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Mark, ASE Certified Technician
Category: Chevy
Satisfied Customers: 847
Experience:  34+ yrs Dealership Exp. - Fully State & ASE Certified - GM Master & GM World Class Tech
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2001 chevy prizm: has a miss on #2 cylinder I changed coils

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2001 chev prizm has a miss on #2 cylinder I changed plugs, and it continued to come up. I switched the top coils from #1 and #2. The enjine still codes a #2 miss. You only feel the miss at idal.
Greetings, my name is Mark.

You need the basics for an engine to run. Gas, Spark, Combustion. The basics. I know that you know that, and I know you know there's a lot more to running it correctly with timing and all that.

But my point is, if you have spark, and you have fuel, then it MUST be engine mechanical. I see by your post, you have handled the spark end of it. Switched plugs, switched coils. etc. I don't see where you have have confirmed fuel. Next would be the fuel injectors. Could be the injector itself, or could be the quad driver inside the PCM. (computer)

Finally, if you have confirmed fuel and spark, you need to considered engine mechanical. Anything from a slightly worn valve seat, to a vacuum leak. First thing I would do is take a static compression test, and then a running compression test. If something shows up, remove the valve cover and look-see.

You mention you only feel the miss at an idle. That's because once the engine gets to reving up the RPM's, things will compensate. Things are moving fast enough that a broken valve spring for faulty lifter will move fast enough to do it's job.

I got a car sent to the shop with the customer prepared for heavy engine mechanical issues. He had the car to several other shops for his rough idle, and mostly it was when the engine was cold. He was told the cylinder head would have to come off because he had a valve problem or seat problem.

I went out, started the car, and sprayed carb clean all around the intake manifold. (4-cylinder, just like yours) and the engine stalled. Removed the intake and installed new intake manifold seals. They are just o-rings. Car was fixed. The customer couldn't believe it. Needless to say, he was very happy.

Even if you don't hear anything, take some carb clean and spray the intake. Spray carefully around the engine and see if you idle improves or gets worst.

Below, I have included a chart for several things to check. A lot of the checks do require the use of a scanner. You sorta have one had tied behind your back with not being to read data, but the chart can at least give you some ideas.

Thanks and good luck to ya !!

Rough, Unstable, or Incorrect Idle, Stalling



DEFINITION: The engine runs unevenly at idle. If severe enough, the engine or vehicle may shake. The engine idle speed may vary in RPM. Either condition may be severe enough to stall the engine. The engine idles at an incorrect speed.

Preliminary Check

Sensor Checks

Inspect the heated oxygen sensors HO2S 1 and HO2S 2 for a condition that can affect sensor activity and response.

Inspect the throttle position (TP) sensor. A sticking throttle shaft or a binding throttle linkage will causes a high TP sensor voltage. Under these conditions the PCM may not control the idle. Monitor the TP sensor angle with the scan tool. The indicated angle should be approximately 11 percent with the throttle closed.
Inspect the engine coolant temperature (ECT) sensor. A fixed or inaccurate ECT sensor reading can cause the engine idle to surge or race. Using the scan tool, compare the engine coolant temperature with the ambient air temperature on a cold engine. If the engine coolant temperature is 5°C (9°F) more than or 5°C (9°F) less than the ambient air temperature, inspect the resistance of the coolant sensor. If the engine coolant temperature is significantly less than the ambient air temperature, inspect the coolant sensor electrical circuit for high resistance.
Inspect the mass air flow (MAF) sensor operation. Run the engine to normal operating temperature. Turn OFF all accessories and observe the MAF sensor at idle with a scan tool. If the MAF sensor reading is more than 3.5 g/s refer to .

Fuel System Checks

Inspect the operation of the fuel system for rich or lean condition. Operate the vehicle under the conditions that caused the concern. Monitor the Fuel Trim parameter on a scan tool in order to identify the problem.
- Lean -- The Long Term Fuel Trim will be more than 20 percent. Refer to XXXXX in
- Rich -- The Long Term Fuel Trim will be less than -20 percent. Refer to XXXXX in .
Test the fuel pressure while the condition exists.
Inspect the evaporative emission (EVAP) control system.
Test the fuel injectors for leaking.

Ignition System Checks

Test for the proper secondary voltage output with a
Inspect the spark plugs for correct operation and good ignition system performance.
Inspect the ignition coil boots for damage.
Observe the IAC Motor Command parameter on the scan tool at idle, while operating the rear defogger, the A/C system, or the headlights. The IAC Motor Command parameter should increase 2-7 percent when operating the engine loads. If the IAC Motor Command parameter remains fixed, check for a short to ground in the Diagnostic Request circuit from the PCM to the TE1 terminal of the underhood diagnostic link connector.

Engine Mechanical Check

Inspect for any of the following engine mechanical problems:
- Any faulty hydraulic lifter assemblies
- Any broken or weak valve springs
- A low compression
- An incorrect valve timing
- Any sticking or leaking valves
- An incorrect or worn camshaft

Additional Checks

Inspect for vacuum leaks. Vacuum leaks can cause a higher than normal idle speed.
Verify the PCM grounds are clean, tight, and in the proper locations.
Check the scan tool to determine if the PCM is receiving an A/C signal. The idle speed should be increased with the A/C ON.
Inspect the battery cables and the ground straps. They should be clean and secure.
Inspect the A/C refrigerant pressure for being too high or for a faulty high pressure switch.
Test for the proper operation of the IAC system.
The engine will run rough and the engine can stall if the CMP actuator solenoid is advancing camshaft timing at idle. Inspect for a fully energized or stuck CMP actuator solenoid. If the operation of the CMP actuator solenoid is suspect,
Inspect the PCV valve for correct operation and the correct part.
Inspect that the idle speed is increasing for changes in electrical loads on the engine.
Verify that the PCM is increasing the idle speed when the P/S system pressure is increased on turns.
Inspect for broken or worn motor and transmission mounts.
Customer: replied 7 years ago.
Thank you, XXXXX XXXXX wondering what is my simple diagnostic reader sensing? What tells it there is a miss? Does it read no spark? Could it read a lack of fuel? Would that come up as a miss or some other type of code?
Hello again and thanks for getting back with me. Your code reader is reading directly from the computer. It's the computer in your car that does all the work in determining what's going on with your engine.

Your code reader is simply telling you the codes stored in the vehicle's computer. Below is a description of the misfire operation and the description of the computer in your car.

Hope this helps........

Misfire Monitor Diagnostic Operation

The misfire monitor diagnostic is based on crankshaft rotational velocity, reference period, variations. The powertrain control module determines the crankshaft rotational velocity using the crankshaft position sensor and the camshaft position sensor. When a cylinder misfires the crankshaft actually slows down momentarily. By monitoring the crankshaft and the camshaft position sensor signals, the control module can calculate when a misfire occurs.

For a non-catalyst damaging misfire, the diagnostic will be required to report a misfire that is present within 1000-3200 engine revolutions.

For a catalyst damaging misfire, the diagnostic will respond to a misfire that is within 200 engine revolutions.

Rough roads may cause a false misfire detection. A rough road will cause torque to be applied to the drive wheels and the drive train. This torque can intermittently decrease the crankshaft rotational velocity and cause a false misfire detection.

On automatic transaxle equipped vehicles, the torque converter clutch (TCC) will be disabled whenever a misfire is detected. Disabling the TCC isolates the engine from the rest of the drive line and minimizes the effect of the drive wheel inputs (torque) on the crankshaft rotation.

When the TCC has been disabled as a result of a misfire detection, the TCC will be re-enabled after approximately 3200 engine revolutions with no misfire is detected. The TCC will remain disabled whenever a misfire is detected. This allows the misfire diagnostic to evaluate the system.

Powertrain Control Module Description

The powertrain control module (PCM) is a precision unit consisting of a one chip microprocessor, an A/D (analog-to-digital) converter, and an I/O (input/output) unit. The PCM is an essential part of the electronic control system. The PCM is responsible for such major functions as control of the fuel injectors, the idle air control (IAC) valve, the fuel pump relay, etc. The PCM performs the OBD II diagnostic tests of the emission related systems. The PCM supplies a buffered voltage, called reference voltage, to the various information sensors and switches. The PCM controls most components with an electronic switch that completes a ground circuit when turned ON. The electronic switch is commonly referred to as an output driver. The PCM is also responsible for a self-diagnosis function and a fail-safe function.

The PCM is located in the center console below the entertainment center (radio/cassette/CD player).

Self-Diagnosis Function

The powertrain control module (PCM) diagnoses any troubles which may occur in the engine control system when the ignition switch is in the ON position with the engine running. The PCM indicates a malfunction by illuminating the malfunction indicator lamp (MIL) when a fault occurs in any of the following systems:

The heated oxygen sensor 1 (HO2S 1)
The heated oxygen sensor 2 (HO2S 2)
The engine coolant temperature (ECT) sensor
The throttle position (TP) sensor (including the CTP switch)
The vehicle speed sensor (VSS)
The intake air temperature (IAT) sensor
The mass air flow (MAF) sensor
The camshaft position (CMP) sensor
The crankshaft position (CKP) sensor
The knock sensor (KS) system
The evaporative emission (EVAP) control system
The idle air control (IAC) system
The CMP actuator solenoid system
The misfire detection
The fuel-trim
The catalyst monitor
The central processing unit (CPU) of the PCM

When the PCM detects a malfunction in one of the above areas, the PCM will illuminate or flash the MIL in order to notify the driver of the occurrence of a fault. The PCM will store a DTC when the PCM illuminates the MIL.

The PCM will turn OFF the MIL after 3 consecutive ignition cycles without the malfunction occurring. The DTC will remain stored in the PCM memory after the MIL is OFF.

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