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Trailblazer
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Discussion Starter #1 (Edited)
Changed the plugs last weekend and now I have a P0300 misfire. It is only misfiring on #1.

Things I have tried so far:
swapped wires from #1 to #3....still misfire on #1 Only
Swapped coils from #1 to #3....still misfire on #1 Only
Pulled and replaced #1 Plug, checked gap, made sure it was at .035....still misfire on #1 Only
replaced all 8 plugs....still misfire on #1 Only
replaced all 8 wires....still misfire on #1 Only
replaced all 8 coils....still misfire on #1 Only
replaced all 8 injectors....still misfire on #1 Only
Cleaned MAF Sensor....still misfire on #1 Only
Cleaned TB....still misfire on #1 Only
checked compression in #1....170#, checked the other 7 just in case, all at 170#....still misfire on #1 Only
pulled the valve cover, loosened the rockers and did a leakdown on #1, held 150 PSI for 30 minutes, no leak....still misfire on #1 Only
Checked battery, 12.64vdc not running, 14.3vdc when running....still misfire on #1 Only
truck will run with the misfire on #1, you can hear it at idle, when truck is at 1k rpm or higher, in park, misfire is gone completely.
when truck is in gear, misfire is terrible, I have to use both feet to keep it running.

Anyone have any new ideas?:dunno::dunno:
 

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TT AWD TBSS Jeep
TT AWD SS Pwrd Jeep
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Vacuum leak at intake near #1

Bad wire going to coil, ohm the wires from plug thru all con. to ecm

Check for corosion in coil where wire goes in

Ohm injector harness wires same way as stated above

G/L
 
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secret squirrel
TrailblazerSS
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bad lifter or bad injector.:burp:
 

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TrailblazerSS
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Worn cam lobe/lifter. Dont see it as much anymore but used to see alot of problems with cylinders 1,7,2,8 (all cylinders at the end of each fuel rail) haveing trash built up in the rail causing miss on only one cylinder. Never seen this on TBSS but pretty much same rail as the 5.3/6.0 trucks.
 

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TrailblazerSS
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Any resolution yet???
 

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Trailblazer
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Discussion Starter #8
Where I am currently, I pulled the rockers on #1, the springs are pristine, the push rods are true and clean, blew them out for kicks.

I'm in the process of checking the injector and coil wiring for continuity. I'm thinking about putting the old plugs back in just for shits, who knows at this point.

I'm still open for suggestions.
 

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Trailblazer
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Discussion Starter #10
Wires are all intact from coil and injector to ecm.....still stumped.....when I ran it late last night it did have a misfire on #6....but it went away after a minute or so....#1 still just misfires away
 

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08 trailblazer SS
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Wonder if your pcm is going bad. Had this happen on a couple GM vehicles in the past. When its running try tapping on the pcm and see if the misfire stops or changes.
 
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TT AWD TBSS Jeep
TT AWD SS Pwrd Jeep
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Wiggle connectors and wires on the inector and coils on the offending cylinders as well. Did you eliminate all the previous suggestions? New plus in the cylinders wil eliminate the possibility you have a cracked one when installing. Inspect the wires carefully for what looks like a pinhole with black around it. That would be the area the spark would be jumping out.
 
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08 trailblazer SS
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Not sure if this will help, diag from GM dealerworld. If you need any more info let me know.

DTC P0300-P0308
Diagnostic Instructions
•Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.
•Review Strategy Based Diagnosis for an overview of the diagnostic approach.
•Diagnostic Procedure Instructions provides an overview of each diagnostic category.
DTC Descriptors
DTC P0300
Engine Misfire Detected

DTC P0301
Cylinder 1 Misfire Detected

DTC P0302
Cylinder 2 Misfire Detected

DTC P0303
Cylinder 3 Misfire Detected

DTC P0304
Cylinder 4 Misfire Detected

DTC P0305
Cylinder 5 Misfire Detected

DTC P0306
Cylinder 6 Misfire Detected

DTC P0307
Cylinder 7 Misfire Detected

DTC P0308
Cylinder 8 Misfire Detected

Circuit/System Description
The engine control module (ECM) uses information from the crankshaft position (CKP) sensor in order to determine when an engine misfire is occurring, and uses information from the camshaft position (CMP) sensor in order to determine which cylinder is misfiring. By monitoring variations in the crankshaft rotation speed for each cylinder, the ECM is able to detect individual misfire events. If the ECM detects a misfire rate sufficient to cause emission levels to exceed mandated standards, DTC P0300 sets. Under certain driving conditions, a misfire rate can be high enough to cause the catalytic converter to overheat, possibly damaging the converter. The malfunction indicator lamp (MIL) will flash ON and OFF when catalytic converter overheating conditions are present and DTC P0300 is set. DTCs P0301–P0308 corresponds to cylinders 1–8. If the ECM is able to determine that a specific cylinder is misfiring, the DTC for that cylinder will also set.

Conditions for Running the DTC
•DTC P0010, P0011, P0101, P0102, P0103, P0106, P0108, P0116, P0117, P0118, P0128, P0220, P0335, P0336, P0502, P1258, P3401, P3425, P3441, and P3449 are not set.
•The engine speed is between 650–6,200 RPM.
•The ignition voltage is between 11–18 volts.
•The engine coolant temperature (ECT) is between −7 and +130°C (+19 and +266°F).
•The fuel level is more than 10 percent.
•The antilock brake system (ABS) and the traction control system (TCS) are not active.
•The power management is not active.
•The transmission is not changing gears.
•The ECM is not in fuel shut-off, or decel fuel cut-off mode.
•Excessive drive wheel slip is not detected.
•The power take-off is disabled.
•The cylinder deactivation is not in progress.
•A manual transmission with a throttle position less than 95 percent.
•DTCs P0300–P0308 run continuously when the above conditions are met.
Conditions for Setting the DTC
The ECM 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
•DTCs P0300–P0308 are Type B DTCs.
•When the MIL is flashing the fuel injector may be disabled for the misfiring cylinder to protect the catalytic converter.
•Depending on the conditions that set the DTC, the engine may go into Open Loop.
Conditions for Clearing the MIL/DTC
•DTCs P0300–P0308 are Type B DTCs.
•The DTC must run and pass under the same operating conditions that were present when the DTC failed.
Diagnostic Aids
•Ensure that each fuel injector is connected to the proper fuel injector harness connector.
•A misfire may only occur when the engine is operating under a load or when the engine is cold.
•A high resistance condition on any ignition control (IC) circuit can cause an engine misfire P0300–P0308 DTC to set without setting an IC circuit P0351–P0358 DTC.
•This test procedure requires that the vehicle battery has passed a load test and is completely charged. Refer to Battery Inspection/Test.
•A misfire DTC could be caused by an excessive vibration from sources other than the engine. Inspect the following for possible sources of vibration:
– A tire or wheel that is out of round or balance
– Variable thickness brake rotors
– An unbalanced drive shaft
– Certain rough road conditions
– A damaged accessory drive component or belt
– A damaged reluctor wheel
– The Crankshaft Position System Variation Learn procedure may need to be performed.
Reference Information
Schematic Reference
Engine Controls Schematics

Connector End View Reference
Component Connector End Views

Electrical Information Reference
•Circuit Testing
•Connector Repairs
•Testing for Intermittent Conditions and Poor Connections
•Wiring Repairs
•Powertrain Component Views
DTC Type Reference
Powertrain Diagnostic Trouble Code (DTC) Type Definitions

Scan Tool Reference
Control Module References for scan tool information

Special Tools
J 26792 HEI Spark Tester

Circuit/System Verification
 1. Verify that no other DTCs are set.
⇒ If a DTC is set, refer to Diagnostic Trouble Code (DTC) List - Vehicle.
 2. Engine idling, verify there is no abnormal engine noise.
⇒ If there is an abnormal engine noise, refer to Symptoms - Engine Mechanical.
 3. Observe the Misfire Current Counters with a scan tool. The Misfire Current Counters should not be incrementing. Random misfires that are not steadily incrementing may be normal.
 4. Operate the vehicle within the Conditions for Running the DTC to verify that the DTC does not reset. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records data.
Circuit/System Testing
Important: You must perform the Circuit/System Verification first.

 1. Verify that the following conditions do not exist:
◦The fuel injector wire harness electrical connectors are connected to the wrong fuel injectors. Refer to Engine Controls Schematics.
◦Engine vacuum leaks
◦Fuel pressure that is too low or too high—Refer to Fuel System Diagnosis.
◦Contaminated fuel—Refer to Alcohol/Contaminants-in-Fuel Diagnosis.
◦Restricted exhaust system—Refer to Restricted Exhaust.
⇒ If you find any of the above conditions, repair as necessary.
 2. Verify that there is no misfire on cylinders 1, 4, 6, or 7 due to cylinder deactivation—Refer to Cylinder Deactivation (Active Fuel Management) System Diagnosis.
 3. Measure the resistance of the spark plug wire, and verify the condition of the spark plug wire. Refer to Ignition System Specifications and Spark Plug Wire Inspection.
⇒ If a fault is found, replace the spark plug wire. Refer to Spark Plug Wire Replacement.
 4. Reinstall the spark plug wire to the ignition module/coil assembly.
 5. Install the J 26792 to the spark plug wire boot.
Important: An erratic or weak spark is considered a no spark condition.

 6. Attempt to start the engine and observe the J 26792. The spark tester should spark.
⇒ If there is no spark, refer to Electronic Ignition System Diagnosis.
 7. Ignition OFF, remove the spark plug from the misfiring cylinder. Verify that the following conditions do not exist with the spark plug:
◦Gas, coolant, or oil fouled
◦Cracked, worn, or incorrectly gapped—Refer to Spark Plug Inspection.
⇒ If there is a condition with the spark plug, replace the spark plug.
 8. Exchange the suspected spark plug with another cylinder that is operating correctly.
 9. Operate the engine under the conditions in which the misfire occurred, this may include putting the engine under a load or allowing the engine to return to a cold condition. Observe the Misfire Current Counters with a scan tool. The misfire should not follow the spark plug exchange.
⇒ If the misfire follows with the spark plug, replace the spark plug.
10. If all conditions test normal, test or inspect for the following:
◦A lean or rich fuel injector—Refer to Fuel Injector Diagnosis.
◦An engine mechanical condition—Refer to Symptoms - Engine Mechanical.
Repair Instructions
•Spark Plug Replacement
•Spark Plug Wire Replacement
•Ignition Coil Replacement
•Engine Control Module Replacement
•K20 Engine Control Module: Programming and Setup
Repair Verification
 1. Install any components that have been removed or replaced during diagnosis.
 2. Perform any adjustments, programming or setup procedures that are required when a component is removed or replaced.
 3. Clear the DTCs.
 4. Turn OFF the ignition for 60 seconds.
 5. If the repair was related to a DTC, duplicate the Conditions for Running the DTC and use the Freeze Frame/Failure Records, if applicable, in order to verify the DTC does not reset. If the DTC resets or another DTC is present, refer to the Diagnostic Trouble Code (DTC) List - Vehicle and perform the appropriate diagnostic procedure.
 6. To verify that the performance of the catalytic converter has not been affected by the condition that set this DTC, perform the Repair Verification for DTC P0420 or P0430. Refer to DTC P0420 or P0430.
© 2015 General Motors. All rights reserved.
 
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Trailblazer
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Discussion Starter #14 (Edited)
Update on this misfiring pig. Replaced the coil pack harnesses, now I have a misfire on #1, 2, 4, 6, 7 & 8, after it warms up the misfires go away except for #1. I have run it with valve cover off and the valves on #1 move the same amount as on #3, 5 & 7. Maybe I should try the crank position & or cam sensor, or even a relearn on both first, then I'll try the sensors. Other than that I think the next try will be the ECM.

Any thoughts??
 

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'08 Silver TBSS
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Man you're having just a hell of a time with this. I guess a new ECM is the next step as you've pretty much done everything. I wish I had more advice for you.
 

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08 trailblazer SS
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3,614 Posts
Doesn't make sense that your getting all those misfires now after replacing a harness til it gets warm. Almost sounds like your dealing with a bad under hood fuse box. But yours is doing the opposite of what mine did, fine when cold then would start giving me issues once up to temp.
 

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TT AWD TBSS Jeep
TT AWD SS Pwrd Jeep
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Did you check for corosion on any of the fuses or the terminals they plug into? A connection that is poor may still run but cause voltage drop which wreaks havoc on the circuit. Take box apart and check for corrossion on the inside as well. Tom may be onto something.
 
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Trailblazer
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Discussion Starter #18
I pulled the fbox...it's clean....I'm thinking ecm
 

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1949 TrailblazerSS
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I still say cam and lifter problem, you said the misfire goes away off idle, good symptom of a damaged cam. If it was a cicuit problem it would set circuit control codes. You already eliminated plugs, wires, coils, injectors, compression, valve springs, rockers, push rods. An ECM issue would also cause circuit codes.
 
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