Why Rain Can Stop Your Car From Starting
Water and vehicle electrics are a poor combination. After heavy rain or in high-humidity conditions, moisture can infiltrate critical components of the ignition and starting system. This is more common on older vehicles but can affect any car with ageing rubber seals, cracked insulation, or missing covers.
The Most Likely Culprits
Distributor cap and rotor (older vehicles): On older vehicles with a distributor-based ignition, moisture inside the distributor cap causes the high-voltage spark to track along the inside of the cap rather than jumping to the correct plug lead. The result: the engine cranks but won't fire, or fires on fewer than all cylinders. Distributor caps can crack over time, letting moisture in. Drying the inside with a rag or WD-40 (used as a water displacer in this case) often gets the car started.
Spark plug leads: Cracked or aged HT (high tension) leads let moisture in, causing the spark to leak to earth rather than reach the plug. The car will misfire badly or refuse to start.
Coil packs: On modern coil-on-plug or coil-pack systems, water ingress into a cracked coil can cause a cylinder to miss or fail to fire at all.
Flooded engine bay: Heavy rain or a pressure wash aimed at the engine can soak connectors, the ECU cover, or the air intake, causing sensors to read incorrectly or water to enter the airbox.
Wet brake drum parking brake on rear-drum vehicles: Not an ignition issue but worth knowing — if the handbrake cable freezes or pads seize to wet drums overnight, the car may feel like it won't move even if it starts.
Quick Fixes to Try
- Open the bonnet and inspect. Look for obvious water pooling on the engine, around the coil packs, distributor (if present), and fuse boxes.
- Use a clean dry rag to wipe the distributor cap, plug lead connections, and any exposed ignition components.
- WD-40 as a water displacer: Spray lightly on ignition leads, distributor cap (outside and inside edge), and coil pack connectors. WD-40 stands for "Water Displacement 40th formula" — it does work for this purpose.
- Wait and try again. On warm days, morning damp often evaporates within an hour.
Longer-Term Fix
- Replace cracked distributor caps, rotors, and spark plug leads — these are wear items that should be replaced every 30,000–60,000 km anyway
- Apply dielectric grease to ignition connector boots when replacing leads or plugs
- Ensure the air filter box lid is properly sealed to keep moisture out of the intake
Cost to Fix
- Distributor cap and rotor: $30–$80 NZD
- Spark plug leads (full set): $60–$180 NZD
- Coil pack (if failed): $80–$250 NZD per coil
Labour for ignition lead and cap replacement is usually 1–2 hours at workshop rates.
WoF Consideration
Damaged spark plug leads and cracked distributor caps are noted as defects by some WoF inspectors as they can cause safety-related misfires. Replacing them proactively is worthwhile on older vehicles.