How to stop moisture in a distributor cap

Updated February 21, 2017

The distributor cap accepts electrical signals from the ignition system and times the release of electricity to the spark plugs to maintain the proper firing order of the plugs. The unit uses a rotating arm attached to the rotating shaft tied to the crankshaft to maintain the timing. The spring-loaded carbon brush conducts the voltage passing the contacts in the distributor to send the voltage to the spark plugs. The system is a sealed system but moisture appears inside of the cap on occasion.

Remove the distributor cap. Mark on the side of the distributor with a white paint marker its current position before removing and label each spark plug wire with masking tape and the order in which the wire is removed.

Inspect the cap for signs of damage. Look for cracks in the housing or plastic, loose contacts, rust or debris, or other signs of damage. If any damage is noted, replace the distributor cap immediately. Some distributor caps use vent screens located on the underside of the cap close to the engine. The screens are small, about the size of a pencil eraser, and are only visible when the distributor cap is removed. Check those carefully for dirt or clogging. Clean the distributor cap with soap and water and allow to fully dry.

Inspect the layout of your engine. Look for A/C lines on or around the cap. Condensation can drip into the cap from these lines. Simple foam tube insulation wrapped around the A/C protects from condensation.

Remove the existing rubber seal on the distributor cap and replace with a factory-approved replacement available at most auto parts stores. A damaged or old seal allows humidity and moisture to accumulate within the cap.

Reinstall the distributor cap by carefully aligning your marks. Reattach the spark plug wires in the proper sequence. No materials are needed to seal the cap. The rubber seal and undamaged cap suffice to block moisture from entering the system.


To maintain optimum function of your ignition system, replace the distributor cap at each tune-up.

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