Mustang Misfire detection?

[Isaac-1]

Last weekend my SES light came on with the code for misfire on Cylinder 2 just after I had my oil changed (I don't know if these were related), I reset the computer and have had more misfires detected in about 300 miles of driving. My question is how do our cars detect misfires and know which cylinder they occur on?

Thanks Ike

 

[enyawix]

easy if a missfire happens the oxygen is not used in that cycle your oxygen sencer can pick that up.

 

[TweekedGT]

It is computer detected. You can through it on a scanner as well and it will pick up every misfire.

 

[sgarlic]

From the FSM:

The Misfire Detection Monitor is an on-board strategy designed to monitor engine misfire and identify the specific cylinder in which the misfire has occurred. Misfire is defined as lack of combustion in a cylinder due to absence of spark, poor fuel metering, poor compression, or any other cause. The Misfire Detection Monitor will be enabled only when certain base engine conditions are first satisfied. Input from the ECT or CHT, MAF and CKP sensors is required to enable the monitor. The Misfire Detection Monitor is also performed during on demand self-test.

1. The PCM synchronized ignition spark is based on information received from the CKP sensor. The CKP signal generated is also the main input used in determining cylinder misfire.

2. The input signal generated by the CKP sensor is derived by sensing the passage of teeth from the crankshaft position wheel mounted on the end of the crankshaft.

3. The input signal to the PCM is then used to calculate the time between CKP edges and also crankshaft rotational velocity and acceleration. By comparing the accelerations of each cylinder event, the power loss of each cylinder is determined. When the power loss of a particular cylinder is sufficiently less than a calibrated value and other criteria is met, then the suspect cylinder is determined to have misfired.

 

[jstreet0204]

Last weekend my SES light came on with the code for misfire on Cylinder 2 just after I had my oil changed (I don't know if these were related), I reset the computer and have had more misfires detected in about 300 miles of driving. My question is how do our cars detect misfires and know which cylinder they occur on?

Thanks Ike

Ahhhhh... It is nice to see a GOOD tech question one in a while. The misfire detection is actually not done the way you would think. One woud think that it is somehow monitored at the ignition level by monitoring the firing of the coils. It isn't though. It is monitored by calculations done on info sent from the crank sensor. The eec monitors the acceleration of the crankshaft wheel. It checks for deviations in the acceleration, and knows what cylinder should be firing at that time. A certain percentage of misfires are considered acceptable based on speed and load. If that threshold is exceeded first the time, the MIL should blink. If the misfire is detected again in the same driving cycle, the MIL will stay on. Misfires can cause raw fuel to go through the system and damage cats. To prevent this, if the eec continues to see misfires on a cylinder, it can disable that injector. Up to two injectors can be disabled.

 

[sgarlic]

Ahhhhh... It is nice to see a GOOD tech question one in a while. The misfire detection is actually not done the way you would think. One woud think that it is somehow monitored at the ignition level by monitoring the firing of the coils. It isn't though. It is monitored by calculations done on info sent from the crank sensor. The eec monitors the acceleration of the crankshaft wheel. It checks for deviations in the acceleration, and knows what cylinder should be firing at that time. A certain percentage of misfires are considered acceptable based on speed and load. If that threshold is exceeded first the time, the MIL should blink. If the misfire is detected again in the same driving cycle, the MIL will stay on. Misfires can cause raw fuel to go through the system and damage cats. To prevent this, if the eec continues to see misfires on a cylinder, it can disable that injector. Up to two injectors can be disabled.

Didn't I just say that?

 

[jstreet0204]

Didn't I just say that?

Yeah but is wasn't there when I first hit the reply button.