... just recently; someone who bought my share of (former) mine Grumman Tiger; had a rough engine and a cold cylinder. He replaced a spark plug; and asked our guru-mechanic about "why".
Here's his response.
To whom it may concern,
There are a number of ways to determine if a plug is bad.
First: Rust on the outside has absolutely nothing to do with the function of the plug.
Second: Plugs are not routinely changed just to be changed.
Third: Plugs ARE changed when the center electrode is worn excessively.
Fourth: Plugs ARE changed when they fail.
Fifth: Plugs are cleaned, inspected, and gapped each annual or 100 hours.
Sixth: If you want to install new plugs each annual, figure around $300+ to change.
It's your money.
Plugs are typically good for 800-1000 hours.
In the last 5 years, the plane has flown approx 350 hours.
At that rate, plugs will last about 10-12 years.
Total cost in 10-12 years to clean and gap is about $1000-$1200.
Total cost to replace each annual for 10-12 years is $3000 to $3600.
- again, it's your money.
- That $2400 would be better invested in a good engine analyzer.
----What to do?
If a plug fails to fire when it should, it could be:
1) The plug is bad.
2) The plug wire is bad. This is extremely difficult to determine.
3) The plug wire is shorting through its shield. You can look at the wire to see.
4) The mag is bad. This is also difficult to determine.
5) Excessive lead/carbon build-up due to running too rich.
6) Excessive oil soak from piston rings being worn.
To determine if the plug is bad:
1) Check the impedance of the plug. This is not definitive.
2) Use a plug tester to test the plug.
3) Visually look for cracks, lead, or oil.
To determine if the plug is fouled:
1) Clean the plug.
2) Reinstall.
3) Check operation.
If the cylinder is not making power, it could be:
1) see above details for plug
2) The intake valve is not closing
3) The intake valve is cracked
4) The intake valve is warped.
5) The cam for the intake valve is bad.
6) The exhaust valve is not closing
7) The exhaust valve is cracked
8) The exhaust valve is warped.
9) The cam for the exhaust valve is bad.
10) The intake tube is blocked
11) The exhaust riser is blocked.
12) Excessive oil soaking of the plugs from piston rings being worn.
13) Both mags are bad on that specific cylinder.
To determine if the valves are not seating/sealing or the rings are worn:
1) Perform a compression test
2) If the compressions are low: Continue troubleshooting
3) If the compressions are good: Continue troubleshooting
Seventh: In the absence of real engine analyzer that will point to a specific plug/cylinder to determine which is not making power, the only solution is to pull the plugs and determine:
1) If the plug firing
2) If the plug lead fouled
3) If the plug oil fouled.
Eighth: If a plug is suspected and the troubleshooter wants to take the simplest approach:
1) Clean the plug(s) and recheck
If the problem persists,
2) Change the plug(s)
It sounds as though you changed suspected plugs. If:
1) You replaced a plug that was
a) oil fouled and not firing
b) lead fouled and not firing
c) damaged and not firing.
and that fixed the problem.......
Problem solved. You did good. Have a beer. Hangar fly with your buddies about how anxious you were when the engine was rough. Talk about how nervous your waif was and probably will never go flying again (a lot of your buddies will identify with this one.) Post it on Facebook.
Gary
PS, regarding spares: I've seen owners who carry extra mags, tires/tubes, plugs, vacuum pumps, alternators, air filters, alternator belts, a case of oil and an extra oil filter, nav lights, strobe lights, flares, fire extinguishers, blankets, water, food, clothing, spare comm radios, spare portable GPSs (4 in one plane!), spare portable Nav radios, spare antennas, spare carpets for the floor (I'm not kidding), ....
I weighed all of the spare parts in one particular Cheetah and it weighed over 200 lbs. The owner and his wife's combined weight was over 500 lbs. The useful load of a Cheetah is roughly 800 lbs. You can't fix stupid.
PPS, what do I carry? 2 quarts of oil, a tow bar, my cell phone and credit cards.
Here's his response.
To whom it may concern,
There are a number of ways to determine if a plug is bad.
First: Rust on the outside has absolutely nothing to do with the function of the plug.
Second: Plugs are not routinely changed just to be changed.
Third: Plugs ARE changed when the center electrode is worn excessively.
Fourth: Plugs ARE changed when they fail.
Fifth: Plugs are cleaned, inspected, and gapped each annual or 100 hours.
Sixth: If you want to install new plugs each annual, figure around $300+ to change.
It's your money.
Plugs are typically good for 800-1000 hours.
In the last 5 years, the plane has flown approx 350 hours.
At that rate, plugs will last about 10-12 years.
Total cost in 10-12 years to clean and gap is about $1000-$1200.
Total cost to replace each annual for 10-12 years is $3000 to $3600.
- again, it's your money.
- That $2400 would be better invested in a good engine analyzer.
----What to do?
If a plug fails to fire when it should, it could be:
1) The plug is bad.
2) The plug wire is bad. This is extremely difficult to determine.
3) The plug wire is shorting through its shield. You can look at the wire to see.
4) The mag is bad. This is also difficult to determine.
5) Excessive lead/carbon build-up due to running too rich.
6) Excessive oil soak from piston rings being worn.
To determine if the plug is bad:
1) Check the impedance of the plug. This is not definitive.
2) Use a plug tester to test the plug.
3) Visually look for cracks, lead, or oil.
To determine if the plug is fouled:
1) Clean the plug.
2) Reinstall.
3) Check operation.
If the cylinder is not making power, it could be:
1) see above details for plug
2) The intake valve is not closing
3) The intake valve is cracked
4) The intake valve is warped.
5) The cam for the intake valve is bad.
6) The exhaust valve is not closing
7) The exhaust valve is cracked
8) The exhaust valve is warped.
9) The cam for the exhaust valve is bad.
10) The intake tube is blocked
11) The exhaust riser is blocked.
12) Excessive oil soaking of the plugs from piston rings being worn.
13) Both mags are bad on that specific cylinder.
To determine if the valves are not seating/sealing or the rings are worn:
1) Perform a compression test
2) If the compressions are low: Continue troubleshooting
3) If the compressions are good: Continue troubleshooting
Seventh: In the absence of real engine analyzer that will point to a specific plug/cylinder to determine which is not making power, the only solution is to pull the plugs and determine:
1) If the plug firing
2) If the plug lead fouled
3) If the plug oil fouled.
Eighth: If a plug is suspected and the troubleshooter wants to take the simplest approach:
1) Clean the plug(s) and recheck
If the problem persists,
2) Change the plug(s)
It sounds as though you changed suspected plugs. If:
1) You replaced a plug that was
a) oil fouled and not firing
b) lead fouled and not firing
c) damaged and not firing.
and that fixed the problem.......
Problem solved. You did good. Have a beer. Hangar fly with your buddies about how anxious you were when the engine was rough. Talk about how nervous your waif was and probably will never go flying again (a lot of your buddies will identify with this one.) Post it on Facebook.
Gary
PS, regarding spares: I've seen owners who carry extra mags, tires/tubes, plugs, vacuum pumps, alternators, air filters, alternator belts, a case of oil and an extra oil filter, nav lights, strobe lights, flares, fire extinguishers, blankets, water, food, clothing, spare comm radios, spare portable GPSs (4 in one plane!), spare portable Nav radios, spare antennas, spare carpets for the floor (I'm not kidding), ....
I weighed all of the spare parts in one particular Cheetah and it weighed over 200 lbs. The owner and his wife's combined weight was over 500 lbs. The useful load of a Cheetah is roughly 800 lbs. You can't fix stupid.
PPS, what do I carry? 2 quarts of oil, a tow bar, my cell phone and credit cards.
