I've got a smoke generator setup now, just need to figure out where/how to best use it. I hooked it up to the line normally going to the crankcase vent valve (at the hose clamp point in the picture below, leaving the hose to the valve cover laying loose) and didn't have any smoke leaking out until I could see it coming out of the air filter. I think I what I need to do is pull the air filter off, put a glove over the inlet pipe where the air filter normally goes, take my hood off and then check everywhere.
Hopefully I'll have some pictures tomorrow of the smoke test setup I made. I hope my misfire issue is as simple as a leak on the intake path.
For reference, this is the service guide info I found on a cobalt forum:
xTooltipElement
Service Information
2007 Chevrolet Cobalt | Cobalt, G5 (VIN A) Service Manual | Document ID: 1823665
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DTC P0300
System Description
The powertrain control module (PCM) uses information from the crankshaft position (CKP) sensor and the camshaft position (CMP) sensor in order to determine when an engine misfire is occurring. By monitoring variations in the crankshaft rotation speed for each cylinder, the PCM is able to detect individual misfire events. A misfire rate that is high enough can cause the 3-way catalytic converter (TWC) to overheat under certain driving conditions. The malfunction indicator lamp (MIL) will flash ON and OFF when the conditions for TWC overheating are present. If the PCM detects a misfire rate sufficient to cause emission levels to exceed mandated standards, DTC P0300 will set.
DTC Descriptor
This diagnostic procedure supports the following DTC:
DTC P0300 Engine Misfire Detected
Conditions for Running the DTC
• DTCs P0107, P0112, P0113, P0117, P0118, P0120, P0121, P0125, P0131, P0132, P0133, P0134, P0171, P0172, P0220, P0336, P0502, P0506, P0507, P0601, P1133, P1621, P2135 are not set.
• The engine speed is between 469-6,400 RPM.
• The ignition voltage is between 9-17 volts.
• The engine coolant temperature (ECT) is between -7°C and +123°C (20-254°F).
• The engine has been running for more than 5 seconds.
• The throttle position (TP) has not increased more than 8 percent in 1 second.
• The TP has not decreased more than 1.5 percent in 1 second.
• DTC P0300 runs continuously when the above conditions are met.
Conditions for Setting the DTC
The PCM is detecting a crankshaft rotation speed variation indicating a misfire sufficient to cause emission levels to exceed mandated standards.
Action Taken When the DTC Sets
• The control module illuminates the malfunction indicator lamp (MIL) on the second consecutive ignition cycle that the diagnostic runs and fails.
• The control module records the operating conditions at the time the diagnostic fails. The first time the diagnostic fails, the control module stores this information in the Failure Records. If the diagnostic reports a failure on the second consecutive ignition cycle, the control module records the operating conditions at the time of the failure. The control module writes the operating conditions to the Freeze Frame and updates the Failure Records.
Conditions for Clearing the MIL/DTC
• The control module turns OFF the malfunction indicator lamp (MIL) after 3 consecutive ignition cycles that the diagnostic runs and does not fail.
• A current DTC, Last Test Failed, clears when the diagnostic runs and passes.
• A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other emission related diagnostic.
• Clear the MIL and the DTC with a scan tool.
Diagnostic Aids
You must perform the CKP system variation learn procedure before proceeding with the diagnostic table. Refer to Crankshaft Position System Variation Learn .
Test Description
The number below refers to the step number on the diagnostic table.
2.If the actual CKP variation values are not within the learned values, the misfire counters may increment.
Step
Action
Values
Yes
No
1
Did you perform the Diagnostic System Check - Vehicle?
--
Go to Step 2
Go to Diagnostic System Check - Vehicle
2
•Start the engine.
•Allow the engine to idle or operate within the conditions listed in the Freeze Frame/Failure Records.
•Monitor all of the Misfire counters with the scan tool.
Are any of the Misfire Current counters incrementing?
--
Go to Step 3
Go to Intermittent Conditions
3
Are any other DTCs set?
--
Go to Diagnostic Trouble Code (DTC) List - Vehicle
Go to Step 4
4
Can any abnormal engine noise be heard?
--
Go to Symptoms - Engine Mechanical
Go to Step 5
5
•Start and idle the engine.
•Compare the manifold absolute pressure (MAP) voltage indicated by the scan tool with the typical scan data. Refer to Scan Tool Data List .
Is the indicated value more than the typical value?
--
Go to DTC P0108
Go to Step 6
6
Inspect the following components:
• The vacuum hoses and seals for splits, restrictions, and improper connections--Refer to Emission Hose Routing Diagram .
• The crankcase ventilation system for vacuum leaks--Refer to Crankcase Ventilation System Inspection/Diagnosis .
• The powertrain control module (PCM) grounds for corrosion and loose connections--Refer to Ground Distribution Schematics .
• The exhaust system for restrictions--Refer to Restricted Exhaust .
• The fuel for contamination--Refer to Alcohol/Contaminants-in-Fuel Diagnosis .
Did you find and correct the condition?
--
Go to Step 20
Go to Step 7
7
•Turn OFF the ignition.
•Remove the fuel pump relay. Refer to Relay Replacement .
•Remove the ignition coil for the affected cylinder, keeping the ignition connected to the harness connector. Refer to Ignition Coil Replacement .
•Install the J 26792 Spark Tester on the ignition coil and a good ground.
•Crank the engine.
Does the spark tester spark?
--
Go to Step 9
Go to Step 8
8
Inspect the affected cylinder spark plug boot for a missing or damaged ignition coil spring.
Did you find and correct the condition?
--
Go to Step 16
Go to Electronic Ignition (EI) System Diagnosis
9
•Remove the spark plug from the cylinder that indicated a misfire. Refer to Spark Plug Replacement .
•Inspect the spark plug. Refer to Spark Plug Inspection .
Does the spark plug appear to be OK?
--
Go to Step 10
Go to Step 11
10
•Exchange the suspected spark plug with another cylinder that is operating properly. Refer to Spark Plug Replacement .
•Operate the vehicle under the same conditions that the misfire occurred.
Did the misfire move with the spark plug?
--
Go to Step 17
Go to Step 14
11
Is the spark plug oil or coolant fouled?
--
Go to Symptoms - Engine Mechanical
Go to Step 12
12
Is the spark plug gas fouled?
--
Go to Step 15
Go to Step 13
13
Did the spark plug show any signs of being cracked, worn, or improperly gapped?
--
Go to Step 16
Go to Step 14
14
Perform the fuel injector coil test. Refer to Fuel Injector Diagnosis .
Did you find and correct the condition?
--
Go to Step 18
Go to Symptoms - Engine Mechanical
15
Perform the fuel system diagnosis. Refer to Fuel System Diagnosis .
Did you find and correct the condition?
--
Go to Step 18
Go to Symptoms - Engine Mechanical
16
Replace or gap the spark plug. Refer to Spark Plug Replacement .
Did you complete the action?
--
Go to Step 18
--
17
Replace the faulty spark plug. Refer to Spark Plug Replacement .
Did you complete the replacement?
--
Go to Step 18
--
18
Was the customer concern the MIL flashing?
--
Go to Step 19
Go to Step 20
19
•Operate the vehicle at the specified value for 4 minutes.
•Operate the vehicle within the Conditions for Running the DTC P0420 as specified in the supporting text. Refer to DTC P0420 .
Does the DTC run and pass?
2,500 RPM
Go to Step 20
Go to DTC P0420
20
•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 21
21
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
