Monday, June 20, 2011

How to troubleshoot Engine Misfires?

Troubleshooting Gasoline Engine Misfires

Misfire issues can generally be categorized as localized or random. A localized misfire in my definition would occur on at most two specific cylinders. In OBD II diagnostic code terms, that would be 2 trouble codes of the form P030x and P030y.; whereas a code P0300 or several different codes P030a, b, c, etc. would indicate a random misfire issue. These two different categories of misfire call for different troubleshooting procedures. In each case, however, the cause of the misfire can be electrical or air/fuel related and can also be continuous or intermittent. I will start with the localized case, so if you have a random misfire, please skip ahead to that section of the post.

Single-cylinder or localized misfire troubleshooting:
The easiest thing to check is always the secondary electrical circuit.
Caution, high voltage ignition sparks can hurt you, but the most serious injuries happen when pulling back and striking something else, like the hood of the car. A single spark is not dangerous to a healthy person, but we always prefer to avoid getting shocked. When testing the ignition wires, coil, or plugs, use an insulating mitt or tool or at least make sure a better ground than your body is available for the spark to jump to.

Electrical checks: If you don't have the diagnostic codes for the misfire or just don't know which cylinder is misfiring, methodically pulling the wires from the plugs on each cylinder in turn is a good way to find the bad actor. When doing this, pull the wire off and away from the plug and let it jump to engine ground. If sparks are jumping, move the wire back and forth from the plug to engine ground and listen for changes in the engine rpm. If there is no change in engine rpm with the wire on versus off the spark plug, that cylinder is misfiring.
Once the particular cylinder(s) misfiring have been identified, you can test both the coil and wire by pulling the wire boot slightly off the plug in question--you can hear the spark jumping to the plug. If your misfire is continuous on this cylinder, you would hear no sparks at all. If the problem is intermittent, listen for missed sparks. If you identify a cylinder with a spark misfire, try swapping wires and coils to determine if the problem is in the wire or the coil. If the problem is not in the wire or the coil, check the distributor (if you have one) for a bad inside terminal and also pull and test the spark plug by putting it back in the wire, grounding the threads, and cranking the engine to check for a good spark at the gap (you might want to pull a few wires off the other p[lugs to avoid having the engine start with the plug out, as this can be very noisy). If all of the electric components seem good, move on to check the cylinder internals.
Fuel injector: Put a long screwdriver on the injector and put the other end against your ear while someone cranks the engine. If the solenoid is not clicking regularly, suspect a bad injector. It is also possible the injector is plugged or dirty. This condition is difficult to diagnose, but if you suspect this you can remove the fuel rail and clean the injectors per the manufacturer's procedure.

Compression test: Buy or borrow a compression tester. Remove several spark plugs, including the one(s) that are misfiring. Screw the compression tester into the plug socket hand light (or hold the tester in the hole if it has a rubber taper end), and have someone crank the engine through several revolutions. Read and record the pressure on the gage, release the pressure in the gage and go on to the next cylinder. A reading of less than 50 or so psi can cause a misfire. A low reading can be caused by a burned valve, a blown head gasket, or possibly an over-tightened rocker arm, among other more remote possibilities. The latter case (rocker arm) usually only occurs after head or valve work was recently done on the engine and was followed by improper valve lash adjustment. Except in this case, the cylinder head must be removed for repair. A low pressure reading on adjacent cylinders is a clue that the head gasket is blown between the two cylinders. Additional clues as to the type of failure may be gleaned from careful inspection of the spark plug.

Random misfire troubleshooting:
The engine computer sets these codes when the crankshaft position sensor indicates rpm hunting under constant external conditions. Therefore, it is very difficult to distinguish genuine misfiring cylinders from generalized rough running.

Random electrical misfire:
If the cause is electrical, it is in the ignition system that produces the signal to the coil(s). In older engines, this might be due to a failing ignition modulator or pickup coil in the distributor. In modern engines, it might be a loose, failing, or poorly connected crankshaft position sensor (or camshaft position sensor in those engines that use this sensor for spark timing). This condition may or may not set an OBD II trouble code. Because of the large variation in ignition systems, I recommend you get the wiring diagram for your ignition system to see what components may be the cause of an inconsistent ignition signal to the coil(s). In rare cases, the problem may be with the modern ignition control, engine control, or powertrain control module. The problem may also be with general deterioration of the ignition wires. This condition can be verified by running the engine in a dark environment and watching for sparks jumping off the coil or wires.

Air/Fuel causes of a random misfire:
For the issue to be identified as a misfire, the changes in conditions in the cylinders must be rapid rather than gradual (such as in the case of a hunting rpm). These kinds of changes may be caused by unsteady airflow in the intake or inconsistent injector operation. Always check for a clean throttle body and idle air passage as well as idle air control motor operation. Also check the EGR valve operation and flow path.There may be specific tests you can run on these valves for your car. Consult a manual if you want to check these out carefully. Most of these components can be cleaned with good old-fashioned carb cleaner. It's also a good idea to make sure your vacuum operated fuel pressure regulator (if you have one) isn't drawing fuel into the intake manifold through a torn diaphragm.
Random injector issues are rare but may be caused by dirty fuel or worn out injectors. Another possibility here is a camshaft position sensor malfunction, but this will usually set a trouble code. In rare cases, injector timing and pulse width irregularity can be caused by the computer itself.

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