When a few pony’s go missing, the coil is the most likely cause. But I’ve seen a ton of coils replaced needlessly. I’ll show you a mechanic hack for nailing it!
It is possible to change just one faulty coil pack. However, when one coil fails, the remaining coils may soon fail also.
In this post, you’ll learn why changing just one coil pack may not fix the misfire. I’ll show you how to successfully diagnose a faulty coil and other possible causes of an engine misfire.
Changing Just One Coil
You can, of course, change just one coil. However, as a mechanic, I’ve seen it so many times, when one coil fails, another isn’t too far behind. If your coils are easy to access, replacing them as they fail is OK, apart from the inconvenience. It is possible to drive a car with a faulty coil, but it isn’t advised.
But on larger engines, you may need to remove engine components like the intake plenum. As it’s part of the intake system, a good seal is essential. Using the old gasket could cause lean rough running and ironically, multiple misfiring. If the intake plenum is removed, ideally, the gaskets need to be replaced too.
Replacing the coil doesn’t always solve the misfire. I’ve been a mechanic for twenty years, and I’ve seen this lots. The car comes into the shop with a new coil fitted, customer complains of a misfire on cylinder X.
I know what’s happened here. The engine developed a misfire. The customer read the misfire code and swapped out the coil only to find the coil wasn’t faulty.
I don’t blame the owner. I genuinely love the DIY approach. The owner’s only mistake is not verifying the fault before committing parts.
How To Diagnose A Faulty Coil
This hack will help you successfully diagnose a failed coil. You’ll need a code reader for this, but they aren’t expensive, and any DIY MacGyver type should have one in the glove box anyway. Check out a neat glovebox-sized code reader I recommended in this review – “Topdon vs Autel”.
A failed coil should set a code P0350, but an intermittent fault may not. But the PCM will record the misfires. Reading the fault codes will likely show one of the following:
- P0302 – Cylinder 1 misfire
- P0302 – cylinder 2 misfire
- P0303 – cylinder 3 misfire
- P0304 – Cylinder 4 misfire
- P0305 – Cylinder 5 misfire
- p0306 – Cylinder 6 misfire
- P0307 – Cylinder 7 misfire
- P0308 – Cylinder 8 misfire
- P0300 – Random cylinder misfire
If you find codes for multiple cylinder misfires, coil failure is not the likely root cause. A bad fuel pump, fuel regulator, bad gas, timing off, bad valve, head gasket, faulty EGR valve, EVAP purge valve, etc.
Remove the engine cover to access the coil packs. Engine cylinder numbering isn’t standardized. Bank 1, also called “A” by some manufacturers, is always the number one cylinder. On V engines, the forward cylinder identifies the number one cylinder and therefore Bank 1 or A.
On straight engines, the first cylinder closest to the drive belt is the number one cylinder. Your engine may be fitted transversely or longitudinal. See the graphic below. When your engine is fitted transversely (front-wheel-drive cars), the front of the engine faces the front right wheel.
Google your engine maker’s cylinder numbering before removing any further components. Best to check we’re working on the correctly identified cylinder.
Record your fault codes and then clear them from the fault memory. Mark the suspect coil, remove it (usually a bolt and block connector with security lock), and swap it with whichever other coil is convenient to access.
Start the engine again for a moment. The check engine light will set. Shut off the engine and read the codes again. If the fault code now points to the faulty coils’ new location (fault followed coil), then you have confirmed a faulty coil.
You’ll find all the tools you’ll need to nail this repair on the “Mechanics tools page”, and you’ll find ignition coils at the Amazon link below.Amazon Ignition Coil
Chasing A Misfire
OK, so you swapped the coil, and the problem stayed with the cylinder. Now what? Begin by reading the codes and clearing the codes once again. The following is a list of likely possible causes in order of how likely it is:
- Faulty spark plug
- Faulty fuel injector
- Compression fault
Now we’ll look at the next likely cause of the cylinder misfire, which is the plug.
Inspect The Spark Plug
Remove the plug and check its condition. We’ll expect it to be fouled as the cylinder is misfiring. However, it shouldn’t be wet as modern cars shut fuel to a misfiring cylinder. An oily deposit on the plug may suggest a mechanical issue.
The simplest hack is to clean the plug (wire brush), and like we did with the coil, swap it with another cylinder that’s convenient. As before, reassemble and start the car, engine light on and read the codes.
If the code follows the plug, swap your plugs out for new ones. Most cars today are fitted with long-life plugs. They typically last about 100k miles and cost between 10 and 20 dollars.
I wrote a post about changing the spark plugs “Do spark plugs need to be torqued”.
Check For Faulty Injector
When the engine is still misfiring on the same cylinder, we’ll turn our attention to the injector. A mechanics hack – place a screwdriver on the injector and place your ear to the handle. The operating injector will click rapidly.
This isn’t the best way to approach diagnosis, and remember, when the MIL (engine light) is on, the injector is shut down. Injector faults range from:
- Wiring issue
- Faulty injector winding
A separate test is needed for each. The swap to another cylinder approach won’t work here, too much work to swap injectors about.
Clear codes from the computer and, with your DVOM set to volts back, probe the injector block connector pins. Note the window to test will be small as the injector is shut down by the computer as it detects a misfiring cylinder, don’t let this lead to misdiagnoses.
Have a helper start the car while you check voltage or use the capture function of the meter if applicable.
Voltage for even a moment and your all good, missing voltage, check to wire open back to the PCM (Power-train Control Unit).
Faulty injector winding
Remove the wiring block connector. Set the meter to resistance and check your Ohms reading. An OL reading means the injector winding is open. A resistance reading between 12-16 ohms is normal, but check the resistance value on neighboring injectors and note a large difference suggests a faulty injector internal circuit.
Injector flow fault
Testing flow is the most amount of work as the injectors need to be removed for spray pattern comparison and fuel volume. This test is dangerous. The fuel is pressurized and without proper test equipment risk of spilled gas and fire is real.
I’d skip this fuel flow test. For now, jump to checking compression. If the compression test is good, come back and explore the injector flow test.
A lack of compression will cause an engine to misfire, but it is the least likely and, unfortunately, the most expensive to repair. The most common causes of low compression include:
- Blown head gasket
- Faulty Valve
- Worn piston rings
The best test here is the leak-down test. The test does require a leak-down test kit and a compressor. The test involves removing the spark plug setting the engine to TDC (Top Dead Centre) on compression stroke on the offending cylinder.
Compressed air is pumped into the cylinder, and the gauge monitors the pressure loss. The more usual way to find the weakness in the cylinder is to use your ears. The air leak points to the failure. See the infographic below.
As an alternative, you can run a compression test. It requires just a compression test gauge. You’ll need to check all cylinders and the max difference between cylinders is 8% and 5% if it’s a high-performance engine.
Running a wet test will help you diagnose if the low cylinder is a valve train (top end) fault or piston rings (bottom end). See the infographic below. You’ll find all these tools on the “Mechanics tools page”.
How long does it take to change an ignition coil pack? Changing the coil pack on most four-cylinder cars won’t take more than 1 hour. A larger engine may take much longer as other components may need to be removed.
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John Cunningham is an Automotive Technician and writer on Rustyautos.com. He’s been a mechanic for over twenty-five years and has worked for GM, Volvo, Volkswagen, Land Rover, and Jaguar dealerships.
John uses his know-how and experience to write fluff-free articles that help fellow gearheads with all aspects of vehicle ownership, including maintenance, repair, and troubleshooting.