Mechanics

Misfires

Step 5 - The spark plugs in your engine can give you valuable information about how the engine is running. The chemical reaction inside the combustion camber will leave deposits on the spark plug that can tell you the condition inside the engine's combustion chambers. For example: if the engine is running rich all of the spark plugs will have a black or grey soot covering the spark plug electrode. If the engine is running lean all of the spark plugs will be clean and white (check your spark plug condition). If one of the spark plugs looks considerably different from the remaining spark plugs that cylinder might be having a problem.

Before you remove the spark plugs, mark the plug wires to identify their positions in the firing order, this will help to make sure they do not get mixed up. Next, use a spark plug socket to remove the spark plugs, we use a spark plug socket because the inside of the spark plug socket is lined with rubber to help cushion the spark plug insulator, which is made of porcelain and is easily cracked or broken. If a spark plug insulator is cracked or broken the engine will misfire because the ignition spark will travel to the engine block (ground) instead of the spark plug gap (between the electrodes)

Step 6 - Engine compression is very important for a smooth running engine. An engine depends on an equal compression reading in each cylinder to run smoothly. If poor compression exists in one or more cylinders it can cause a rough idle condition and low engine power output. A compression test of all cylinders is needed to check for engine wear and internal damage. Remove ignition coil connector or ignition system or fuel pump fuse to disable power to the ignition or fuel system to avoid fire. 

Compare cylinders compression reading, typical compression readings are between 125 p.s.i. and 160 p.s.i. all cylinder readings should be within about 5% of each other. If low compression exists a cylinder malfunction exits and further troubleshooting is required. Possible causes for a low compression condition are: burned intake or exhaust valve, broken piston or piston ring, broken valve spring or a blown head gasket. If engine compression tests "okay" proceed to the next step.

Step 7 - The ignition system in your car ignites the fuel inside the engine's combustion chamber at the optimal time in the piston stroke to produce the most power while emitting the least amount of emissions as possible. There are many configurations of ignition systems but all operate on the same principle, create a low energy field and collapse it onto a high energy coil and that transfers the electrical energy into the secondary ignition system, i.e. coil wire, distributor cap and rotor (if equipped) plug wires and finally the spark plug.


Testing the Ignition System Video

This system is triggered by the primary ignition system, this system varies depending on manufacturer but all operate on the same principle, use some kind of low voltage trigger system i.e. crankshaft position sensor (CKP), camshaft position sensor (CAS). This low voltage system (1.5 to 3.0 volts) is amplified to 12 volts by using an ignition module (amplifier) and then transferred to the primary side of the ignition coil. The PCM (Powertrain Control Module) controls the engine ignition timing by advancing and retarding the primary trigger signal. In older cars points and condenser with a vacuum advance unit performed this job. We have listed the most common causes for an ignition system to fail or misfire.

Step 8 - Test for proper fuel pressure with a test gauge on the fuel rail or in line somewhere in the system, most throttle body injection cars (TBI) are between 13 psi and 17 psi. and most (DPI) direct port inject systems are between 40 psi and 55 psi. If good fuel pressure is present continue to next step. If no or little fuel pressure is present check the fuel pump fuse and fuel pump control relay located in the fuse panel, you can find this fuse and relay by checking your owners manual, back of the fuse panel cover diagram, or an online auto repair manual, if the fuse or relay has failed replace it a new unit and re-test. Have a helper crank over the engine while you place your fingers over the relay, does the relay click under your fingers? If so the relay could be working, there is a chance the relay has burned contacts inside causing the problem but we will get back to that.

Next, access the fuel pump power feed wire, there are a few ways to do this, first you need a wiring schematic to find the color wire needed for testing, the best way to do this is with a service manual. Once you have found the color wire it should be located in the wiring harness near the fuel tank were the pump is located. Ground the test light and probe (pierce the wire's outer coating with the test light point) the wire, have a helper crank the engine over. If the test light illuminates and you have no fuel pressure the fuel pump had failed and needs to be replaced.  If the test light doesn't illuminate the fuel pump control relay has probably failed, replace it with a new unit and re-test, in most cases this relay is under thirty dollars. There is an outside chance the power feed to the relay has failed but it doesn't happen very often. If this is the case trace the power source to the relay.


How to Test Fuel Pressure Video

 Step 9 - While an engine is running you should be able to hear the injectors clicking as the electronic valve opens and closes. To aid in this inspection use a stethoscope and touch it against each injector. If no audible sound is heard test fuel injector pulse and supply voltage output (this test is used for most cars).  This test will tell you if the computer system has operating voltage and injector trigger signal. Remove an electrical connector from a fuel injector (all injectors need to be tested) probe both sides of the connector with a grounded test light or multimeter (voltmeter) (black lead) switched to DC voltage (there are only two terminals).

 Have a helper turn the key to the "on" position without cranking the engine and observe the test light. The test light should illuminate one side of the connector only and the multimeter should be at about 12 volts. Next, switch the test light or multimeter (voltmeter) lead (black) or to the positive side of the battery to test the system ground injector trigger, probe the side of the connector that did not light up, have a helper crank the engine over and observe the test light, it should blink on and off or the multimeter (voltmeter) should bounce from 0 to 12  volts. (Note: if no injector pulse is present try disconnecting the remainder of injectors and re-test, if a fuel injector is shorted it can shut down the injector driver causing no injector pulse.

If injector pulse returns plug the injectors electrical connectors in one at a time until the pulse fails and then replace that injector). If this test revealed that there was no pulse but system has power, the PCM is not generating a fuel injector trigger. If there is no trigger to the fuel injector it will not allow fuel to enter into the engine. Some of the most common reasons that can cause this condition include a shorted fuel injector, injector wiring damage or shorted PCM. While the injector trigger wire is off, test the fuel injector windings by setting the multimeter (voltmeter) to ohms and attach the leads to either side of the injector.

AUTHOR


Written by
Co-Founder and CEO of 2CarPros.com
35 years in the automotive repair field, ASE Master Technician, Advanced Electrical and Mechanical Theory.


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Article first published (Updated 2013-08-16)