Failing spark plug?

Katamarino

Katamarino

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Katamarino
About 2 hours out of Hawaii, cylinder 3 EGT rose about 70f, and CHT dropped 30f. A mag check showed that one spark plug was not operating so I assumed it had failed.

A few minutes later the temps went back to normal, and another mag check was normal too.

Can a spark plug start to show failure in this way, or can they experience temporary fouling? Trying to decide if it should be changed. They were all cleaned/gapped about 50 hours back.
 
Gravity suxx so it’s likely lead in the lower plug.

Re- gap should not be needed yet.

Just putting in upper position and let heat/ gravity clean deposits may be fine,

Or pick n blast.
 
Could also be a failing mag. If they're not inspected regularly the distributor can accumulate carbon dust and other stuff that can short the spark to ground on one plug.
 
I have had more spark plug leads go bad than spark plugs. I will say when a lead went bad it did not fix its self.
 
Lead failures seem to occur from improper installation.

Preloading the spark plug end or being too close to the exhaust causes problems.

Old , thick black leads are particularly susceptible to heat damage.

Most Leads made in the last 50 years are silicone.

Not all created equal though.
 
Resistance check should not be overlooked when evaluating spark plugs.
 
I had a plug that was flakey at high temps. It would quit firing when I ran at peak EGT (best economy). It would fire fine at 100 deg ROP. I didn't notice it until a post-flight analysis of my engine data. The EGTs were all similar until I leaned to peak. Then one EGT would go about 100 deg higher than the others. On the next flight I did an enroute mag check and confirmed it was a bad plug. My A&P checked the plug on his tester -- it tested fine. I told him to change the plug. He did, and the problem went away.
 
pburger said:
My A&P checked the plug on his tester -- it tested fine. I told him to change the plug. He did, and the problem went away.
Click to expand...

Another reason to think it may have been a high resistance plug. Bomb testers do not check for that.
 
Lean of Peak mag checks can be a very good thing.
 
We cleaned and tested all plugs, including resistance, and they were fine. There was a bit of lead in the relevant plug which may have been the issue.

10 of the 12 plugs now have more than 600 hours on them so will be replaced with new soon anyway.
 
Not approved - but, there is a fine wire option for my engine. Thoughts...?
 
Recently had #3 bottom plug foul with some lead deposits. Others were mostly clean.
Stumbled (no pun intended) on this article and thought I would share, as I found a couple of bits most interesting. (Red text)
I think I understand the article for the most part, but want to confirm, is it just a catch 22 to run lean during cruise? Because I do, but don't have an engine monitor other than the ol' cessna 4-in-1 EGT to really monitor anything else.

Thoughts?

Lead Fouling of Spark Plugs​

Lead fouling of aviation spark plugs is a condition likely to occur in any engine using leaded fuels. Lead is added to aviation fuel to improve its anti-knock qualities. The lead, however, has the undesirable effect of forming lead oxide during combustion. This lead oxide forms as a solid with varying degrees of hardness and consistency. Lead deposits on combustion chamber surfaces are good electrical conductors at high temperatures and cause misfiring. At low temperatures, the same deposits may be good insulators. In either case, lead formations on aircraft spark plugs prevent their normal operation. To minimize the formation of lead deposits, ethylene dibromide is added to the fuel as a scavenging agent that combines with the lead during combustion.

Lead fouling may occur at any power setting, but perhaps the power setting most conducive to lead fouling is cruising with lean mixtures. At this power, the cylinder head temperature is relatively low and there is more oxygen than needed to consume all the fuel in the fuel-air mixture. Oxygen, when hot, is very active and aggressive. When all the fuel has been consumed, some of the excess oxygen unites with some of the lead and some of the scavenger agent to form oxygen compounds of lead or bromine or both. Some of these undesirable lead compounds solidify and build up in layers as they contact the relatively cool cylinder walls and spark plugs. Although lead fouling may occur at any power setting, experience indicates that the lead buildup is generally confined to a specific combustion temperature range. Combustion temperatures outside this specific range minimize the lead fouling tendency.

If lead fouling is detected before the spark plugs become completely fouled, the lead can usually be eliminated or reduced by either a sharp rise or a sharp decrease in combustion temperature. This imposes a thermal shock on cylinder parts, causing them to expand or contract. Since there is a different rate of expansion between deposits and metal parts on which they form, the deposits chip off or are loosened and then scavenged from the combustion chamber by the exhaust or are burned in the combustion process.

Several methods of producing thermal shock to cylinder parts are used. The method used depends on the accessory equipment installed on the engine. A sharp rise in combustion temperatures can be obtained on all engines by operating them at full takeoff power for approximately 1 minute. When using this method to eliminate fouling, the propeller control must be placed in low pitch, or high rpm, and the throttle advanced slowly to produce takeoff rpm and manifold pressure. Slow movement of the throttle control provides reasonable freedom from backfiring in the affected cylinders during the application of power.

Another method of producing thermal shock is the use of excessively rich fuel-air mixtures. This method suddenly cools the combustion chamber because the excess fuel does not contribute to combustion; instead, it absorbs heat from the combustion area. Some carburetor installations use two-position manual mixture controls that provide a lean mixture setting for cruising economy and a richer mixture setting for all powers above cruising. Neither manual mixture control setting in this type of configuration is capable of producing an excessively rich fuel-air mixture. Even when the engine is operated in auto-rich at powers where an auto-lean setting would be entirely satisfactory, the mixture is not rich enough.
 
I'd love to see data that supports those statements. I have a hard time believing that running lean exacerbates lead fouling. Sample of one, I stay pretty clean when lean.
 
If you lean to best power (max rpm) or best economy (lean to lean misfire onset and enrichen until smooth) during cruise the cylinders should be hot enough to minimize lead fouling. In a carbureted engine only 1 or two cylinders will likely be lean of peak at best economy. You can also lean aggressively on the ground to keep lead oxides from accumulating as quickly as otherwise. Running rich for extended periods will just encourage carbon deposits. I lean aggressively in cruise and taxi, and I very rarely see significant lead deposits on my plugs. They usually look squeaky clean at annual. When I ran rich on the ground (before I knew better) I cleaned up carbon and lead deposits on plugs frequently.
 
Katamarino said:
About 2 hours out of Hawaii, ...
Click to expand...
I have to ask, where were you heading and in what?
Assuming 100 kt airspeed, any of the islands except Kaui are accessible from any other within 2 hours. The only inter-island flight over two hours would be from the Big Island to Kaui.
 
Domenick said:
I have to ask, where were you heading and in what?
Assuming 100 kt airspeed, any of the islands except Kaui are accessible from any other within 2 hours. The only inter-island flight over two hours would be from the Big Island to Kaui.
Click to expand...
To the Mainland

www.pilotsofamerica.com

Katamarino's Round the World flight

Finally, the big day arrived. Today I set off from KPJC (Zelienople PA), via CYTZ (Toronto, Canada), and have made it as far as CYMT (Chibougamau, Canada). Doors open, this is it: All the gear: A good send-off! First stop Toronto, to collect my friend Mike. Goodbye, Toronto. Within...
www.pilotsofamerica.com www.pilotsofamerica.com
 
chemgeek said:
during cruise the cylinders should be hot enough to minimize lead fouling.
Click to expand...

That's what I found curious.
Are the cylinders are being ran at a CHT based (partially) on mixture, but there is a point where the ratio of fuel/oxygen creates a higher temp than the cyl/plugs, and makes them get 'stickier'?

A lot of other articles talk about CHT and engine longevity, etc. Is there a transition where higher CHT's are better? Helps avoid some lead buildup, but not too high to affect engine longevity?
I don't know..
It's just a bit of unique info I'd never come across before.
Wondering if Mike Busch mentions it anywhere.

I rarely see much lead fouling either, and am very aggressive with leaning at all times. This may just be an underperforming plug that needs to be tested. Plus it's the first time it's fouled, and could have been just some breaking off the cyl head and getting wedged, etc. Not changing anything I do at this point, just thought the article was interesting.
I too, would love to see their data for these findings.
 
Dave Theisen said:
To the Mainland

www.pilotsofamerica.com

Katamarino's Round the World flight

Finally, the big day arrived. Today I set off from KPJC (Zelienople PA), via CYTZ (Toronto, Canada), and have made it as far as CYMT (Chibougamau, Canada). Doors open, this is it: All the gear: A good send-off! First stop Toronto, to collect my friend Mike. Goodbye, Toronto. Within...
www.pilotsofamerica.com www.pilotsofamerica.com
Click to expand...

Actually, inbound from Kiritimati!
 
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