Fouled spark plug

[Wistarmo]

Went on a short flight in my 1993 M20J.  Preflight magneto check was perfectly normal.  During the flight my JPI engine monitor alarmed for excessive EGT differential. The engine was otherwise running perfectly. My #3 cylinder EGT was 120 degrees hotter than the others.  Performed an inflight magneto check and on the right magneto the engine ran severely rough.  There was power loss on the #3 cylinder.  Returned to both magnetos and landed.  Did a runup at 2000 rpm and repeated the magneto check with the same results.  Leaned the engine several times to clear what I presumed was a fouled spark plug.  This worked and all indications returned to normal.  Took the plane up for a test flight and all was again perfectly normal; the engine ran smoothly on the right magneto and the EGT differential was 40 degrees, which is normal for my aircraft.

My question is: what is the significance of a fouled spark plug that is cleared?  What is the cause, and what further steps should I take?

[carusoam]

Wistarmo,

Things to consider for fouled spark plugs...

[1] The reason for the slightly higher EGT on the cylinder with the fouled plug is because there is a small amount of unburned fuel escaping the cylinder and continuing the burn in the exhaust pipe (where the EGT probe is)

[2] When the plug is cleared everything returns to normal.  Because it is normal.  No further worries.  If the same plug fouls again, that is worth investigating.

[3] Lower plugs fowl more often than upper plugs.  Little beads of lead form and collect in the lower plugs.  any oil that is dripping in a cylinder will most likely collect in the lower plugs.  Running rich can also foul a plug.

[4] Reasons for plug fouling:  lead deposits form from running rich and cool, especially while taxiing.  Oily plugs are another issue.

[5] Things that appear as fouled plugs can be related to spark plug gap, ignition wire age, or time to overhaul the magnetos.

If it is just a simple fouled plug, lean the mixture, watch it clear on the JPI....

I believe with the JPI, you can determine exactly what plug is fouled.  The left mag powers the top left bank and right lower bank.  The right magneto powers the right top bank and lower left bank.

As an owner, you are allowed to pull plugs and inspect them.  Be aware of the torque values and the requirement for new crush washers.  If you are not up to the task, ask your A&P to help you.

To avoid most fouling issues lean agressively on the ground.  For leaning in the air, search for ROP and LOP discussions on this board.....

Best regards,

- anthony -

[GeorgePerry]

I'm a big fan of TCP fuel additive by Alcor.  It does a great job of reducing the instances of plug fowling and lead build up.

 

http://www.aircraftspruce.com/catalog/appages/alcortcp.php

 

[richardheitzman]

I agree, lean it out and clear it up. You might have been running too rich on the ground and fouled it up without knowing it.

Agressive lean on the ground. You can lean it out in the air to clear a plug, but be careful.