general misfire CEL won't go away!

[FiremarshalRob]

My 1996 Geo Metro illuminated the CEL about 3 months ago. I need an inspection sticker at the end of July, so I need to get this bugger fixed.

The 2 codes I get are "General Misfire Detected" and "Crank Position Sensor malfunction".

The car is running great, getting 40mpg, no misfire can be felt whatsoever.

I pulled the CPS, cleaned the connections, and re-installed.
I pulled the distributor, plugs, installed new wires, re-installed everything as should be. I went to the library and read on their alldata data base about possible problems. I can't find one.

My mechanic told me if the CPS was the problem, the car wouldn't run. He said the general misfire detection is triggering the CPS code. So I'm hesitant to purchase a 50 dollar CPS if that's not the issue.

Is their a fancy diagnostic tool for an older model 96 Metro that can PINPOINT the EXACT root cause for the general misfire?

I also have a relatively new PCV valve. All evap canister connections are tight. Plugs are new. All spark plug and distributor connections tight.

I won't be able to get a new inspection sticker with this aggravating CEL that keeps coming back within minutes of clearing it.

 

[eljefino]

On saturns there is often an annoying CPS code that is for the CAM position sensor. Bigger joke-- there isn't one! It's software inferred. Still got the POXXX numeric codes written down somewhere?

The saturn fix is stocker copper spark plugs, NOT platinum like the parts stores all have now. The ignition module "scopes" the spark and looks for a proper amount of inductive decay... or whatever.

 

[FiremarshalRob]

P0300 General/Random misfire

I THINK the other one is P0340, I will double check this afternoon.

 

[sciphi]

Copper plugs in GM vehicles are generally a good idea.

 

[GMBoy]

Your crank pos. sensor monitors the rotation of the crank and can sense misfires by the hestitation of the crank's movement. When misfire is detected it sets the code P0300 for random meaning many and often effecting all cylinders. P0301, 0302 etc tell you exactly which cyl misfired. If this sensor goes bad, it will send bad info to the PCM and you'll get the code. If you have new plugs, wires, cap and rotor etc and there are not vacuum leaks or EGR issues than this is your problem. Often times, as your mechanic said, when it finally fails your car won't start. n

As for diagnostic tools, GM's own Tech 2 can pinpoint this problem as can some other comparable tools.

If it was me, I'd replace the sensor. $50 is not too bad actually. HOWEVER - I did have the same issue once with a 2002 Trans AM where the car had P0300 random misfire codes immediatley after putting in those dumb E3 plugs. (My friends car, she bought them and I reluctantly installed them). Next day, I pulled those E3 plugs and put in Autolite Platinums that she bought to replace the new E3's....problem solved. So, with just 3 sparkplugs - maybe you should try and change them again?

Lastly, another possible cause is a fuel injector. You really need a scan tool to pinpoint your issues. I wish you were nearby I would be glad to help you w/my Tech 2.

 

[FiremarshalRob]

Originally Posted By: GMBoy
Your crank pos. sensor monitors the rotation of the crank and can sense misfires by the hestitation of the crank's movement. When misfire is detected it sets the code P0300 for random meaning many and often effecting all cylinders. P0301, 0302 etc tell you exactly which cyl misfired. If this sensor goes bad, it will send bad info to the PCM and you'll get the code. If you have new plugs, wires, cap and rotor etc and there are not vacuum leaks or EGR issues than this is your problem. Often times, as your mechanic said, when it finally fails your car won't start. n

As for diagnostic tools, GM's own Tech 2 can pinpoint this problem as can some other comparable tools.

If it was me, I'd replace the sensor. $50 is not too bad actually. HOWEVER - I did have the same issue once with a 2002 Trans AM where the car had P0300 random misfire codes immediatley after putting in those dumb E3 plugs. (My friends car, she bought them and I reluctantly installed them). Next day, I pulled those E3 plugs and put in Autolite Platinums that she bought to replace the new E3's....problem solved. So, with just 3 sparkplugs - maybe you should try and change them again?

Lastly, another possible cause is a fuel injector. You really need a scan tool to pinpoint your issues. I wish you were nearby I would be glad to help you w/my Tech 2.

When I first had the motor installed last year, they put AC Delco plugs in. They glopped oil in the threads (instead of antiseize) and it looked tacky and oil was all in the holes. I thought it was an oil leak. I replaced them with OEM NGK plugs per the Isuzu motor. I have the 1.3L 4 cylinder "big block". lol

I guess you have me thinking about going ahead with the crank sensor replacement. This model is throttle body injected. I've recently changed the air filter and cleaned the housing, throttle body, etc.

It just appears that the primary code is the random misfire, and the secondary code is the CPS code.

 

[GMBoy]

Due to OBDII and emmissions rules, the misfire code takes priority because when you are misfiring you're polluting more. We are guessing (can't see the car) its the CPS because you are not feeling any misses and everything else checks out and the CPS looks for the misses so it reports the P0300 codes. Seems a high chance that the CPS is your problem. I haven't checked my repair data yet, but I beleive there may be a special reset or calibration you have to do when removing/replacing this sensor so it works right. I will check and repost. If so, you will need the appropriate scan tool to do this.

 

[GMBoy]

Here is the P0340 code info:


xTooltipElement
Service Information
1996 Geo Metro | Firefly, Metro (VIN M) Service Manual | Document ID: 1563944
--------------------------------------------------------------------------------

DTC P0340 Camshaft Position (CMP) Sensor Circuit




Circuit Description
The powertrain control module (PCM) receives voltage pulses from the camshaft position (CMP) sensor. The PCM uses these pulses as reference signals indicating engine speed and number 1 cylinder identification. The CMP sensor generates the voltage pulses from the rotation of the distributor shaft. A signal rotor, which rotates with the distributor shaft, passes through the air gap of the CMP sensor. The movement of the signal rotor through the air gap creates a flux in the magnetic field generated by the CMP sensor. The CMP sensor responds to the change in the magnetic field and sends this information to the PCM.

Conditions for Setting the DTC
• No signal from the CMP sensor while cranking the engine.

• The condition present for 2 seconds.

Action Taken When the DTC Sets
• The PCM illuminates the malfunction indicator lamp (MIL).

• The PCM records the operating conditions at the time the diagnostic fails. This information is stored in the Freeze Frame buffer.

Conditions for Clearing the MIL/DTC
• The MIL turns OFF after three consecutively passing trips without a fault present.

• A History DTC clears after 40 consecutive warm-up cycles without a fault.

• Use the scan tool Clear DTC Information function or disconnect the PCM battery feed in order to clear the DTC.

Diagnostic Aids
Check for any of the following conditions:

• Check for a DTC P1500 Starter Signal Circuit and if found, diagnosis the DTC P1500 first. A starter motor that fails to operate normally may set a DTC P0340. Check the starter motor and starter motor circuits if a DTC P0340 is set and there are problems with the operation of the starter motor.

• Check the CMP sensor signal rotor for proper alignment and damage. Refer to Engine Electrical.

• Check for faulty electrical connections at the PCM and circuit components.

• Inspect the wiring harness for damage or electrical faults.

For proper electrical circuit repair procedures, refer to Wiring Repairs.

If a DTC P0340 cannot be duplicated, the information included in the Freeze Frame data can be useful in determining vehicle operating conditions when the DTC was first set.

Test Description
The numbers below refer to the step numbers in the Diagnostic Table.

The Powertrain OBD System Check prompts the technician to complete some basic checks and store the freeze frame data on the scan tool if applicable. This creates an electronic copy of the data taken when the fault occurred. The information is then stored in the scan tool for later reference.

This step determines if fault is present.

The CMP sensor resistance can range from 185 ohms to 325 ohms at -10° to 100°C (14° to 212°F).

This step checks the CMP sensor for the correct air gap.

Step
Action
Values
Yes
No

1
Did you perform the Powertrain On-Board Diagnostic System Check?
--
Go to Step 2
Go to Powertrain On Board Diagnostic (OBD) System Check

2
Turn ON the ignition, leaving the engine OFF.
Clear the scan tool information.
Operate the vehicle within Freeze Frame conditions as specified. Crank the engine if the vehicle will not start.
Does the scan tool indicate a DTC P0340?
--
Go to Step 3
Go to Diagnostic Aids

3
Turn OFF the ignition.
Check the vehicle at ambient temperature.
Disconnect the distributor electrical connector.
Measure the resistance of the CMP sensor (distributor side) with a DMM.
Is the resistance within the specified value?
185 to 275 ohms at -10° to 50°C (14° to 122°F)
Go to Step 4
Go to Step 11

4
Measure the A/C voltage from the CMP sensor (distributor side) with the DMM, while cranking the engine.

Is the voltage within the specified value?
0.10-0.40 volts
Go to Step 5
Go to Step 8

5
Turn OFF the ignition.
Reconnect the distributor electrical connector.
Disconnect the PCM electrical connectors.
Measure the A/C voltage from the ignition reference high signal circuit and the ignition reference low signal circuit with the DMM, while cranking the engine.
Is the voltage within the specified value?
0.10-0.40 volts
Go to Step 10
Go to Step 6

6
Check for any of the following conditions:
• An open in the ignition reference high or low signal circuits

• A short to ground in the ignition reference high or low signal circuits.

• A short to voltage in the ignition reference high or low signal circuits.

Repair as necessary. Refer to Wiring Repairs in Wiring Systems.
Was a repair necessary?
--
Go to Step 13
Go to Step 7

7
Repair the faulty electrical connection at the distributor.

Is the action complete?
--
Go to Step 13
--

8
Check the CMP sensor air gap. Refer to Distributor Overhaul in Engine Electrical.

Is the CMP sensor air gap within the specified value?
0.2 to 0.4 mm (0.008 to 0.016 in)
Go to Step 11
Go to Step 9

9
Adjust the CMP sensor air gap. Refer to Distributor Overhaul in Engine Electrical.

Does the engine start and run?
--
Go to Step 13
Go to Step 11

10
Check for a faulty connection at the PCM.
Repair as necessary.
Was a repair necessary?
--
Go to Step 13
Go to Step 12

11
Replace the CMP sensor. Refer to Distributor Overhaul in Engine Electrical.

Is the action complete?
--
Go to Step 13
--

12
Replace the PCM. Refer to Powertrain Control Module Replacement .

Is the action complete?
--
Go to Step 13
--

13
Perform the scan tool Clear DTC Information function and road test the vehicle within the Freeze Frame conditions that set the DTC.
Review the scan tool data and check for DTCs. The repair is complete if no DTCs are stored.
Are any DTCs displayed on the scan tool?
--
Go to the Applicable DTC Table
System OK

© 2009 General Motors Corporation. All rights reserved.

 

[mechtech2]

That CPS code shows a problem, and a RESULTANT misfire, not the other way around.

 

[FiremarshalRob]

Thanks for the great info guys!

I guess my first step as of now would be to replace the CPS and then see what happens?
My local NAPA has one for about 40 dollars.