zcler59

On a road trip I dropped into 3rd to pass and the car goes into limp mode (ENG PWR REDUCED on the DIC) throwing out code P0068 - throttle position and engine load do not agree. Probable causes:

1. Check engine for vacuum leak
2. Check engine condition - mechanical condition
3. Plugged exhaust or intake
4. Transmission condition

If I stop the car and leave it for about a minute, I can run it fine at full power but the CEL is still on.

Weird thing is if I leave the car overnight the CEL disappears and it runs fine. Then totally randomly it happens again, CEL and limp mode.

I've got an 05 SS/SC, everything stock except K&N intake.

Any ideas of things I can try to fix before I take it in?

Gene Culley

iTrader: (1)

xTooltipElement
Service Information
2007 Chevrolet Cobalt | Cobalt, G5 (VIN A) Service Manual | Document ID: 1618999
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DTC P0068
Circuit Description
The powertrain control module (PCM) uses the following information to calculate an expected airflow rate:

• The throttle position (TP)

• The barometric pressure (BARO)

• The super charger inlet pressure (SCIP)

• The intake air temperature (IAT)

• The mass air flow (MAF)

• The engine RPM

If the PCM detects the airflow rate is more than expected, DTC P0068 sets.

DTC Descriptor
This diagnostic procedure supports the following DTC:

DTC P0068 Throttle Body Airflow Performance

Conditions for Running the DTC
• DTCs P0641, P0651, P1516, P2101, P2119, P2176 are not set.

• The engine is running and the engine speed is more than 800 RPM.

• DTC P0068 runs continuously when the above conditions are met.

Conditions for Setting the DTC
The PCM detects that the calculated airflow rate is more than expected for more than 0.2 second.

Action Taken When the DTC Sets
• The control module illuminates the malfunction indicator lamp (MIL) when the diagnostic runs and fails.

• The control module records the operating conditions at the time the diagnostic fails. The control module stores this information in the Freeze Frame and/or the Failure Records.

• The control module commands the TAC system to operate in the Reduced Engine Power mode.

• A message center or an indicator displays Reduced Engine Power.

• Under certain conditions the control module commands the engine OFF.

Conditions for Clearing the MIL/DTC
• The PCM will turn OFF the malfunction indicator lamp (MIL) during the third consecutive trip in which the diagnostic has run and passed.

• The history DTC will clear after 40 consecutive warm-up cycles have occurred without a malfunction.

• The DTC can be cleared by using a scan tool.

Test Description
The numbers below refer to the step numbers on the diagnostic table.

6.This step will determine if the SCIP sensor voltage is within the proper range with the key ON, engine OFF.

7.This step will determine if the sensor responds properly to the change in super charger inlet pressure.

10.When the PCM detects a condition within the ETC system, other DTCs may set due to the many redundant tests run continuously on this system. Locating and repairing one individual condition may correct more than one DTC. Keep this in mind when reviewing captured DTC info.

Step
Action
Values
Yes
No

Schematic Reference: Engine Controls Schematics

Connector End View Reference: Engine Controls Connector End Views or Powertrain Control Module Connector End Views

1
Did you perform the Diagnostic System Check - Vehicle?
--
Go to Step 2
Go to Diagnostic System Check - Vehicle

2
Are DTCs P0101, P0120, P0220, P1516, P2101, P2119, P2135 or P2176 also set?
--
Go to Diagnostic Trouble Code (DTC) List - Vehicle
Go to Step 3

3
•Observe the Freeze Frame/Failure Records for this DTC.
•Turn OFF the ignition.
•Start the engine.
•Operate the vehicle within the Conditions for Running the DTC. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records.
Did the DTC fail this ignition?
--
Go to Step 4
Go to Testing for Intermittent Conditions and Poor Connections

4
Inspect for the following conditions:

• Vacuum hoses for splits, kinks, and proper connections as shown on the Vehicle Emission Control Information label--Inspect thoroughly for any type of leak or restriction.

• Air leaks at throttle body mounting area and intake manifold sealing surfaces

Did you find and correct the condition?
--
Go to Step 10
Go to Step 5

5
Important: The Altitude vs. Barometric Pressure (BARO) table indicates a pressure range for a given altitude under normal weather conditions. Weather conditions consisting of very low or very high pressure and/or temperature may cause a reading to be slightly out of range.

•Accurately determine the altitude.
•Turn ON the ignition, with the engine OFF.
•Observe the BARO Sensor kPa parameter with a scan tool.
•The BARO sensor pressure should be within the specified range for your altitude. Refer to Altitude Versus Barometric Pressure .
Is the BARO sensor pressure within the specified range as indicated on the Altitude vs. Barometric pressure table?
--
Go to Step 6
Go to DTC P2228 or DTC P2229

6
•Allow the engine to reach operating temperature.
•Turn OFF the engine.
•Turn ON the ignition, with the engine OFF.
•Observe the super charger inlet pressure (SCIP) sensor voltage parameter with a scan tool.
Is the SCIP sensor voltage within the specified range?
2.2-2.4 V
Go to Step 7
Go to DTC P1182

7
•Idle the engine.
•Observe the SCIP sensor kPa parameter with a scan tool.
•Increase the engine speed slowly and then back to idle.
Does the SCIP sensor kPa change smoothly and gradually as engine speed is increased and returned to idle?
--
Go to Step 8
Go to DTC P1182

8
•Idle the engine.
•Take a snapshot of the Engine Data List while performing the following action:
• Increase the engine speed slowly to 3,000 RPM, then back to idle.

• Exit from the snapshot and review the data.

• Observe the MAF g/s sensor parameter frame by frame with a scan tool.

• Refer to Scan Tool Snapshot Procedure .

Does the MAF sensor g/s change smoothly and gradually as the engine speed is increased and is returned to idle?
--
Go to Step 9
Go to DTC P0101

9
Caution: Turn OFF the ignition before inserting fingers into the throttle bore. Unexpected movement of the throttle blade could cause personal injury.

•Inspect the throttle body for the following conditions while modulating the throttle using the scan tool:
• Loose or damaged throttle blade

• Broken throttle shaft

• Drive mechanism damage

•If any of these conditions exist, replace the throttle body assembly. Refer to Throttle Body Assembly Replacement .
Did you find and correct the condition?
--
Go to Step 10
Go to Testing for Intermittent Conditions and Poor Connections

10
•Clear the DTCs with a scan tool.
•Turn OFF the ignition for 30 seconds.
•Start the engine.
•Operate the vehicle within the Conditions for Running the DTC. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records.
Did the DTC fail this ignition?
--
Go to Step 2
Go to Step 11

11
Observe the Capture Info with a scan tool.

Are there any DTCs that have not been diagnosed?
--
Go to Diagnostic Trouble Code (DTC) List - Vehicle
System OK

© 2009 General Motors Corporation. All rights reserved.

jgarciarivera

iTrader: (1)

^^^ Where can I find this information? This is a series of good troubleshooting steps. I've seen this at GM dealers but not sure where to get it from the web.

Thanks

Jose

mrbelvedere

iTrader: (3)

try cleaning the throttle plate

jgarciarivera

iTrader: (1)

^^ I have the same P0068 and just waiting for my TB

kobe77

Should I replace my tb? I have same code. I got baro pressure and tps. 05 ss/sc stage 2 and typhoon intake. Inspecting for vacuume leaks later this week.