874 Engine Control Systems II Technician Handbook Engine Misfire Diagnosis To monitor misfires, the ECM uses both the Crankshaft Position CKP sensor and the Camshaft Position CMP sens
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Engine Misfire
Diagnosis
To monitor misfires, the ECM uses both the Crankshaft Position (CKP) sensor and the Camshaft Position (CMP) sensor The Crankshaft Position (CKP) sensor is used to measure variations in the crankshaft rotation speed On the power stroke, the crankshaft accelerates If the cylinder misfires, the crankshaft decelerates The Camshaft Position (CMP) sensor is used to identify specific misfiring cylinders
When the engine misfires, high concentrations of hydrocarbons (HC) enter the exhaust gas High HC concentration levels can cause increased exhaust emission levels High concentrations of HC can also cause increases in the Three-Way Catalytic Converter (TWC) temperature, which may cause damage to the TWC To prevent this increase in emissions and to limit the possibility of TWC thermal damage, the ECM monitors the misfire rate
The misfire monitor is designed to detect increases in emissions caused by cylinder misfires, not to reduce driveability concerns
Misfires are counted when the crankshaft rotation speed variations exceed predetermined thresholds When the temperature of the TWC reaches the point of thermal degradation, the ECM blinks the MIL If the misfire exceeds the threshold levels, and could cause emission deterioration, the ECM illuminates the MIL and sets a DTC
The engine misfire monitor is a continuous monitor
The catalyst temperature is calculated by the ECM based on driving conditions and percentage of misfire
NOTE
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There are two types of misfire monitors: Type A and B
• A type A misfire is one that is severe enough to damage the catalytic converter
• The ECM will blink the MIL if the misfire will result in catalyst temperatures of 1832 degrees F
(1000 degrees C) or more
• The ECM will blink the MIL on the first trip that the malfunction
is detected but the MIL will not illuminate steady (DTC Stored) until conditions are met on the 2nd trip
• The Misfire Monitor Type A duration is based on 200 crankshaft revolutions
• The type B or otherwise known as an emission deterioration misfire is less severe than the type A
• A type B misfire will not damage the catalytic converter
• The ECM will turn the MIL ON after two consecutive trips that the malfunction is detected
• The Misfire Monitor Type B duration is based on 1000 crankshaft revolutions
Misfire Monitor Types
A and B Misfire Monitor Type A
Misfire Monitor Type B
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Ignition Counter The ignition counter (Ignition parameter) represents the number of
ignition events, as reported to the TIS Techstream, that have occurred during the current ignition cycle The ignition cycle is 1000 revolutions for most 1998 and earlier vehicles, and 200 revolutions for most 1999 and later vehicles
The TIS Techstream will display the ignition counter recycling at different rates based on the number of cylinders Each cylinder in a four-stroke engine will fire 500 times in 1000 revolutions Therefore, four-cylinder engines will produce 2000 ignition counts in 1000 revolutions, six-cylinder engines will produce 3000 ignition counts in
1000 revolutions, and eight-cylinder engines will produce 4000 ignition counts in 1000 revolutions
The TIS Techstream will display the ignition counter recycling at different rates based on the number of cylinders Each cylinder in a four-stroke engine will fire 100 times in 200 revolutions Therefore, four-cylinder engines will produce 400 ignition counts in 200 revolutions, six-cylinder engines will produce
600 ignition counts in 200 revolutions, and eight-cylinder engines will produce 800 ignition counts in
200 revolutions
1998 and Earlier Models
1999 and Later Models
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The Misfire Data List provides selected information related to cylinder misfire The Misfire Data Lists from OBD II and CAN OBD II
vehicles will be different
The Ignition parameter represents the ignition counter and indicates
how many ignition events (all cylinders) have occurred in the current
200 revolution cycle (1999 and later vehicles) When the ignition counter is cycling, the misfire monitor is operating and reporting to the TIS Techstream
The Cylinder # Misfire Rate parameter indicates how many times
(0–100) the cylinder has misfired during the current ignition counter cycle A value above 0 indicates a misfiring cylinder For six- and eight- cylinder engines, the ECM may not set specific misfiring cylinder DTC(s) at high engine RPM and may only set DTC P0300
in this condition Diagnose the cylinder(s) with the highest Cylinder # Misfire Rate first
The Misfire Margin parameter indicates the percentage chance that
a misfire will NOT occur A negative percentage indicates a misfire is occurring A high positive percentage indicates a low chance for a misfire to occur under the current conditions
Catalyst OT MF F/C stands for Catalyst Over Temperature Misfire Fuel Cut History and individual cylinder parameter will be turned
ON when a Type A misfire is present and fuel injector(s) has been
shut down
Misfire Data List
Ignition
Cylinder # Misfire Rate
Misfire Margin (CAN OBD II vehicles)
Cat OT MF F/C
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Misfire detection due to a catalyst over temperature [Data list → Cylinder # Misfire Count]
The misfire ratio is evaluated every 200 engine revolutions, and a DTC is detected when a misfire has taken place three times at or above the threshold misfire ratio that causes a thermal deterioration
in the catalyst (Two trip detection logic) While a misfire is taking place under these conditions, the MIL
blinks, regardless of the number of trips On some engines, the MIL
will continue to blink after the catalyst over temperature conditions have passed On the next key cycle, the MIL will not blink until over
temperature conditions occur in the catalytic converter
Misfire detection for a deteriorated level of emissions [Data list
→ All Cylinder Misfire Count]
The misfire ratio is evaluated every 1,000 engine revolutions, and a DTC is detected when a misfire has occurred at or above the threshold misfire ratio for a judgment of emission deterioration (Two trip detection logic)
If this condition happens on the second consecutive trip, the MIL is
turned ON
Misfire Detection
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The Misfire Monitor counts the number of misfires for all cylinders, individual
cylinders, and tracks average misfires from previous trips
The ECM counts misfires and is available under misfire test details
MISFIRE RATE – total misfire count for all cylinders MISFIRE RATE 1 – number of misfire counts for cylinder 1 Each count represents a cylinder misfire KOEO the test details provides the number of misfires from the last trip This is reset to zero after the engine has started
After the engine is started the previous misfire counts are averaged and stored in
the Misfire Monitor as EWMA Misfire EWMA stands for Exponentially Weighted
Moving Average (EWMA) Misfire which represents the average number of counts for all cylinders as well as individual cylinder misfires This value represents 10%
of the last trip’s misfires plus 90% of the average of the previous trips
Exponentially Weighted Moving Average (EWMA) Misfire Formula
0.1 x (current counts) + 0.9 x (previous average)
EWMA MISFIRE – total average for all cylinders EWMA MISFIRE 1 – average for cylinder 1 The cylinder and misfire count can be useful for the following situations
1. A customer reports the check engine lights was flashing, but now it does not flash There is no DTC and the MIL is off The EWMA can report which cylinder(s) had the misfire
2. The vehicle has P0300 DTC Which cylinder had the most misfires
3. The vehicle has a P0300 and multiple misfire DTCs Again, which cylinder had the highest misfire counts
Misfire Monitor
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Duplicating Misfires All Cylinders Misfire Count displays the total number of misfires for all
cylinders The available range is from 0 to 255 misfire counts This counter line resets to zero every 1000 crankshaft revolutions
Misfire RPM and Misfire Load populate when misfire criteria is met on
the first trip Parameters will not update on the second trip but Freeze
Frame Data will be saved for the Current DTC This information can
be very useful in determining vehicle operating conditions while
misfires were taking place
To duplicate an engine misfire, use the Misfire RPM and Misfire Load values from the Engine Live Misfire data list Reference the Freeze Frame data to duplicate engine operating conditions, such as
calculated load, coolant temperature, etc., at the time the DTC set
Unfortunately, Misfire Load is displayed in g/rev (grams per
revolution) and is derived from the MAF signal To convert g/rev to gm/sec follow the procedure below:
1. Divide Misfire RPM by 60 sec to get Revolutions per Second
2. Multiply Revolutions per Seconds by Misfire Load (g/rev) to get grams per second (g/sec)
• Example: Misfire RPM shows 2275 RPM and Misfire Load shows 0.28 g/rev
2275rpm/60=38Hz; 38Hz * 0.28g/rev=10.64 g/sec The misfire was detected at 2275 RPM with a MAF Sensor signal of 10.64 g/sec
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Misfire Margin is the difference between the predicted change in
crankshaft rotation speed represented electronically by NE signal to the actual change in the NE signal
The Misfire Margin will move towards the negative region when
misfires are detected If it is under 30% there is a chance misfire is occurring If it is in the negative zone, misfire is occurring
This parameter is useful for it will change as the chance of a misfire increases When trying to duplicate the misfire conditions, the misfire margin percentage will decrease as the engine nears or experiences cylinder misfire
Misfire Margin
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Single Cylinder
Misfire Diagnosis
Use the following steps to diagnose a single cylinder misfire (examples based on DTC P0302):
1. Duplicate the misfire using Engine Live and Freeze Frame data
2. Compare the spark plug firing tip of the misfiring cylinder to spark plug firing tips from cylinders with no misfires present
3. Swap the igniter from the misfiring cylinder to a different cylinder with no misfires present
(swap the igniter with cylinder # 4)
4. Swap the spark plug from the misfiring cylinder to a separate different cylinder with no misfires present (swap the spark plug with cylinder # 6)
5. If the misfire moves to a cylinder with a swapped component, replace the component
(e.g if cylinder # 4 begins to misfire, replace the igniter, etc.)
6. If the misfire stays with the original cylinder, check the cylinder compression pressure and fuel delivery
The diagnostic procedure outlined here is for training purposes only and may not be effective for every issue Always refer to Service Bulletins (SB) and Repair Manuals (RM) for information on specific vehicles and issues
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Compression Test Proper cylinder compression pressure verifies the misfire is not
related to mechanical components
When performing a cylinder compression pressure test, fuel injection and ignition must be turned OFF The procedure to turn OFF the fuel injection and ignition, as well as check the
compression, varies with vehicle model Always refer to the vehicle Repair Manual for the proper cylinder compression pressure test procedure and pressure specifications
Performing a Cylinder Compression active test (some 2005 and
later model vehicles) will turn OFF fuel injection and ignition so that
a compression pressure test can be performed While this active test may be available, the Repair Manual cylinder compression pressure inspection procedure may not utilize it The fastest way to
find out if a Cylinder Compression active test can be performed on a
specific vehicle is to check the active tests available within the ECM
using the Techstream by clicking Active Test from the Engine Live
main menu Performing this active test, if available, will allow a cylinder compression pressure test to be performed without disconnecting all fuel injector connectors and/or removing the EFI fuse or circuit opening relay
Active Test
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Cylinder Speed
Data List
Some 2006 and later model vehicles will display the speed of each
cylinder During a Cylinder Compression active test with all spark
plugs installed, if a single cylinder is rotating faster than other cylinders, this may indicate a mechanical issue with that cylinder
In the illustration, the spark plug was removed from cylinder number
4 and a Cylinder Compression active test was performed
It may be necessary to crank the engine for up to 20 seconds before accurate Engine Speed of Cyl # readings populate.
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Multiple Cylinder
Misfire Diagnosis
Misfires can be the result of other system malfunctions If other system DTC(s) are present, diagnose those DTC(s) / systems first
If multiple cylinder misfires are present, check for commonalities between cylinders Some commonalities include fuel, mechanical issues, inputs,
and outputs
Improper fuel pressure, restricted injectors, restrictions in the fuel line, incorrect fuel such as E-85 or diesel, or water in the fuel are all possible fuel issues that may cause multiple cylinder misfires
Broken or misadjusted valve train components, incorrect valve timing and carbon buildup are some possible mechanical issues that may cause multiple cylinder misfires
Some inputs that may cause multiple cylinder misfires include the crankshaft position sensor, camshaft position sensor, engine coolant temperature sensor, intake air temperature sensor, A/F and/or O2 sensors, and the MAF sensor
Some outputs that may cause multiple cylinder misfires include fuel trim and valve timing (VVT-i)
Rough roads, oversized wheels, etc may also cause multiple cylinder misfires It is very important to talk to the customer and understand the actual
driving conditions when the misfire DTC(s) occurred
If several cylinders are misfiring, drive the system rich (+25) with an Injector Volume or A/F Control active test If the misfiring cylinders stop misfiring, this
may indicate restricted injectors for these cylinders
If all cylinders but one on a single bank are misfiring, check fuel trims If the fuel trims for the misfiring bank are in the double-digit negatives, perform an
Injector Volume active test and drive the system rich (+25%) If the misfiring
cylinders stop misfiring, this may indicate a leaking injector in the cylinder that was not misfiring The cylinder with the leaking injector may actually start to misfire with the injection volume commanded rich (+25%) This may apply to single cylinder misfires on four-cylinder engines with two banks
NOTE NOTE NOTE
Fuel
Mechanical
Inputs Outputs