Soot on spark plugs

Daniel Ybarra

Daniel Ybarra

Filing Flight Plan
Joined
Oct 26, 2019
Messages
10
Location
Menifee, CA
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Display name:
Daniel
For the past 19 years I am the single owner/flyer of a 1963 Piper Cherokee 180, with a Lycoming O-360-A3A. TTSN on the engine is 2699, 241 since SMOH, and 20 hours since annual and oil change. I lean aggressively during taxi, and use my JPI EDM930 to lean to peak EGT while I cruise at 2400-2450 RPM (roughly 65% power). I have been doing this for years. I fly the airplane to/from work (20 minute flight) usually once a week.

On Friday, 02-25-2022, I did a 30 minute flight. All seemed normal during the run-up and during the flight. However, when I reduced throttle for the descent I noted a little roughness in the engine. It felt like I had a fouled spark plug. When I pulled the throttle to idle on short final the roughness increased significantly. The JPI EDM930 said cylinder #2 was about 500 degrees different (cooler) See JPI download below.

upload_2022-3-3_6-18-34.png


I pulled the spark plugs and noted the cylinder #3 plugs were clean, while cylinders #1, #2, and #4 all had significant soot accumulation.
upload_2022-3-3_6-18-49.png


I cleaned and verified gap on each plug (.018). I checked impedance on each plug and the Tempest plugs were 800-900 ohms, and the Champions were nominally 1200 ohms. I bore scoped the cylinders and all looked good. A cold compression check found #1-74, #2-76, #3-72, #4- 74. I also checked the timing and found it spot on at 25 degrees. I checked the intake tubes for each cylinder and found no signs of an induction leak.

This is actually the second time this has happened. It also happened when I was flying home from work a few weeks ago. That time I did an in-flight mag check and found one of the #2 cylinder spark plugs not firing (I think it was the one attached to the left magneto). I didn't think to do that this time.

When I pulled the plugs they appeared the same as they do this time (#3 clean, all other sooty). I cleaned the plugs, verified gap, check impedance, rotated them, and returned them to service. MY A&P/AI suggested I run about 50 degrees rich of peak in cruise, which I did. When I returned the plugs to service it ran smoothly until this second occurrence.

At first, despite the impedance check, I thought I might have a bad plug. But since I rotated them after the first occurrence and have the same result, I now doubt that.

Could it be the ignition leads or the magnetos?

At this point I'm out of ideas.Thank you for any guidance you can provide.
 
Your spark plug resistance has to be really high before plugs start to have issues if your magneto is supplying a good spark. I have pulled old Champion plugs with
10-20 kohm resistance that were still firing. I had a couple plugs giving trouble that read over 100 kohms.

The plugs in my old Bonanza will soot up much like yours. I have to lean to the verge of cutoff when taxiing. Mine will soot up a bit, but no ignition issues.

Check your air filter.
 
First, make sure #2 & #3 JPI leads are not swapped (not sure how that could happen since they are on opposite banks, unless erroneous from the day of install). You could either pull #2 or 3 and run the engine to see if the monitor jives with the one you pulled, or dip the one you pulled into hot water or use a torch or lighter to confirm.

As for the actual cause, check for an induction leak on #3.

My non-A&P 2 cents.
 
I also suspected an induction leak on #3. OP says that issue was inspected with no leak found. I would look again to verify .
 
Maybe so, especially since it happens during low MP (high vacuum).
 
Ignoring everything else and just going by what happened...Stumbled on power reduction...id say carb ice.
 
Two posters mention induction leak, and that is most likely. Those sparkplugs tell a tale. A flashlight and mirror to get a good look at the gasket at the head, and a good examination of the rubber hose coupling the induction tube to the sump's manifold. Those gaskets can get blown out by a backfire caused by switching the mags accidentally to OFF and then back to ON during the runup. The concussion in the exhaust travels back through any cylinder that has both valves open at the top of the exhaust stroke. The gaskets take a set and the flange and head lose their grip on them, allowing sections to be blown out.
 
HOW was it inspected?

My technique is:

Carb Heat ON

Throttle. OPEN

Disconnect Carb Heat Duct at source and plug in the Leaf Blower.

Pressurize system and check for leaks with soap and water.


There may be better ways to do this; but it seems many leaks can be identified by this method.
 
Daniel Ybarra said:
For the past 19 years I am the single owner/flyer of a 1963 Piper Cherokee 180, with a Lycoming O-360-A3A. TTSN on the engine is 2699, 241 since SMOH, and 20 hours since annual and oil change. I lean aggressively during taxi, and use my JPI EDM930 to lean to peak EGT while I cruise at 2400-2450 RPM (roughly 65% power). I have been doing this for years. I fly the airplane to/from work (20 minute flight) usually once a week.

On Friday, 02-25-2022, I did a 30 minute flight. All seemed normal during the run-up and during the flight. However, when I reduced throttle for the descent I noted a little roughness in the engine. It felt like I had a fouled spark plug. When I pulled the throttle to idle on short final the roughness increased significantly. The JPI EDM930 said cylinder #2 was about 500 degrees different (cooler) See JPI download below.

View attachment 105088


I pulled the spark plugs and noted the cylinder #3 plugs were clean, while cylinders #1, #2, and #4 all had significant soot accumulation.
View attachment 105089


I cleaned and verified gap on each plug (.018). I checked impedance on each plug and the Tempest plugs were 800-900 ohms, and the Champions were nominally 1200 ohms. I bore scoped the cylinders and all looked good. A cold compression check found #1-74, #2-76, #3-72, #4- 74. I also checked the timing and found it spot on at 25 degrees. I checked the intake tubes for each cylinder and found no signs of an induction leak.

This is actually the second time this has happened. It also happened when I was flying home from work a few weeks ago. That time I did an in-flight mag check and found one of the #2 cylinder spark plugs not firing (I think it was the one attached to the left magneto). I didn't think to do that this time.

When I pulled the plugs they appeared the same as they do this time (#3 clean, all other sooty). I cleaned the plugs, verified gap, check impedance, rotated them, and returned them to service. MY A&P/AI suggested I run about 50 degrees rich of peak in cruise, which I did. When I returned the plugs to service it ran smoothly until this second occurrence.

At first, despite the impedance check, I thought I might have a bad plug. But since I rotated them after the first occurrence and have the same result, I now doubt that.

Could it be the ignition leads or the magnetos?

At this point I'm out of ideas.Thank you for any guidance you can provide.
Click to expand...

50 degrees ROP is the worst possible place to run your engine at cruise. I would go back to running at peak or slightly LOP if possible.
Like the other posters have stated this looks like a classic induction leak. I would however try one other thing. The next time you have this issue lean the engine slightly. I have seen this happen especially on hot days where a cylinder gets to rich to fire and EGT drops rapidly. Poor ignition system performance can increase the chances of this happening. In one case I saw it was bad plug wires.
 
Guys, look again at the #3 sparkplugs. Both are clean. The others are all black from a rich mixture. They're not oily. That says only one thing: #3 is getting a really lean mixture compared to the rest, and on a carbureted engine that can only come from an induction leak. The sooty plugs are like that because the pilot had to run rich enough that #3 cylinder didn't cut out and make the engine rough. Reducing power for descent can cause some leaning, causing roughness there, too. And closing the throttle lowers the MP (raises manifold vacuum), sucking evem more air past that induction leak and causing lean misfire.
 
It’s the number 2 cylinder that’s going cold. I agree that number 3 is running lean but I suspect number two is going cold because it’s to rich. He probably has two issues going on or perhaps if he gets number 3 in balance it will lean the other cylinders. A clogged injector can push more fuel to the other injectors.
 
Jeff767 said:
It’s the number 2 cylinder that’s going cold. I agree that number 3 is running lean but I suspect number two is going cold because it’s to rich. He probably has two issues going on or perhaps if he gets number 3 in balance it will lean the other cylinders. A clogged injector can push more fuel to the other injectors.
Click to expand...

It’s carbed, not injected.

The empirical data (spark plug) shows #3 is lean, it’s the one with the problem for sure. You don’t need an engine monitor to see that. Engine monitor is possibly wrong. Sure, it could be something totally different going on in #2, but simplest explanation is that it’s wired wrong.

500 degrees difference is huge. Perhaps bad thermocouple, likely 2 & 3 are swapped.

I’m usually wrong at home, sometimes or more at the airport, almost never with engines (a bit cocky perhaps).
 
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I would agree the first thing to check is the probes. Takes seconds to check if the engine is decowled. Just unp,ug number 2 and see which readout goes bad.
 
Waiting for a follow up from the OP ...
 
Problem solved! Thank you all for your comments and suggestions!

It seemed like an induction leak was most probable, so I took another look. I pulled the Number 3 induction tube and the gasket came off in pieces. It was stained and very brittle.

I can't say for sure that part of it was missing, but that seem very likely, thus the leak. As a precaution, I replaced the gaskets on each induction tube, and they also were brittle and came off in pieces. I was probably not far away from a leak on one or more of the other cylinders. And while I was there I also replaced the intake hoses, which all seemed to be in good shape, but was an inexpensive item to proactively replace.
 
Thanks for the follow up. Reading through the thread, I agreed it had to be an imbalance in the cylinder mixtures and glad to see my understanding was correct. The black deposits are classic running too rich, #3 is what you want all your plugs to look like. That they were different said something is going on unique to #3.

Pushing harder for understanding - Is there any other way for this to happen? For carbs, is there any way for the fuel distribution to be out of balance? I think no, but I'm having a difficult time right now picturing the connection between the carb and the intake valve. It's just piping, right?
 
Thanks for the followup.

Carbs rarely give as even fuel distribution as fuel injection. I've heard a few methods to improve carb fuel distribution, but I cannot vouch for either of them.
-- crack the carb heat. It introduces a bit of turbulence into the intake flow causing better mixing.
-- if you are running WOT, pull off WOT a bit to slightly cant the throttle plate. This also increases turbulence.
 
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Magman said:
HOW was it inspected?

My technique is:

Carb Heat ON

Throttle. OPEN

Disconnect Carb Heat Duct at source and plug in the Leaf Blower.

Pressurize system and check for leaks with soap and water.


There may be better ways to do this; but it seems many leaks can be identified by this method.
Click to expand...

What level of pressure can a leaf blower produce? Enough to reveal an induction leak. Versus "burping" with 80 PSI, during the annual leak down or differential test.
 
Your post said it. “ Enough to reveal an induction leak”.

Every system has its limits. I consider the Leaf Blower a step above unaided visual inspection. It is NOT a replacement for established procedures. Unlike methods that require disassembly and adapters this method can be set and used conveniently which means it can be done often. It has helped me find many defects.

Why would I ever want to check a normally aspirated induction system with 80 psi?
I don’t believe 50 in MAP is even equal to 80 psi.

Perhaps if you could explain the process here maybe we can gain insight into the advantages . And limits.
 
Magman said:
Why would I ever want to check a normally aspirated induction system with 80 psi?
I don’t believe 50 in MAP is even equal to 80 psi.
Click to expand...
Yup. If the pressure is too high you won't find the leak unless it's blowing right in your face. Too much air blows the soapy water away and you see no bubbles. A vacuum cleaner outlet is plenty strong enough for the purpose.

50" MAP is about 24.6 psi. 80 PSI could blow those gaskets right out. They're not intended for that sort of pressure.
 
bflynn said:
Pushing harder for understanding - Is there any other way for this to happen? For carbs, is there any way for the fuel distribution to be out of balance? I think no, but I'm having a difficult time right now picturing the connection between the carb and the intake valve. It's just piping, right?
Click to expand...

The typical O-320 induction system. The carb bolts to the bottom of the oil sump, and there are tubes cast into the sump to take the air/fuel mix to the induction risers. #6 is the gasket that was leaking. The heat of the cylinder head cooks them.

upload_2022-3-14_20-32-30.png

A Continental IO-550N has its induction up above the engine.

upload_2022-3-14_20-38-18.jpeg

There are a lot of different layouts, with the newer ones having a bit more science to them, like equal-length risers and no abrupt changes of direction.
 
Good point. It would move the needle!

I would think either monitor in limits or disconnect.
 
Magman said:
Your post said it. “ Enough to reveal an induction leak”.

Every system has its limits. I consider the Leaf Blower a step above unaided visual inspection. It is NOT a replacement for established procedures. Unlike methods that require disassembly and adapters this method can be set and used conveniently which means it can be done often. It has helped me find many defects.

Why would I ever want to check a normally aspirated induction system with 80 psi?
I don’t believe 50 in MAP is even equal to 80 psi.

Perhaps if you could explain the process here maybe we can gain insight into the advantages . And limits.
Click to expand...

Lots of food for thought in your and Dan's reply. Thanks.
See
. About 2:50 the "burp" is explained.
In the video the fellow says 80 psi, but looking at the second dial, the indicated pressure seems to read 20 or so.
 
No time to watch now. I think I saw this before and thought it needed a couple people to do it . Also moving the prop off of TDC @ 80 psi is quite hazardous and OSHA likely frowns on it. I’ve done enough scary things on aircraft and decided this would not be another.

it would also appear that if you have 80 psi in a small combustion chamber and then allow air to also escape to the much larger induction system then you really only have 20 psi or so for testing. Maybe that’s good?

it may make more sense after viewing. Always good to learn new techniques.
Thanks for posting!
 
21541803 said:
About 2:50 the "burp" is explained.
In the video the fellow says 80 psi, but looking at the second dial, the indicated pressure seems to read 20 or so.
Click to expand...
80 psi going into the differential pressure tester - pressure on the cylinder side is going to be much lower when it "burps" because of the restriction from the orifice in the tester (between the gauges). I suspect that the pressure in the intake manifold is anybody's guess - it will depend on how far the intake valve opened and the restriction through the throttle body and the relative volumes of the cylinder and intake manifold. But it did blow away the bubble solution and not make bubbles.

 
Watched the vid and don’t see any advantage to the technique. A constant pressure with 1 person as opposed to a “burp” while testing plus needing 2 people seals the deal for me.

Just watch the MAP , if applicable.
 
Probably safer to do the leak down test with two people. Then it make sense to "burp" at the time of doing the test instead of rigging up the vacuum later on.

Thread drift. Is it more dangerous to do a leak down test on a O-540 engine (80 psi) than a O-200 engine (80 psi) at TDC? Does the O-540 bigger cylinder volume at 80 psi mean more potential energy "stored" that can cause greater injuries?
 
The 540 has a larger bore with pressure on each square inch. Either can hurt you.

You can begin the test at lower pressure until TDC is confirmed under pressure.
Some tester have a “ slow pressurize “ feature.

To me; key is comfortably holding the prop to control movement in either direction but being prepared to release and get out of the way quickly if needed.
 
If a cylinder is pressurized to 80psi inlet pressure, and you wiggle the prop looking for TDC, and the piston flips, you are likely not fast enough to get out of the way.

I got 7 staples in my scalp to prove it. (Looked way worse than it was--no concussion, no loss of senses. Worst part was calling my wife to pick me up at the ER--again).

My NEW SOP for differential compression test is to do all 4 cylinders from the same side of the engine. Which ever side gives you a better/safer hold on the prop. Rotate the prop to approximate TDC, then pressurize to 40psi cylinder pressure. Rock the prop gently to find the maximum reading, then release and pressurize to 80psi inlet pressure.
 
I agree with Dom’s method of positioning.

Also have had to call the wife under similar circumstances. Last time I was still laying on the ground and to get the bandages out.

Note: It seems you get a little sympathy if you address them as Nurse Nightengale
rather than Nurse Ratched.
 
I built my own differential pressure tester. It has a regulator actuated by a squeeze-lever setup, like a caulking gun. One controls the pressure with that squeeze effort. It also has a quick-release valve that dumps the cylinder pressure instantly when the lever is released. If that prop starts to get away, just let go of the tester. Pressure is gone. Built everything but the two gauges and the hose fittings. Never patented it. It would be far more expensive than the typical aviation diff tester since there is so much machining and fabricating involved, so sales would be tiny and would never even pay for the patent.
 
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