Bruce2

1999 Honda Civic DX D16Y7 engine

Symptoms:
Starts and runs well at light throttle.
Slight miss was noticeable when accelerating from stop.
Missed when throttle opened fully at 1800 - 3000 rpm.
If full throttle was maintained, "check engine" light was set and scanner showed code P1399 (random misfire).
The misfire was sufficiently random that a specific cylinder was not identified by trouble code.

Diagnosis:
Plugs good condition and color. Gaps correct.
Distributor clean inside & out, rotor new, no carbon tracks.
No corrosion on electrical connections inside distributor and the distributor connector to engine wiring harness (See note below).
No shocks from high-voltage ignition wiring. No missing when sprayed with water.

With engine idling, disconnected ignition wires from one plug at a time.
On #2 cylinder, as I was withdrawing the plug connector, I could see a spark jumping from the insulating tube to the inside of the recess in the valve cover.

Examination of the insulating tube revealed a burn-through smaller than a pin head about 2 cm. above the plug end.

With the increased cylinder pressures of full throttle at low rpm, the resistance across the plug gap was higher than through the defective (carbonized) insulating sleeve to the inside of the valve cover recess.

Solution:
Set of ignition wires (NGK) from a 1998 Civic at the local auto recycler for $10.

Note:
To open the distributor wiring connector to the engine wiring harness, you must first remove the connector from the metal support bracket. The tab on the support bracket locks the two halves of the connector together.