The Professional Way To Diagnose an Engine Misfire

The Smart Way To Diagnose an Engine Misfire: A Comprehensive Guide

Engine misfires are one of the most common and frustrating problems a car owner can experience. An engine misfire occurs when one or more cylinders in your engine fail to produce power at the right time. This issue can lead to reduced engine performance, increased fuel consumption, and, in severe cases, damage to the engine itself. Diagnosing an engine misfire involves understanding its symptoms, identifying the potential causes, and following a step-by-step approach to pinpoint the problem.

The Right and Wrong Ways To Diagnose An Engine Misfire

If you’ve read the trouble codes and find that the engine misfire is limited to one or two cylinders, the Internet advice columns will tell you to start by replacing the spark plug in those cylinders. If that doesn’t fix the problem, they then suggest swapping ignition coils around to see if the engine misfire follows the coil swap. From a DIY point of view, this seems to make sense because the cost of a few plugs or a single coil is pretty low compared to the cost of a shop diagnostic charge.

However, the downside to this approach is that replacing a spark plug or ignition coil is a pretty crude diagnostic procedure that doesn’t make sense in the big picture because single plugs usually don’t fail. If all the plugs and coils were installed at the same time, single plugs and coils usually don’t fail unless there is some underlying mechanical issue. Replacing a plug or coil is often a temporary solution to a more serious problem.

Single cylinder codes are usually not caused by a bad spark plug or coil

If your code(s) indicates a single-cylinder engine misfire, and all the plugs and coils were replaced simultaneously, the root cause of the engine misfire is usually NOT caused by a single defective spark plug or defective coil. While it is possible to get a defective spark plug or coil, it’s rare, and the failure happens immediately after installation, not thousands of miles down the road. Likewise, it’s also possible to damage a spark plug during installation due to over-tightening, which causes a porcelain-to-shell seal leak. But again, that type of failure usually shows up immediately after installation.

Mechanical issues, such as a worn camshaft lobe, cracked or leaking intake or exhaust valve, or fuel injector failure, are more often the root causes of a single-cylinder engine misfire. That mechanical failure often damage the ignition coil, tricking DIYers and some technicians into replacing the plug and coil, only to have those fail again.

• The wrong way to diagnose an engine misfire— Replace the spark plug, erase the trouble code, and take if for a ride. If the code comes back, swap the ignition coil to a different cylinder, clear the code and take it for a ride. If the trouble code follows the swap, replace the coil, clear the code and take it for a ride. If the code returns, get really frustrated and take it to a shop for a diagnosis.

• The Right Way to Diagnose an Engine Misfire— Buy or rent the proper diagnostic tools and find the underlying problem without buying any parts.

Diagnosing an Engine Misfire: The Step-by-Step Process

Diagnosing the cause of an engine misfire requires a systematic approach. Below are the primary steps you should take to determine the source of the problem:

Step 1: Retrieve Diagnostic Trouble Codes (DTCs)

The first step in diagnosing an engine misfire is to connect a code reader or OBD-II scanner to your car’s onboard diagnostic port. This device will display trouble codes that can help identify the problem. For instance, a code like P0301 indicates a misfire in cylinder 1, while P0302 points to a misfire in cylinder 2, and so on.

Step 2: Check for spark using a spark tester

Buy or rent a coil-on-plug spark tester (Lisle $20700 is one I recommend). Remove the coil from the misfiring cylinder. Insert the tester into the coil and connect the clamp to a good ground. Dial in the spark plug gap. Then, have a friend crank the engine and watch for a consistent spark. This test confirms a good ignition coil.

If you don’t get a consistent spark, buy or rent a noid light set to check for wiring harness operation. Disconnect the electrical connector from the harness and insert a noid light. A consistently flashing noid light confirms there’s no problem with the wiring harness.

Spark Plugs: Remove and inspect each spark plug. Look for worn electrodes, excessive carbon buildup, or oil fouling. Replace any damaged or worn-out spark plugs.

Step 3: Check For Fuel Injector Operation

There’s no easy way to check for a clogged fuel injector without removing it from the engine and testing it on a flow bench. While checking for consistent fuel pressure drop during each fuel injector operation can help narrow down a clogged fuel injector, it’s not foolproof, and it’s time-consuming. Professional technicians often use an automotive stethoscope to listen to the sound of each injector and compare the sounds that might indicate a malfunctioning injector. To do this, simply touch the stethoscope probe to each fuel injector and compare the sounds. The injector that sounds different is a likely cause of the engine misfire.

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

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, watching the gauge to ensure the pressure rise stops. 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 to bring the cylinder you’re testing to TDC on the compression stroke. This is when both the intake and exhaust valves are closed.
Tip: Insert a long screwdriver or a similar tool into the spark plug hole and rotate the crankshaft until the tool stops moving up, 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 threaded properly and snugly to avoid 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 to 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.

Step 6: 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.