The Smart Way to Diagnose an Engine Misfire

How to Diagnose an Engine Misfire Step by Step

Quick Summary
The ECM detects an engine misfire by monitoring the crankshaft position sensor. It knows when it commanded the plug to fire and then monitors crankshaft speed to determine whether it increases. If the cylinder misfires, the crankshaft will slow because the cylinder has failed to contribute its share of power. That failure can be momentary or constant, isolated to one cylinder or spread across several. Regardless, every engine misfire has a root cause, and finding it requires discipline, not guesswork. Here are the most common causes of an engine misfire:

1) Worn or fouled spark plugs or failing ignition coils are the most common causes. That’s especially true if the misfire occurs under load.
2) Air and fuel mixture issues come in a close second. They can be caused by a faulty MAF or MAP sensor, a vacuum leak, a clogged/faulty fuel injector, or low fuel pressure.
3) Mechanical issues are third on the list, with issues such as low compression from leaking valves, worn piston rings, or camshaft wear that can also prevent proper combustion and are frequently overlooked.

When I diagnose an engine misfire, I follow a logical process to verify spark, fuel, and mechanical integrity before replacing parts. Guessing costs money; testing finds the real cause. If you want a permanent fix—not a temporary one—you need to diagnose an engine misfire correctly.

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How to locate the cause of a misfire

The internet loves shortcuts. If a scan tool indicates a single-cylinder misfire, such as P0301 or P0304, the common advice is to replace the spark plug in that cylinder. If that doesn’t work, the advice is to swap the ignition coil and see if the engine misfire follows.

Step 1: Check for quality spark and good coil operation

The professional way to check for a good spark is with a spark tester hooked in series with the spark plug.

But we know you won’t do that. Instead, you’ll automatically replace the spark plug. If that doesn’t fix the misfire, you’ll swap the ignition coil to another cylinder. Ok, you’ve done that, and you found the misfire didn’t move to the new cylinder after you swapped the coil.

So what’s next?

Well, you skipped a few steps, like checking for corrosion, power, and ground at the coil and fuel injector. So, let’s backtrack and check those things now.

Step 2: Check the wiring for the ignition coil at the misfiring cylinder

Swapping the coil to another cylinder proved that the coil is working. But it doesn’t prove out the wiring at the misfiring cylinder. So go back to the misfiring cylinder and visually inspect the ignition coil connectors and wiring harness for loose terminals, corrosion, damaged insulation, or broken wires.

Using a digital multimeter, verify battery voltage in the coil connector with the ignition in the RUN position. Then check for a pulsed ground while cranking. If you see battery power and pulsed ground, that eliminates a wiring issue as the cause of the misfire.

Step 3: Check for power and ground at the misfiring cylinder’s fuel injector

Using a multimeter, check for battery voltage and pulsed ground at the misfiring cylinder’s fuel injector. In addition, professional technicians often use an automotive stethoscope to listen to each injector and compare the sounds for indications of a malfunctioning injector. To do this, touch the stethoscope probe to each fuel injector and compare the sounds. The injector that sounds different is likely causing the engine misfire.

Fuel Pressure—Don’t waste your time checking fuel pressure for a single-cylinder misfire. Low fuel pressure affects all cylinders equally.

Step 4: Conduct a compression test 

Buy or rent a cylinder compression tester. Then test the compression for all cylinders. Here’s how to do that:

1) Remove all the ignition coils and spark plugs. Check each plug for signs of coolant wetness on the electrodes or oil on the porcelain. A leaking spark plug tube seal can allow oil to enter the spark plug tube. That oil can cause engine misfires. The fix is a new valve cover gasket and spark plug tube seals.

2) Remove the in-tank fuel pump fuse and install the compression gauge on one cylinder. Have a friend crank the engine while you watch the gauge to ensure the pressure rises to the stop. Write down the pressure. Repeat with each cylinder.

If you find a cylinder with low compression (more than 20% lower than the others), conduct a cylinder leak-down test.

Step 5: Conduct a cylinder leak-down test

1) Position the Cylinder at Top Dead Center (TDC) — Refer to the engine’s firing order to determine the sequence. Turn the crankshaft: Using a wrench or ratchet, rotate the crankshaft until the cylinder you’re testing is at TDC on the compression stroke. This is when both the intake and exhaust valves are closed.
Tip: Insert a long screwdriver or similar tool into the spark plug hole and rotate the crankshaft until the tool stops rising, indicating TDC. Be careful not to damage the cylinder.

2) Connect the Leak Down Tester — Attach the leak-down tester to the cylinder: Connect the hose from the leak-down tester to the spark plug hole of the cylinder you’re testing. Ensure it is properly threaded and snug to prevent air leaks.

3) Connect the air compressor —  Attach the tester to the air compressor and set the compressor’s regulator to the appropriate pressure, typically between 80 and 100 psi.

4) Pressurize the cylinder — Open the valve on the leak-down tester to allow compressed air to enter the cylinder.

5) Read the gauge — The primary gauge on the tester shows the percentage of leakage. A healthy cylinder should have less than 10-15% leakage. Anything above this range indicates potential problems.

Identify the Source of the Leak — If the test shows significant leakage, you need to determine where the air is escaping:

• Hissing from the exhaust pipe— Indicates a leaking exhaust valve.
• Hissing from the throttle body or intake manifold— Points to a leaking intake valve.
• Air escaping from the oil filler cap— Suggests worn piston rings or a damaged cylinder wall.
• Bubbles in the radiator or coolant reservoir— Could indicate a blown head gasket or a cracked cylinder head.

©, 2024 Rick Muscoplat