Rick's Free Auto Repair Advice

Diagnose a misfire

How to diagnose a misfire trouble code

You’re probably here because your computer set a misfire code and now you want to know how to diagnose a misfire. Pros use some pretty sophisticated tools to diagnose a misfire, but there are some things you can do yourself to track down the most likely candidates. But first, here’s some background on misfire codes.

How does the computer know which cylinder is misfiring?

Contrary to popular believe, engine rotation is not smooth. Instead, it’s a series of power pulses. Every time a piston/cylinder enter the power cycle (right after ignition), it speeds up the rotation of the crankshaft. The computer reads the crankshaft position sensor to obtain the timing of the crankshaft but also its speed. The computer knows when it commanded ignition and fuel injection for a particular cylinder and it expects to see an increase in rotation speeds immediately after ignition.

If the cylinder misfires, its contribution to crankshaft rotation speed will be less than expected and the crankshaft will slow slightly. If the speed decreases regularly after the computer commands ignition and fuel injection to a specific cylinder, the computer concludes that that particular cylinder is misfiring.

What are the most likely causes of misfire?

#1 Worn spark plugs. In a perfect world, all spark plugs should wear evenly. But we don’t live in a perfect world. Spark plugs can wear unevenly and a spark plug in one cylinder can misfire before the others.

Here are the most common causes of misfiring spark plugs:

oil fouled spark plug that causes a misfire

Fouled spark plug

excessively worn spark plug that causes misfires

Worn spark plug

broken side spark plug electrode causes misfires

Missing side and center electrodes.











1)     You haven’t changed them according to the car makers schedule

2)     The spark plugs weren’t torqued properly during installation. Over-torqueing can distort the shell can cause a leak between the shell and the porcelain. Under torqueing, on the other hand, causes combustion gasses to leak around the spark plug threads. On aluminum heads, proper torque is mandatory. You simply MUST use a torque wrench. The days of torqueing by feel are long gone.

3)     Worn spark plug wires. Early wear on spark plug wires causes random misfires. Those misfires cause buildup on the spark plug electrodes causing them to “ground” out. Once the ignition coil “fires” the voltage has to go somewhere. If it can’t jump across the spark plug gap, it’ll jump down the outside of the spark plug between the boot and the porcelain. If the problem isn’t addressed, the spark will eventually fire right through the spark plug wire insulation. That can damage other electronic components under the hood.

4)     Weak spark plug coil

new and damaged coil on plug ignition coils. Damaged ignition coil fires through side of coil and causes a misfire

Misfiring coil on plug ignition coil. Notice the arcing on the bottom coil. The spark has been shooting righ through the side of the coil above the boot.

5)     Oil and coolant leaks can foul spark plug.

6)     Leaking valves can foul spark plugs

7)     Incorrect spark plug or incorrect gap

8)     Stuck intake or exhaust valve

9)     Improper seat or crack on intake or exhaust valve

10) Vacuum leak near the cylinder

Fix Multiple misfire codes

These same items can cause multiple misfire codes. For example; if all plugs and wires have the same wear, they can misfire at the around the same time. BUT, you should also check these other items as well:

• Vacuum leak can cause lean misfires on multiple cylinders—even cause all cylinders to misfire

• Unmetered air caused by a tear in the air duct between the air filter box and the throttle body; again causing lean misfire

• Low fuel pressure that causes a lean misfire

• Clogged fuel injectors

• Carbon buildup on the intake valves that reduces the air charge coming into the cylinder or prevents the complete valve closure.

• Bad gas

Check fuel pressure and fuel delivery volume

Attach a fuel pressure gauge and check fuel pressure at pump prime and running. Then perform a fuel volume test to rule out the fuel pump and fuel pressure regulator

Check fuel trims on your scan tool

View long and short term fuel trims. High fuel trim reading usually indicate a vacuum leak or low fuel pressure. Low or negative fuel trim indicates leaking fuel injectors.

Step 1 Swap coils

If you have a coil-on-plug ignition system, swap the coil from the cylinder that’s misfiring with a coil from a different cylinder. Clear the trouble code and drive the vehicle. If the misfire now occurs in the swapped cylinder, replace the coil.

If you have a waste-spark distributorless ignition system DIS ignition system, the misfires should appear in the two cylinders powered by the same coil pack.

distributorless igntion coil pack

DIS coil pack. Each coil powers two cylinders

The spark jumps across the gap (center to side electrode) in the firing cylinder and returns to ground by firing side to center electrode on the partner cylinder which is on the exhaust stroke. When the partner cylinder is on the ignition cycle, the power flow reverses. So, if the spark can’t jump the gap on both plugs, you would see two misfire codes on the two partner cylinders. If you have coil packs and can swap them, move one coil pack to a different position. If the misfire follows to the new cylinders, replace that coil pack. If the misfire stays in the original cylinders, suspect the plugs and wires.

Check and replace worn plugs and worn plug wires

Next, check the condition of the spark plugs and spark plug wires (if your engine uses spark plug wires). But here’s a warning: The threads on most spark plugs are treated with a nickel coating to prevent them from seizing in the cylinder head. That coating eliminates the need to coat the threads with anti-seize compound. BUT, that coating is a one-time protection. You cannot remove a spark plug and reuse it without applying anti-seize during the re-installation. If you skip that step, you greatly increase the chances of that plug seizing in the head.

You can test spark plug wire resistance using a DVOM meter. If there’s an open condition in the wire, that test will disclose it. But the DVOM test is not definitive. A wire can test good but still misfire through the insulation in actual use. If the wires have many miles on them, replace them along with the spark plugs.

If you’ve replaced the spark plugs and wires and swapped the coils or replaced them and still have a misfire code on a specific cylinder, your job just got a bit harder. Now you could be looking at a fuel injector problem, a vacuum leak, or a valve issue. Here are some DIY tips to help you diagnose a misfire at this point

Perform a compression test and cylinder leakdown test

A dry/wet compression test will identify a valve/ring problem. Remove all the plugs and check the compression on each cylinder without adding oil. Repeat the test by adding three oil pumps into the spark plug hole. Then compare readings. If you have a weak cylinder, you should see much lower readings on that cylinder. However, since this test is done at low RPM, it isn’t definitive. You could still have a sticky valve that’s simply not closing fast enough at higher speeds. Before you conclude you need a valve job, do an upper intake cleaning.

Perform an Upper Intake Cleaning and test the fuel injectors

Since there’s no downside to cleaning upper intake valves, you may want to do this first.

spray intake valve cleaner by CRC

CRC Intake Valve Cleaner

This procedure introduces a cleaning solvent into the intake to clean the valve stems and possibly eliminate a sticking valve. There are several ways to do this. The most common technique is to remove the vacuum hose from the brake booster unit and use it to suck liquid cleaner into a running engine. I’m not fond on this idea. If you feed the solvent too quickly, you can hydrolock the engine, causing expensive damage. Here’s a much safer way.

By a few cans of CRC Intake Valve Cleaner from any auto parts store. Then follow these instruction to the letter:

(From the CRC website)

It is recommended to read the entire product label before use.

1. Start engine and meet operating temperature.

2. Remove air filter cover.

3. Locate MAF sensor between the air box and throttle body.

4. Engage (lift) the straw on the PermaStraw Dual-Action Spray System and insert PAST THE MAF SENSOR. To avoid throwing a code, DO NOT spray product in front of the MAF sensor. You may need to disassemble air intake to administer past the MAF sensor.

5. With engine running at 2,000 RPM, spray product into air intake, behind the MAF sensor, continuously for 30 second intervals until can is empty. If necessary, accelerate to avoid engine stalling during spray period. DO NOT exceed 3,500 RPM. Take care not to burn arm or hand on hot engine surfaces while dispensing can.

6. Once can is empty, accelerate the engine 2 to 3 times without exceeding 3,500 RPM.

7. Run at idle for one minute, then turn engine off.

8. Reassemble air intake system and let engine heat-soak for one hour.

9. Restart engine and drive at highway speeds for at least 10 minutes.

After cleaning, redo the compression and cylinder leakdown test. If the results look good, you’ve most likely corrected the problem. Clear the codes and take if for a drive. If the code doesn’t return, you’re done. If the results still how a weak cylinder, it’s time to take it to a shop.

Check for a plugged, leaking or misfiring, fuel injector

Most car makers provide power to all the injectors and fire each one by connecting the ground at the proper moment. You can check power at the injectors with a DVOM. With the key in the RUN position, you should see battery voltage at the power wire of each injector. If an injector doesn’t have power, that would cause a misfire. Track down the cause of the missing power.

To check ground operation, you’ll need a fuel injector noid light. Hold the noid light near the injector while the engine is idling. The light should blink every time the injector is grounded by the computer. If you see that, increase the idle speed to 2,000 rpm. Now look for misses in the blinking pattern. If you see gaps, you might have a weak solenoid coil or a ground issue between the injector and the PCM. If the injector passes the power and noid light test, it could still be plugged or leaking. At that point, you’ll probably have to take it to a shop and have a flow test done. Or, if you’re willing to gamble, you can replace the suspect fuel injector.

Test for a vacuum leak

It’s rare, but a vacuum leak near the misfiring cylinder can cause only that cylinder to misfire. So check for vacuum leaks in hoses near the misfiring cylinder and along the head and intake gaskets around that cylinder.

What not to replace

Fuel pump

You should never replace a fuel pump without first conducting a fuel pressure test, a fuel volume test, and a leakdown test. And, while a bad fuel pump might produce a P0300 random misfire code, it generally doesn’t produce a single cylinder misfire code.

Fuel pressure regulator

Some wanna-be mechanics think a fuel pressure regulator might cause a cylinder specific misfire. Ah, I don’t think so. If you have a fuel pressure regulator mounted on the fuel rail (most newer systems have the regulator in the gas tank), you don’t have much to lose by removing the vacuum hose and checking for fuel in the vacuum line. If you see or smell raw fuel, the regulator diaphragm is bad and you must replace the regulator. But if there’s not leak, don’t replace the regulator.

©, 2014 Rick Muscoplat



Posted on by Rick Muscoplat

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