1991-1993 BMW 318i Ignition Coil Guide: Misfires, No-Starts, and Testing

Symptoms of a Failing 1991-1993 318i Ignition Coil

A failing ignition coil on the M42 engine will cause noticeable running problems. Because a single coil provides spark for all four cylinders, its failure affects the entire engine, not just one cylinder. Watch for these common signs.

• Engine Misfires: This is the most common symptom. A misfire can feel like a shudder, stumble, or jerking while driving. When the car is stopped at a light, a misfire may cause a rough, vibrating idle.
• Engine Cranks but Won't Start: If the coil has failed completely, it cannot produce the high voltage needed for a spark. The engine will turn over with the key, but it will never fire up and run.
• Loss of Power and Poor Acceleration: A weak or intermittently failing coil produces a weak spark, leading to incomplete combustion. This results in sluggish acceleration and a general lack of power, making the car feel slow and unresponsive.
• Reduced Fuel Economy: When the air-fuel mixture isn't burned efficiently due to a weak spark, the engine uses more fuel to do the same amount of work. A sudden drop in your MPG can be a sign of ignition problems.
• Check Engine Light: A misfire will almost always trigger the Check Engine Light on the dashboard. On these OBD-I vehicles, the code may be a general misfire code rather than pointing to a specific component, but it's a clear signal to investigate the ignition system.

Diagnosing the Ignition Coil

Before buying a replacement, you can perform a few tests to confirm the coil is the problem.

1. Visual Inspection: Check the coil's plastic housing for any cracks, melting, or burn marks. Inspect the low-voltage electrical connector for corrosion and the high-tension towers (where the spark plug wires connect) for corrosion or damage.
2. Spark Test: You can use an in-line spark tester between the main coil wire and the distributor cap (or a spark plug wire and a grounded spark plug) to see if a spark is being generated while cranking the engine. If there is no spark, and you've confirmed the coil is receiving 12V power with the key on, the coil is likely bad.
3. Resistance Test (Multimeter): You can test the coil's internal windings using a multimeter set to measure resistance (Ohms). Disconnect all wires from the coil. Measure the primary resistance between the positive (15) and negative (1) terminals. Then measure the secondary resistance between the negative (1) terminal and the center high-voltage output tower. While specific values can vary, you are looking for readings that are not infinite (open circuit) or zero (short circuit). A coil that tests good when cold can still fail when hot, but a bad reading definitively confirms failure.

Buying a Used Coil/Ignitor for Your 318i

For a vehicle of this age, a quality used OEM ignition coil is an excellent, budget-friendly choice. Here’s what to look for.

• Why Used OEM is a Smart Choice: The original coils, typically made by Bosch, were built to a very high standard. A used OEM Bosch coil is often more reliable and durable than a cheap, new aftermarket part from an unknown brand. The quality of the internal windings and insulation material is critical for longevity, and OEM suppliers get this right.
• Physical Inspection Checklist:
  • No Cracks: Carefully inspect the entire black plastic housing. Hairline cracks can let moisture in, causing the coil to short out.
  • Clean Terminals: Look at the 3-pin electrical connector and the four high-voltage towers. They should be free of green or white corrosion.
  • No Heat Damage: Check for any signs of melting, warping, or discoloration on the housing, which would indicate overheating.
• Verify Compatibility: The same ignition coil was used in the 1991 E30 318i and the 1992-1993 E36 318i, as both shared the M42 engine. Look for the Bosch part number 0 221 504 474 or BMW part numbers like 12 13 9 066 468 or 12 13 7 599 218 to ensure you have the correct part.
• Mileage vs. Condition: For an ignition coil, physical condition and age are more important than mileage. A well-preserved coil from a 100,000-mile car is better than a cracked, corroded one from a 60,000-mile car.

Known Issues, TSBs, and Recalls

While there have been several safety recalls issued for the 1991-1993 BMW 318i, none of them pertain to the ignition coil. Recalls for this vehicle concerned issues like potential fuel hose leaks, sticking throttle bodies, and heater core problems. Failure of the ignition coil is generally considered a standard wear-and-tear issue for a vehicle of this age, resulting from years of heat cycles and vibration.

Cost of Replacing the Ignition Coil

Replacing the ignition coil on a 1991-1993 318i is very affordable, especially if you do it yourself.

Replacement Guide

Replacing the ignition coil on the M42 engine is a straightforward job suitable for any DIYer.

• Difficulty: 1 out of 5 (Beginner)
• Estimated Time: 20-30 minutes
• Tools Needed: Basic metric socket set, ratchet.

Step-by-Step Replacement

1. Disconnect the Battery: Always disconnect the negative battery terminal before working on electrical components.
2. Locate the Coil: The ignition coil is typically mounted on one of the inner fenders in the engine bay.
3. Disconnect Wires: Unplug the 3-pin electrical connector. Then, carefully pull off the four spark plug wires from the coil's towers. It's a good idea to label the wires (1, 2, 3, 4) with tape to ensure they go back in the correct order.
4. Unbolt the Coil: Use a socket to remove the bolts or nuts holding the coil bracket to the car's body.
5. Install the New Coil: Bolt the new coil into place. Reconnect the spark plug wires to their correct towers. Plug in the 3-pin electrical connector.
6. Reconnect Battery: Reconnect the negative battery terminal and start the car to test your work.

Frequently Asked Questions (FAQ)