Troubleshooting misfires
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Tech Droid
Joined: Aug 2005
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From: North East PA
Troubleshooting misfires
Engine Misfire and Rough Running Condition
[/b]There are several types of engine misfire. Under load misfire, engine idling misfire, continuous misfire, and random misfire.
Engine misfires exist because one or more of these three reasons. In order of probability:
1)Ignition system has failed, or is failing intermittently, or is firing at the wrong time.
a. spark plug has fouled or is worn out
b. ignition coil has failed
c. spark plug wires have shorted
e. engine control module coil driver has failed
2)Fuel air mixture is incorrect on one or more cylinders. Way off air fuel mixture will not burn.
a. vacuum leak at the intake manifold
b. fuel injector has failed
c. EGR valve is stuck open
d. mass air flow sensor has failed
e. oxygen sensor has failed
f. air intake boot is cracked
3) One or more cylinders lost compression. A certain amount of compression is needed to support combustion. Also, if compression is too low and the fuel mixture actually ignites, there will be little power produced, which would feel the same as a misfire.
a. burned or leaking intake or exhaust valves
b. worn or broken piston or piston rings
c. worn out camshaft
d. wrong weight motor oil was installed holding the cam followers from adjusting
e. broken valve spring, bent push-rods, damaged rocker arms
f.failed head gasket.
Newer cars with OBDII systems cab detect misfires set a trouble code and light the MIL/CEL light. A code reader or preferably a scanner can be used identify the misfiring cylinders. Older cars using OBDI systems may[/b] cause the MIL/CEL light to flash with the misfire. Non computer controlled cars leave it up to your trouble shooting skills, although ignition scopes are available than can identify misfires by looking and comparing ignition traces on the screen.
Common problems:
[/b] 1.Shorted, burned or otherwise bad spark plug wire
2.Worn out, cracked, fouled, or otherwise damaged spark plug
3.Failed or failing ignition coil.
4.Failed or failing ignition control module
5.Distributor problems, such as carbon tracking, worn or damaged rotor & cap
6. BadKPS or CPS
7. Bad fuel pressure due to bad fuel pump, pressure regulator, fuel filter
8. Bad fuel injector
9. Bad MAF
10. Bad O2 sensor/s
[/b]There are several types of engine misfire. Under load misfire, engine idling misfire, continuous misfire, and random misfire.
Engine misfires exist because one or more of these three reasons. In order of probability:
1)Ignition system has failed, or is failing intermittently, or is firing at the wrong time.
a. spark plug has fouled or is worn out
b. ignition coil has failed
c. spark plug wires have shorted
e. engine control module coil driver has failed
2)Fuel air mixture is incorrect on one or more cylinders. Way off air fuel mixture will not burn.
a. vacuum leak at the intake manifold
b. fuel injector has failed
c. EGR valve is stuck open
d. mass air flow sensor has failed
e. oxygen sensor has failed
f. air intake boot is cracked
3) One or more cylinders lost compression. A certain amount of compression is needed to support combustion. Also, if compression is too low and the fuel mixture actually ignites, there will be little power produced, which would feel the same as a misfire.
a. burned or leaking intake or exhaust valves
b. worn or broken piston or piston rings
c. worn out camshaft
d. wrong weight motor oil was installed holding the cam followers from adjusting
e. broken valve spring, bent push-rods, damaged rocker arms
f.failed head gasket.
Newer cars with OBDII systems cab detect misfires set a trouble code and light the MIL/CEL light. A code reader or preferably a scanner can be used identify the misfiring cylinders. Older cars using OBDI systems may[/b] cause the MIL/CEL light to flash with the misfire. Non computer controlled cars leave it up to your trouble shooting skills, although ignition scopes are available than can identify misfires by looking and comparing ignition traces on the screen.
Common problems:
[/b] 1.Shorted, burned or otherwise bad spark plug wire
2.Worn out, cracked, fouled, or otherwise damaged spark plug
3.Failed or failing ignition coil.
4.Failed or failing ignition control module
5.Distributor problems, such as carbon tracking, worn or damaged rotor & cap
6. BadKPS or CPS
7. Bad fuel pressure due to bad fuel pump, pressure regulator, fuel filter
8. Bad fuel injector
9. Bad MAF
10. Bad O2 sensor/s
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