Exhaust and Intake Leaks

[INA201]

Hopefully this will help others with this question.   I keep a decent inspection of my engine, change the oil, and am mechanically above average but I don’t get the issues with exhaust and induction leaks.  I’m told by a highly reputable mechanic that I have some small exhaust leaks where the pipes fit together and an induction leak where the seals are not particularly tight. I’m just wondering how having some minor leaks is an issue.  What are the advantages of having everything perfectly tight. I did request to have everything looked at closely. I could see the exhaust pipes not fitting perfectly tight but never thought it was an issue. The induction leaks they tested for with some type of fluid I believe. Can someone explain how this effects performance and the importance of a tight system.  It seems to me that an induction leak would just mean the engine was getting air but not 100% through the pipe leading to the engine. Exhaust leak just seems like a tiny bit might leak around the edges of the pipe while most of the gas goes on out. I don’t fully get it or is it overkill to have it perfect. Thanks 

[N201MKTurbo]

I don't think they mean anything. You would just be wasting money chasing them down.

Others disagree.

[Hector]

I would not want leaks in either induction or exhaust. Induction leaks will result in lean mixture for the affected cylinder. It will eventually develop into backfire or popping sound at idle settings like when you pull the throttle in short final. You will see a rapid EGT increase in that cylinder while leaning (or maybe not if you don’t have an engine monitor or it’s not cylinder #3). You will have an overly lean cylinder while running the others too rich. Not a very efficient way to run your engine.


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[carusoam]

1) Air leaks into the intake will show up  in the Gami spread...

2) exhaust leaks can be important as the CO starts to show up in the cabin...

Where these leaks are and how easy they are to fix depends on how mechanically apt you may be?

Is it a slip joint or something that can be taken apart cleaned up and reassembled?

Is it a seal that needs to be replaced?

Somethings are pretty easy to identify.

Somethings are pretty easy to understand, when pointed out.

Other things get a pic posted on MS...  titled what causes this stain?

PP thoughts only, not a mechanic...

Best regards,

-a-

 

[INA201]

The intake seal was a little off and a new gasket put in and sealed up with the intake pipes sanded and repainted. The exhaust was sent out for repair as it didn’t fit together tightly.  I have noticed a little bit of rough idle sort of popping and light backfire but was very mild and at really low RPM. I didn’t worry about the low RPM popping sound because it was very mild and usually only occurs after a hot start when taxiing.  Weird, the light rough idle that I thought was normal only happens after refueling or restarting after sitting a few minutes but I bet that is the small induction leak.  I don’t get why it matters where the engine gets its air from? Doesn’t the engine/cylinder just suck in the air it needs no matter where it comes from? 

BTW thanks for the responses above, makes sense.

[Hector]

The intake seal was a little off and a new gasket put in and sealed up with the intake pipes sanded and repainted. The exhaust was sent out for repair as it didn’t fit together tightly.  I have noticed a little bit of rough idle sort of popping and light backfire but was very mild and at really low RPM. I didn’t worry about the low RPM popping sound because it was very mild and usually only occurs after a hot start when taxiing.  Weird, the light rough idle that I thought was normal only happens after refueling or restarting after sitting a few minutes but I bet that is the small induction leak.  I don’t get why it matters where the engine gets its air from? Doesn’t the engine/cylinder just suck in the air it needs no matter where it comes from? 
BTW thanks for the responses above, makes sense.

Not exactly, your fuel injection or carburetor (don’t know what plane you fly) is trying to feed a correct mixture of air/fuel to each cylinder. You can adjust that air/fuel ratio using the mixture control in your plane. If you have an induction leak in a cylinder then that cylinder is getting a higher air/fuel ratio than your other cylinders; an overly lean mixture. In a fuel injected engine this will result in a lousy GAMI spread, badly tuned injection system, and you may not be able to run LOP. In a carbureted engine, which already have uneven fuel distribution by nature of the carburetor it will make fuel distribution even worse.


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[Bob - S50]

35 minutes ago, INA201 said:

I don’t get why it matters where the engine gets its air from? Doesn’t the engine/cylinder just suck in the air it needs no matter where it comes from? 

BTW thanks for the responses above, makes sense.

It can make a difference at takeoff power.  We always run very rich at takeoff to prevent detonation.  If you have an induction leak, the mixture will be leaner than it should be which will reduce your safety margin.  A very small leak?  Probably OK.  A small leak?  Maybe OK.  A big leak?  Who knows.  If you have fuel flow, check it at full power.  I believe it is supposed to be in the 18 - 20 GPH range at full power at SL.  If you are seeing something significantly lower than that (16 GPH?), you could be asking for trouble.

At cruise power, if all the cylinders leak the same amount it won't matter.  However, that is unlikely to be the case.  Given uneven leaks, the cylinders with the biggest leaks we be the leanest.

If you have a 4 cylinder engine monitor, lean the engine and check the spread.  The easiest is to see the difference in degrees LOP.  That is, let's say that by the time the last cylinder peaks, the first cylinder is 100F LOP.  That's a big difference and the engine probably won't run smoothly LOP.  If all 4 are within 30F or so, you're probably good to go.

If you run ROP, just make sure the leanest of the cylinders is 100F ROP.  Using the example above, the richest may be 200F ROP which is pretty inefficient.

[carusoam]

Nice detailed explanations, Hector and Bob...

 

The other issue... things that occur after a restart... the engine and surrounding areas have been heated up.  The chances of vapor bubbles occurring in the fuel line is increased.  Things that are on the hot-start procedure may help...

Popping in the exhaust is usually a signature of excess fuel making its way to the exhaust system and igniting there...

 

Good to have an engine monitor for these kind of issues...

might be as simple as a champion spark plug not working, or FF asking for an adjustment...

Speaking of FF, are you familiar with the rpm rise when going to idle cut-off when killing the engine?

The engine is no longer in rpm control at low power settings, leaning the mixture slowly to cut-off will normally cause an rpm rise with the leaner mixture......

I don’t remember popping being normal for an IO360 while idling...  a few various engines will experience popping when reducing power to descend for a landing... a time when excess fuel is more challenging to control... (not steady state)

PP thoughts only, not a mechanic...

Best regards,

-a-

[INA201]

It all makes sense.  I'm flying a 1978J with an IO-360.  The engine runs really well imop but does exhibit symptoms of slight popping at low RPM after a hot restart and on the power decrease close to landing.  I just got a JPI w fuel flow installed so I can figure out the FF on takeoff and see where it is before and after the repairs.  I think i'm less than 18gph on takeoff but I need to download the data to be sure.

[carusoam]

Often, people post pics of their JPI graphs...

It helps to send them to Savvy first, Using their website...

Their website also includes a few hints on how to collect the most usable data, not just any data.

MSers eyes are pretty good at analyzing any data.  But, start-up to shutdown with a flight in between can be a lot of data.

Performing a good clean run-up always helps...

My run-ups got better execution after I saw some of my own data...

Some MSers graphs have an envious amount of data available... including FF and MP...  

Now you have a JPI with lots of data... do you check MP and FF before airspeed comes alive? (Because you can)

PP thoughts only, not a CFI....

Best regards,

-a-

[Hector]

It all makes sense.  I'm flying a 1978J with an IO-360.  The engine runs really well imop but does exhibit symptoms of slight popping at low RPM after a hot restart and on the power decrease close to landing.  I just got a JPI w fuel flow installed so I can figure out the FF on takeoff and see where it is before and after the repairs.  I think i'm less than 18gph on takeoff but I need to download the data to be sure.

Might be interesting to do a GAMI test before and after repair of the induction leak. If the leak is significant it should show a much closer spread after the repair


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[carusoam]

Ina,

https://www.savvyanalysis.com/home

If you need details for Gami spreads and things like that...

We have an MSer at the core of the Savvy group...  

Of course the center of the Gami lean test comes from these good people...

http://www.gami.com/gamijectors/leantest.php

Best regards,

-a-

[N201MKTurbo]

I’ve explained it a few times before, but induction leaks in a fuel injected lycoming will not make the mixture uneven. The fuel is injected after any leaks. 

Any leaks will affect the MP in the intake manifold, making it a bit higher, but it will be the same for all cylinders. The fuel injected will be equal, so no difference in mixture.

[Hector]

Induction leak test
This in-flight test is an effective method for detecting leaks in the engine's induction system. It is best accomplished in level cruise flight at about 5,000 feet MSL. It consists of a pair of tests: the high-MP test and the low-MP test.
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).
Here's the principle behind this test: During the high-MP test, the induction manifold pressure is very close to outside ambient pressure, so any induction leak will have little or no effect on engine operation. During the low-MP test, the manifold pressure is significantly lower than outside ambient (by about 10 inches), so any induction leak will cause the affected cylinder (or cylinders) to run substantially leaner than the others, resulting in a smaller drop in EGT than the others.

Above from Savvy.


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[N201MKTurbo]

Your exhaust leaks can make it pop with a very rich mixture and low power. It will suck air in through the leaks which will react with the hot rich mixture.

It doesn’t hurt anything. If it is leaking at the slip joints, you can clean them up real good and reassemble them with anti sieze. It will seal them up for a while. Or you can spend hundreds or thousands of dollars to get your exhaust overhauled.

[N201MKTurbo]

4 minutes ago, Hector said:

Induction leak test
This in-flight test is an effective method for detecting leaks in the engine's induction system. It is best accomplished in level cruise flight at about 5,000 feet MSL. It consists of a pair of tests: the high-MP test and the low-MP test.
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).
Here's the principle behind this test: During the high-MP test, the induction manifold pressure is very close to outside ambient pressure, so any induction leak will have little or no effect on engine operation. During the low-MP test, the manifold pressure is significantly lower than outside ambient (by about 10 inches), so any induction leak will cause the affected cylinder (or cylinders) to run substantially leaner than the others, resulting in a smaller drop in EGT than the others.

Above from Savvy.


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This would be true for a carbureted engine. I doubt you would see any difference in a fuel injected Lycoming.

[Hector]

This would be true for a carbureted engine. I doubt you would see any difference in a fuel injected Lycoming.

This is the procedure recommended by Savvy for a fuel injected engine. I would think it should work for both but have never tried it in my C.


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[N201MKTurbo]

I would be very curious to see the numbers from someone with leaky gaskets, before and after.

I have had an intake riser come compleatly off. Other than the mixture knob being further in than normal, the engine ran just fine.

[carusoam]

Rich,

any idea what allows the fuel injector to adjust FF on the fly?

Anything to do with the vent hole it has... the hole that is supposed to point one way, but leaks fuel out if it is pointed the other way...?

Just wondering...

Best regards,

-a-

[N201MKTurbo]

7 hours ago, carusoam said:

Rich,

any idea what allows the fuel injector to adjust FF on the fly?

Anything to do with the vent hole it has... the hole that is supposed to point one way, but leaks fuel out if it is pointed the other way...?

Just wondering...

Best regards,

-a-

The RSA-5 adjusts fuel flow according to air flow through the servo. It measures air flow by the pressure difference between the four tubes at the inlet and a port in its throat (Venturi). The four tubes act as Pitot tubes and the pressure increases with air flow. The pressure decreases on the venturi port with air flow. These two pressures go to opposite sides of the air diaphagram in the pressure regulator.

The hole you mention is used to admit atomization air into the injector nozzle. It has nothing to do with controlling the fuel pressure to the nozzles.

[INA201]

8 hours ago, N201MKTurbo said:

I would be very curious to see the numbers from someone with leaky gaskets, before and after.

I have had an intake riser come compleatly off. Other than the mixture knob being further in than normal, the engine ran just fine.

Just got the JPI 900 about 2 months and 20 flight hours ago.  I'm getting the plane looked at by an MSC for some minor issues and my distrust of the mechanic who did the last annual.   I haven't downloaded data off the JPI so I'm guessing all the before and after info will be stored on it.  We are doing Dual mag rebuild, new ignition harness,  in addition to tightening up the intake and exhaust.  Apparently there is an AD on the Bendix fuel servo that we are complying with as well.   It'll be interesting to see with all this being done at once.

 

25 minutes ago, N201MKTurbo said:

The RSA-5 adjusts fuel flow according to air flow through the servo. It measures air flow by the pressure difference between the four tubes at the inlet and a port in its throat (Venturi). The four tubes act as Pitot tubes and the pressure increases with air flow. The pressure decreases on the venturi port with air flow. These two pressures go to opposite sides of the air diaphagram in the pressure regulator.

The hole you mention is used to admit atomization air into the injector nozzle. It has nothing to do with controlling the fuel pressure to the nozzles.

So if the RSA-5 adjusts the fuel flow then any induction leaks after it will effect the mixture going to the cylinders?  

[Bob - S50]

10 hours ago, N201MKTurbo said:

I’ve explained it a few times before, but induction leaks in a fuel injected lycoming will not make the mixture uneven. The fuel is injected after any leaks. 

Any leaks will affect the MP in the intake manifold, making it a bit higher, but it will be the same for all cylinders. The fuel injected will be equal, so no difference in mixture.

Can you point me at the thread where you explained that?  It seems to me that the amount of fuel being sent to each cylinder would be the same regardless of induction leaks.  After all, it's just being metered by the spider.  However, the amount of air going to the cylinders might be different if there is an induction leak.  And as explained by Hector, that would me most noticeable at low MP (and thus low power) settings.

And after reading Hector's comment, I guess my earlier comments about problems at takeoff really don't apply.  At takeoff, the throttle will be wide open, the difference between MP and ambient will be only slight, and very little extra air will be drawn in.  The mixture should be fine.  By the time there is a significant difference between ambient and MP, the power will be so low it won't hurt anything.  The only result will be that the mixture will be getting leaner as the power is reduced.

[N201MKTurbo]

43 minutes ago, Bob - S50 said:

Can you point me at the thread where you explained that?  It seems to me that the amount of fuel being sent to each cylinder would be the same regardless of induction leaks.  After all, it's just being metered by the spider.  However, the amount of air going to the cylinders might be different if there is an induction leak.  And as explained by Hector, that would me most noticeable at low MP (and thus low power) settings.

And after reading Hector's comment, I guess my earlier comments about problems at takeoff really don't apply.  At takeoff, the throttle will be wide open, the difference between MP and ambient will be only slight, and very little extra air will be drawn in.  The mixture should be fine.  By the time there is a significant difference between ambient and MP, the power will be so low it won't hurt anything.  The only result will be that the mixture will be getting leaner as the power is reduced.

You are compleatly correct. 

[kortopates]

On 7/3/2018 at 9:12 PM, Hector said:

Induction leak test
This in-flight test is an effective method for detecting leaks in the engine's induction system. It is best accomplished in level cruise flight at about 5,000 feet MSL. It consists of a pair of tests: the high-MP test and the low-MP test.
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).
Here's the principle behind this test: During the high-MP test, the induction manifold pressure is very close to outside ambient pressure, so any induction leak will have little or no effect on engine operation. During the low-MP test, the manifold pressure is significantly lower than outside ambient (by about 10 inches), so any induction leak will cause the affected cylinder (or cylinders) to run substantially leaner than the others, resulting in a smaller drop in EGT than the others.

Above from Savvy.


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Most of the commenters have a good understanding of the impacts of an induction leak. To clarify some the discussion above, we publish the above test because it works for all engines, carbureted and fuel injected (FI), but its actually a pretty poor test for FI engines. I have a much better test based on gami spreads for FI engines that I pass on to clients whenever we suspect an induction leak since the 10" MAP test isn't always conclusive at all - its just so-so but has the advantage of being doable for any engine. 

An induction leak will most certainly alter the mixture, but the degree is entirely dependent on MAP versus ambient pressure. as Bob @Bob - S50, Rich @N201MKTurbo and Hector @Hector were pointing out so well above. Operating  a FI engine at WOT, will mask any effects of an induction leak, pretty much entirely. Why? Its basic physics since their is no differential pressure gradient across the intake system and ambient atmospheric pressure - they are equal and air will follow the path of least resistance. But the symptoms present themselves whenever ambient atmospheric pressure is very different from MAP. This is most apparent in a normally aspirated engine operating at idle when MAP is about 15+" less than ambient making the symptoms much worse. In the NA example operating at idle, the leak allows a greater amount of air to enter the cylinder causing an overly lean mixture. In a turbo, though we can see an overly rich mixture at altitude since a much higher MAP than ambient atmospheric pressure is leaking out the air resulting in a overly rich mixture.

Furthermore, a leak near the cylinder intake will effect just that cylinder. A leak earlier in the intake system could effect one entire side and a leak in a cross over pipe could effect a couple cylinders (the latter mostly apply to carbureted induction systems). We can also see leaks from leaky sniffle valves/cylinder drains. Leaks can be very troublesome to find/detect since they may only leak with a large pressure differential and only when a vacuum is applied. The latter can make them very hard to find given one of the most popular test is to pressurize (rather than apply a vacuum) to the induction system and the spray it with soapy water to look for bubbles. 

The above is only entirely true for FI engines. Carbureted engine induction leaks are even more complicated in a sense because a carbureted engine isn't just leaking air, its leaking a mixture of air and fuel which is in different states of fuel atomization depending on how close the leak is to the carburetor vs the cylinder. But unlike with the FI engine, the notion that a leak at WOT doesn't present symptoms doesn't apply to carbureted engines - it sure can. 

Edited July 6, 2018 by kortopates

[Ragsf15e]

On 7/3/2018 at 9:32 PM, N201MKTurbo said:

I would be very curious to see the numbers from someone with leaky gaskets, before and after.

I have had an intake riser come compleatly off. Other than the mixture knob being further in than normal, the engine ran just fine.

So before your intake riser came off, did it look like this?  The injector above is dry and clean, I think this is coming out the intake gasket.  My gami spread is great, but it seems there’s an intake leak here?