=2ahUKEwjcl7Sc8Nj5AhU0KFkFHfEGDkgQxa8BegQIChAB&usg=AOvVaw1-MPuTrggzpc41tqGr6zGp

I’m still confused by this. TDC would be the smallest volume the piston ever has, so you’d be very limited by the total amount of air available to “charge” it. If it was rolled backwards from TDC until just before the intake valve opens (“in reverse”), to BDC or thereabouts, you’d have the max volume for the charge. Then, rolling it back just a bit more would open the intake valve with the largest volume for the test.

At least that’s what it seems like to me, which is why I question using TDC as the start rather than BDC between the intake and compression strokes.

Again, I’m open to the idea I may be missing the obvious. Being at BDC rather than TDC seems much closer to the Cheetah Tank analogy.

 

[Magman]

OP. Are you concerned with a particular aircraft /engine symptom or just general
knowledge? O-rings, gaskets, Alternate Air can be significant variables?

An O-320 has a Bore of 5 + inches . O-470 in vid even more. Calculate the area of the piston by your favorite method ( .7854 x D squared ?) and multiply by 80 #
on every square inch. You will find you are dealing with enough force to result in a
trip to the Emergency Room.

No need for the engine to start to hurt you. BAD!

 

[Checkout_my_Six]

Not sure what is going on here....but it shouldn't be that complicated to pressurize the induction system and let the soapy bubbles do the work for finding the leak.

 

[Llewtrah381]

OP. Are you concerned with a particular aircraft /engine symptom or just general
knowledge?

Kinda more general knowledge.

My plane (a Warrior but with an O360-A4M) is in the shop for an upgrade and I had noted that #4 tends to run about 15-20 degrees hotter CHT and 100-150 or so hotter EGT than the others while in cruise. It drops more in line when I lean heavily but then tends to run rougher, so I asked them to check things out at their discretion. Haven’t seen any blue stains. I was just curious re how they test the induction system on the ground and ran into that video - and not a lot more.

So, more for my education than anything else. Of course, I could just ask them how they do it but what’s the fun in that?

 

[Llewtrah381]

Not sure what is going on here....but it shouldn't be that complicated to pressurize the induction system and let the soapy bubbles do the work for finding the leak.

Agree. That’s basically the technique in the video but with a differential compression tester providing the air via a spark plug hole rather than a leaf blower in the air intake. I saw this technique - didn’t see one with the leaf blower.

Just curious about the procedure in general

 

[William Pete Hodges]

I’m still confused by this. TDC would be the smallest volume the piston ever has, so you’d be very limited by the total amount of air available to “charge” it. If it was rolled backwards from TDC until just before the intake valve opens (“in reverse”), to BDC or thereabouts, you’d have the max volume for the charge. Then, rolling it back just a bit more would open the intake valve with the largest volume for the test.

At least that’s what it seems like to me, which is why I question using TDC as the start rather than BDC between the intake and compression strokes.

Again, I’m open to the idea I may be missing the obvious. Being at BDC rather than TDC seems much closer to the Cheetah Tank analogy.

After thinking this out a little farther, you could start at bottom dead center at the beginning of the compression stroke. Turn the air on flowing air into the cylinder but with the intake valve open it won't charge. Now move the crank in the direction of rotation until the intake valve closes, This causes the cylinder pressure to charge and tends to push the piston down in the reverse direction thus opening the valve. All of this motion should take place between BDC and about 45 degrees after BDC. Does that sound closer to what you think should be happening?

 

[Llewtrah381]

After thinking this out a little farther, you could start at bottom dead center at the beginning of the compression stroke. Turn the air on flowing air into the cylinder but with the intake valve open it won't charge. Now move the crank in the direction of rotation until the intake valve closes, This causes the cylinder pressure to charge and tends to push the piston down in the reverse direction thus opening the valve. All of this motion should take place between BDC and about 45 degrees after BDC. Does that sound closer to what you think should be happening?

Exactly! Thanks

 

[Chip Sylverne]

This reads like the perfect illustration of academic exercise vs. real world solutions i.e. the engineer (OP) vs the technician. (Look for bubbles)

I

 

[Llewtrah381]

Thanks to those who provided constructive inputs.

Given that I was curious about the test and the only video I could find on the subject was this one - and it was cited by several reputable sites - I was hoping to better understand what was going on. Although the leaf blower idea makes sense, I haven’t seen that in a video or referenced by a reputable source. Not saying it’s not a good idea - just that it didn’t come up much in my looking around.

I agree re the risk with pressurizing the cylinder but if you watch the video, Mike Kobylik doesn’t seem to be rocking it back and forth very far and doesn’t seem to be working too hard (that prop makes a good lever, I guess).

My A&P apparently didn’t find any leaks and is sealing up some cooling gaps. I plan to look closer at the results and formally run some of the “in-flight” tests for induction leaks I’ve also seen online if the same observations persist.

In any case, thanks again to those who were constructive

 

[Magman]

The baffles are supposed to force air between the cooling fins and carry the heat away. Loose baffles will allow air to bypass the fins.

Many Pipers have what looks like a” scoop” on the lower side of the rear cylinders. This baffle directs air over the fins in the exhaust port area of #4 cylinder. It also seems to get distorted which leaves a gap of approximately 3:/8 in and 2in long. This condition would allow that cylinder to have high CHT.

Worth taking a look.