#3 Exhaust valve?

[airguy]

Yesterday my C-172 partner sent me this pic of the engine monitor after a 2.5 hour flight home, WOTLOP 2400 rpm at 6500, IO-360 with matched GAMI-jectors and FP prop.

Two possibilities immediately come to mind - leaking/stuck exhaust valve on #3, and possibly a clogged injector on #3. Since this was WOTLOP if that cylinder went any leaner, this is what I would expect to see. Any other suggestions?

I'm going to do a compression test this afternoon, if that looks good then I'll take it up for a very close lean-test to check peak on each cylinder against fuel-flow spread. Hopefully one or the other will give me a clue.

 

[Skylane81E]

Compression tests are easy.

So is swapping the lead between 3 and 1

But if you are LOP wouldn't a blocked injector make that cylinder run cooler not warmer?

 

[airguy]

Compression tests are easy.

So is swapping the lead between 3 and 1

But if you are LOP wouldn't a blocked injector make that cylinder run cooler not warmer?

If already lean and going leaner, the mixture is very slow to ignite and burn, with the result that it's still burning much later into the exhaust stroke (higher EGT) and not as much heat gets transferred to the cylinder (lower CHT). The top bar is the EGT indication, the gap in the bars is the CHT indication.

 

[Tom-D]

If already lean and going leaner, the mixture is very slow to ignite and burn, with the result that it's still burning much later into the exhaust stroke (higher EGT) and not as much heat gets transferred to the cylinder (lower CHT). The top bar is the EGT indication, the gap in the bars is the CHT indication.

When you are LOP, going leaner will cause the cylinder to quit firing and the CHT/EGT will go cooler because there is no fuel to burn.

The amount of heat is equal to the amount of fuel.

 

[airguy]

When you are LOP, going leaner will cause the cylinder to quit firing and the CHT/EGT will go cooler because there is no fuel to burn.

The amount of heat is equal to the amount of fuel.

It's a matter of degrees - if you go too lean, yes the fire will go out. If there is a minor blockage in the injector such that this one cylinder is just slightly leaner than the others, this would be the expected result.

As you lean from peak, the EGT's rise and the CHT's drop, because the mixture is slower to light (more time from the spark event before full combustion) and the flame front moves slower (more total time for combustion, results in still-burning mixture as the exhaust valve opens). The cylinder eventually will get lean enough that it simply won't ignite, or ignites too late to produce any real power, which is what you're describing. With my matched injectors I've always had very consistent CHT/EGT spread because the mixture across all 4 was very consistent. The mixture was not lean enough at this point to cause any of the cylinders to mis-fire, unless the #3 injector is partially clogged.

 

[Tom-D]

It's a matter of degrees - if you go too lean, yes the fire will go out. If there is a minor blockage in the injector such that this one cylinder is just slightly leaner than the others, this would be the expected result.

As you lean from peak, the EGT's rise and the CHT's drop, because the mixture is slower to light (more time from the spark event before full combustion) and the flame front moves slower (more total time for combustion, results in still-burning mixture as the exhaust valve opens). The cylinder eventually will get lean enough that it simply won't ignite, or ignites too late to produce any real power, which is what you're describing. With my matched injectors I've always had very consistent CHT/EGT spread because the mixture across all 4 was very consistent. The mixture was not lean enough at this point to cause any of the cylinders to mis-fire, unless the #3 injector is partially clogged.

To raise the burn temps by leaning, you must be on the rich side of peak headed toward peak. not on the lean side headed to flame out.

 

[airguy]

To raise the burn temps by leaning, you must be on the rich side of peak headed toward peak. not on the lean side headed to flame out.

You're not raising the TEMPERATURE of the combustion event by leaning - you're changing where that combustion happens. The EGT sensor ain't in the cylinder, it's in the manifold. The combustion event occurs later and last longer, putting hotter exhaust gases in the manifold than it normally sees, which results in higher EGT indications. Likewise, because less time is spent in the cylinder by the burning mixture (and at a lower pressure since the piston is already dropping before peak chamber pressures are reached) less heat is leached into the cylinder head, which shows up as lower CHT.

Being on the rich side of peak, oxygen is the limiting reagent and you are cooling the burning mixture by having excess fuel. If you lean from that point you have less inactive cooling mass and more active burning mass, resulting in higher combustion temps and pressures, and a maximum chamber pressure at peak mixture where it does the most good. You have more burning mixture in contact with the cylinder walls for a longer period of time at a higher pressure, which results in high CHT's. The mixture combustion is complete (or nearly so) before exiting into the exhaust manifold, so the EGT is lower at the point where the sensor is located.

As you continue to lean past peak the mixture flame front and ignition process (initiated by the spark plug) both slow down, resulting in less combustion in the chamber and more in the manifold, which shifts the heat load from the cylinder to the exhaust indicators. With a properly matched set of injectors, it's possible to operate the engine at wide-open-throttle and control power (RPM) by mixture alone, down to about 50% total power before the mixture refuses to light without a rough-running engine. I know in my plane I can easily run WOTLOP as low as 2200 rpm without a hint of roughness - or at least I could last week. Now, with a potentially clogged injector, the fire would go out in #3 if that's the cause and I'll know it.

 

[comanchepilot]

probably just a lead fouled plug - run it full power at 2000' for 10 min leaned out a little bit. If that does not work, let the engine cool and then remove the bottom plug and see if its lead fouled. Take a brush or a pick and clean it out - reinstall and torque the plug and see what happens.

Horse - not zebra.

 

[alfadog]

I thought that when we spoke LOP and ROP, that the "peak" referred to is peak EGT so it would follow that EGT is lower either side. (scratches head)

 

[airguy]

I thought that when we spoke LOP and ROP, that the "peak" referred to is peak EGT so it would follow that EGT is lower either side. (scratches head)

It is, and "net net" the EGT's will drop on either side of peak - but we are talking here (or I was anyway) about relative EGT's one cylinder versus another with different mixture. Perfect stoichiometric mixture will result in highest EGT's and nearly the highest CHT's, as well as the highest peak chamber pressure, and is just about the worst place to run your engine at full power.

Horse - not zebra.

Roger that.

I'll go fly it this afternoon and see what I can figure out.

Here's a link from another board with the John Deakins data, it will help you get a grip on LOP operations better than I can explain it.
http://www.vansairforce.com/community/showthread.php?t=75132

 

[alfadog]

It is, and "net net" the EGT's will drop on either side of peak - but we are talking here (or I was anyway) about relative EGT's one cylinder versus another with different mixture. Perfect stoichiometric mixture will result in highest EGT's and nearly the highest CHT's, as well as the highest peak chamber pressure, and is just about the worst place to run your engine at full power.

My feeling is that you are overcomplicating why the EGT is up then if it is a leaking exhaust valve. The simpler explanation being that leaking exhaust valve means more and hotter gasses leaking past into the manifold.

 

[comanchepilot]

its only 75F hotter - I also see that the CHT is lower - it could be fouling it could also be a little bit of crap in the injector - its dirt simple to take out and look through - get a bottle of hoppes #6 - drop the injector in over night but it smells like a plug rather than a valve - those engines are pretty bulletproof - even for Contis.

The reason why I lean toward a plug is that with only 1 plug firing the cylinder will not run as hot and the fuel will still be burning a little when the exhaust valve opens meaning the sensor is gonna see a hotter temp.

The FIRST thing I would do before even starting the engine is pull the bottom plug - do you have massive or fine wires? Fine wires foul less and its easier to see the fouling - if it is clean - then brush it off anyway- feel around with the pick [lead is obvious, its not like you need to find it] and if its clean check the top one -but I'd be surprised.

Then reinstall and run it up - if it runs up fine - go fly it. Do a spiral climb over the airport - and see how it behaves. Lean it to peak - then bring LOP and then make sure that cylinder goes LOP and see how it behaves LOP.

If you are STILL seeing the same thing - then you pull the injector [this is on the ground obviously! ] and see if it is clear - if it is - drop it in the Hoppes over night and come the back the next day and reinstall and fly again over the airport. If its STILL doing it - THATS when I would pull the cylinder head off and look at the valve. Make sure you have a new gasket for it.

PS: you are not getting full rated hp unless you are operating at peak . . . combustion pressure means power .. . which is why when you operate LOP you slow down. You can operate LOP all you want but a Skyhawk is slow enough. . . .

 

[Tom-D]

You're not raising the TEMPERATURE of the combustion event by leaning - you're changing where that combustion happens.

Not really, have you ever tried to figure out how long it takes any engine to travel 28 degrees at 2700 turns.

you will not change the timing of the combustion event even 1 degree. but reduction of fuel in the combustion will be greater and the loss of heat will be greater than any timing effect.

If your theory worked, you could adjust the EGT by changing the timing of the mag.

 

[Capt. Geoffrey Thorpe]

And you will change EGT if you change the timing.

One shortcoming of "typical" aircraft engines is the fixed timing when you run lean. As you get leaner you would ideally advance the spark to make up for the slower burn rates.

In the old job I had piles of dyno data on this topic, but I no longer have access.

 

[Tom-D]

And you will change EGT if you change the timing.

One shortcoming of "typical" aircraft engines is the fixed timing when you run lean. As you get leaner you would ideally advance the spark to make up for the slower burn rates.

In the old job I had piles of dyno data on this topic, but I no longer have access.

When you run LOP it won't make that much difference.
It's a theory that works in the lab, but not in practice.

When you are LOP the EGT gauge has already seen the timing heat change, and the next heat change you will see will be fuel starvation.

So, when you see Peak you also are seeing the timing heat change, any thing after that is a fuel issue.

 

[Edward Kollin]

Yesterday my C-172 partner sent me this pic of the engine monitor after a 2.5 hour flight home, WOTLOP 2400 rpm at 6500, IO-360 with matched GAMI-jectors and FP prop.

Two possibilities immediately come to mind - leaking/stuck exhaust valve on #3, and possibly a clogged injector on #3. Since this was WOTLOP if that cylinder went any leaner, this is what I would expect to see. Any other suggestions?

I'm going to do a compression test this afternoon, if that looks good then I'll take it up for a very close lean-test to check peak on each cylinder against fuel-flow spread. Hopefully one or the other will give me a clue.

Did you do a mag check? It looks like a bad plug to me. Looks the same when I perform a LOP mag check.
I've seen both the EGT and CHT drop with a partially plugged injector when running LOP.

Ed

 

[kmead]

If the engine is smooth, you have lost an EGT probe. I see this a lot.

 

[airguy]

ComanchePilot hit it on the head. In the spirit of "look at the easiest fix first", I did a runup first and #3 failed on the bottom plug. Decowled and found this....

Of course, it's gonna be a cold plug if you can't get fire to it....

I pulled the plug and cleaned it (because of course it was fouled at this point), reinstalled and connected the wire, runup and subsequent flight were good. After talking to my plane partner on the phone, he said the mag checked fine on takeoff but enroute he noticed 100rpm loss that was intermittent over several minutes, then stayed steady and that's when he noticed the EGT/CHT difference on #3. That was about 1.5 hours into a 2.3 hr flight home.

 

[Tom-D]

OBTW the "Manifold' is on the intake side of the engine, Franklin is the only flat engine that has an exhaust manifold, the rest only have exhaust pipes and mufflers. some call them header pipes.

glad it was an easy fix.

 

[airguy]

OBTW the "Manifold' is on the intake side of the engine, Franklin is the only flat engine that has an exhaust manifold, the rest only have exhaust pipes and mufflers. some call them header pipes.

Since I recently jumped on Henning for inaccurate use of specific terminology, I will accept this flogging...

As for the engine, I've seen enough bad experiences with other owners having to do a top end or valve-guide work on a mid-time engine that now I'm hearing monsters under the bed. With 700 hours on this engine when I got that pic on my cellphone I immediately spring-loaded to "exhaust valve, here we go..."

 

[airguy]

do you have massive or fine wires? Fine wires foul less and its easier to see the fouling .

These are massive electrodes - what is a good plug number for the equivalent of REM 38 massives in a fine-wire? I was going to replace all the plugs at annual (end of August) anyway, could easily change to fine-wire.

 

[Tom-D]

These are massive electrodes - what is a good plug number for the equivalent of REM 38 massives in a fine-wire? I was going to replace all the plugs at annual (end of August) anyway, could easily change to fine-wire.

Simply add an "S" to the model of the plug REM38 "S"

 

[comanchepilot]

So the plug lead fell off? How is that remotely possible if the screw on leads are properly torqued?

Now you have to go to every single other spark plug lead and check that they are torqued to the proper specs.

700 hours is still just past the break in phase - I would not expect to see lead fouling to after 1400 hours -

 

[airguy]

So the plug lead fell off?

Yup.

How is that remotely possible if the screw on leads are properly torqued?

Good question, one I don't have an answer for. Last time they were touched (by my knowledge) was at last annual.

Now you have to go to every single other spark plug lead and check that they are torqued to the proper specs.

I put a wrench on them all and lightly wiggled to check for any looseness, all the others were good. I don't know offhand what the torque specs are for the wire attachment, I'll have to look that up and give it a whirl. I'm also going to have a short discussion with the A&P that did the last annual.