- Location
- New Mexico
Trying to help a friend with his '96 Cherokee. It keeps throwing Misfire codes on random cylinders. It seems to be running great so I am not convinced it really is misfiring.
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How does the ECM determine a Misfire?
- Thread starter DaJudge
- Start date
Digger87xj
NAXJA Forum User
- Location
- Buffalo Creek/Lakewood CO
it can detect the slow-down in the speed the crankshaft in milliseconds between firing times of cylinders.
Original_MudButt
NAXJA Forum User
- Location
- In the eighth dimension
It's my understanding it uses the CPS in combination with with the oxygen sensor.
Also wondering if the knock sensor has anything to do with it??
I'm at work so don't have time to read this, but it looks technical:
http://www.testroete.com/car/Toyota...raining/01 - Engine and Engine Control/16.pdf
Also wondering if the knock sensor has anything to do with it??
I'm at work so don't have time to read this, but it looks technical:
http://www.testroete.com/car/Toyota...raining/01 - Engine and Engine Control/16.pdf
cruiser54
NAXJA Forum User
- Location
- Prescott, Az
91 and later have no knock sensor.
Original_MudButt
NAXJA Forum User
- Location
- In the eighth dimension
Got it. Thanks for the heads up.
- Location
- Antioch, CA
The PCM uses the crank sensor to take a sample of the normal crankshaft speed and identify the location of the marks used on the flexplate/flywheel. The sample picture is taken on a hard deceleration event after a battery disconnect or the PCM is reset. It then uses this picture as a reference to the actual rotation of the engine during idle and to a certain RPM. Later model vehicles can detect a misfire over the entire RPM range of the engine, our XJs aren't that sophisticated so the cutoff RPM for detection is slightly lower. When the engine misfires, the amount of time it takes for 2 complete revolutions is different than when the engine is running on all 6 cylinders. The PCM can detect this difference in time/speed and uses this information to set a misfire fault code.
So, with that in mind, the PCM will detect anything that throwns off the crankshaft speed even slightly and will cause a misfire code to set. I have seen a fan clutch that only had 1 bolt holding it on set a multiple misfire code. Fixed the fan and the code went away.
A good compression test will normally tell you if there is a engine problem, swapping injectors and spark plugs will help isolate the problem. If the misfire follows the parts, then replace the parts and try again.
So, with that in mind, the PCM will detect anything that throwns off the crankshaft speed even slightly and will cause a misfire code to set. I have seen a fan clutch that only had 1 bolt holding it on set a multiple misfire code. Fixed the fan and the code went away.
A good compression test will normally tell you if there is a engine problem, swapping injectors and spark plugs will help isolate the problem. If the misfire follows the parts, then replace the parts and try again.
