CODE 48 MISSFIRE DIAGNOSIS

Please help me test this Wiki. If we decide to change it later we can easily copy and paste this content into another format.

Moderator: Wiki Moderator

Post Reply
steveX

CODE 48 MISSFIRE DIAGNOSIS

Post by steveX » Sun Apr 20, 2008 5:24 pm

if multiple codes are set, go to the lowest code first. repairing for a code 13,44 or 45 may correct code 48

TEST DESCRIPTION:code 48 will set if the following:
* TPS is between .48 and 1.30 volts.
* RPM is between 1300 and 2100.
* MPH is between 50 and 60.
* o2 cross counts greater than 21.
* ALL of the above for 30 seconds.

O2 SENSOR TEST:
code 48 could be set if the o2 sensor is degraded and cannot travel over the full rich to lean voltage range, This narrowed range could allow o2 cross counts to be above the valve necessary to set the code.

* with "scan" tool installed, verify engine is at normal operation temperature and in "closed loop"

* Engine idling in park.
* Select O2 sensor position on "scan"
* rapidly flash the throttle from idle to near wide open throttle and back while observing 02 voltage.
* Repeat if necessary to confirm voltage range, and "closed loop"

- If voltage exceeds 250-750mV range---The o2 sensor is OK

- IF voltage remains within 250-750mV range--- replace o2 sensor.


DIAGNOSTIC AIDS:

1. Ignition System Checks: Remove each spark plug and inspect (fouled, cracked, worn)
Fouled -- check ignition wire (hi resistance, damage, poor connections, grounds).
check coil and module operation
check basic engine problem

cracked or worn --- replace as necessary

2. Fuel system checks: restricted fuel system (injectors, fuel pump, lines and filter)
Injectior -- perform injector balance test, verify each injector circuit with tool J-34730-2 or equivalent.
Fuel pump -- verify proper fuel pressure and fuel quality
Lines anf filter -- verify no restriction in lines or filter.

3. Basic Engine Checks: unless spark plug(s) inspection identifies a specific cylinder(s), road test vehicle under test conditions to reverify code 48 prior to engine disassembly.
basic engine (valves, compression,camshaft,lifters)
compression -- check rings, pistons, valves
Valves -- check for burned, weak springs, broken parts, worn or loose guide.
camshaft -- check for worn or broken.
lifters -- check for worn , broken

ELECTRONIC SPARK CONTROL (ESC) SYSTEM

PURPOSE Varying octane levels in todays gasoline can cause detonation in high performance engines. Detonation is caused by a uncontrolled explosion (burn) in the combustion chamber. This uncontrolled explosion could produce a flame front opposite that of the normal flame front produced by the spark plug.
The "rattling" sound normally associated with detonation is the result of two or more opposing pressure (flame fronts) colliding within the combustion chamber. Though "light" detonation is sometimes considered normal, "heavy" detonation could result in engine damage.
To control spark knock an electronic spark control (ESC) system is used. This system is designed to retart spark timing up to 10 degree to reduce spark knock in the engine. this allows the engine to use maximum spark advance to improve driveability and fuel economy.

Operation The ESC system has two major component:
* ESC module (part of MEM-CAL/ECM Chip)
* ESC knock sensor.
The ESC knock sensor detects abnormal vibration (spark knocking) in the engine. the sensor is mounted in the engine block near the cylinder #2 by the oil pump hosing. the sensor produces an A/C output voltage which increases with the severity of the knock. this signal voltage inputs to the ECM. the ECM then adjusts the electronic spark timing (EST) to reduce spark knock.

DIAGNOSIS
The "scan" tool will have two position to check for diagnosing this circuit. Knock signal is used to monitor the input signal from the knock sensor. This position should display "YES" to indicate knock is being detected. knock retart is the indication of how much the ECM is retarting the spark.
Code 43 is designed to diagnose the knock sensor and wiring, so problem encountered with this circuit should set the code. however, if no code 43 was set but the ECM system is suspect because detonation was the customer`s complaint.

Circuit Description
The knock sensor is used to detect engine detonation and the ECM will retart the electronic spark timing based on the signal being received. The circuity within the knock sensor cause the ECM`s supplied 5 volt signal to be pulled down so that under a no knock condition, circuit (dark blue wire) would measure about 2.5 volts. the knock sensor produces an A/C signal which rides on the 2.5 volts DC voltage. the amplitude and frequency are dependent upon the knock level.
The MEM-CAL used with this engine contain the functions which were part of the remotely mounted ESC modules used on other GM vehicles. The ESC portion of the MEM-CAL then send a signal to other parts of the ECM which adjust the spark timing to retart the spark and reduce the detonation.

Test description: Number(s) refer to circled number(s) on the diagnostic chart.

1. with engine idling, there should not be a knock signal present at the ECM because detonation is not likely under a no load condition.

2. Tapping on the engine lift hook should simulate a knock signal to determine if the sensor is capable of detecting detonation. If no knock is detected, try tapping on engine block closer to sensor before replacing sensor.

3. If the engine has an internal problem which is creating a knock, the knock sensor may be responding to the internal failure.

4. This test determines if the knock sensor is faulty or if the ESC portion of the MEM-CAL is faulty. If it is determined that the MEM-CAL is faulty, be sure that it is properly installed and latched into place. if not properly installed, repair and retest.




SOURCE: GM SERVICE MANUAL.

Post Reply