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Bob, Auto Tech
Category: Chevy
Satisfied Customers: 4087
Experience:  Plus 40 years GM, over 30 certified with Chevrolet, ASE certified Master Tech. (Expired, Retired)
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5.7L Tahoe 2wd4dr. Extremely rough idle, misfire, dies out ...

Customer Question

5.7L Tahoe 2wd4dr. Extremely rough idle, misfire, dies out if you don't give it throttle, running rich. P0300 Code no others. New plugs, wires, cap, rotor, fuel injector on #5 cylinder, fuel pressure reg. removed cats. No joy =( please help.
Submitted: 8 years ago.
Category: Chevy
Expert:  Bob replied 8 years ago.
I am going to post some info for you from the GM service manual regarding your problem. You are going to need a class 2 scan tool to run some function tests. You may be able to rent one from a local parts store. But, a code reader can't do the job we need to do to find the problem. It has to be a class 2 scan tool similar to what the dealers use. Other wise, you are going to continue to throw parts at it and waste a lot of money. If you do not have the experience to use the tool to run the tests, it may be better to pay a professional to get to the bottom of the problem rather than continue to throw parts at it. Below is some basic info for testing. There is a lot to it, to try to find this kind of a problem, it is not a cut and dried type of thing. Sorry I can't just give you some simple test to do with a volt or ohm meter, etc. But considering how complex these new units are, sometimes it is better to just bite the bullet and pay the pro. If you have the experience and the tools, I will do my best to help you track it down. Thanks
Expert:  Bob replied 8 years ago.

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

  1. Wetting down the secondary ignition system with water from a spray bottle may help locate damaged or deteriorated components. Look and listen for arcing or misfiring as you apply the water.
  2. If the Misfire Current counters are incrementing and there is no apparent misfire, an erratic CKP sensor signal could be the cause. Perform the diagnostic table for DTC P0335 first if this condition is suspected.

    If a misfire is present and you suspect a fuel control problem, force the fuel system into Open Loop using the scan tool and allow the engine to run for a few minutes. If this eliminates the misfire, refer to any fuel control related DTCs which are set. If no other DTCs are set, refer to the Engine Scan Tool Data List.

    A misfire may not be apparent at idle. The misfire may only occur above idle under a load. Road test the vehicle and monitor the misfire current counters.

    If more than one cylinder is misfiring, the misfire current counters may only increment for one cylinder. Example: Cylinders 1 and 8 are both misfiring, yet only cylinder 8 increments on the misfire current counter.

    If one of the injector fuses is open, only two or three misfire current counters may increment for the corresponding side of the engine.

  3. The cylinder with the more significant misfire may cause another cylinder counter to increment only by a small amount.
  4. If the engine misfire moves with the spark plug, this is good indication that you should replace the spark plug.
  5. An engine mechanical problem can cause a spark plug to gas foul. Inspect for loose rockers, collapsed lifters, or worn camshaft lobes.
  6. If the customer concern is the MIL flashing, this indicates that a Catalyst Misfire has occurred. Drive the vehicle in the conditions to run the catalyst diagnostic.
Expert:  Bob replied 8 years ago.

Running the vehicle out of fuel causes sufficient misfire to set DTC P0300. A vehicle that is out of fuel may have fuel level DTCs also set.

Water contamination in the fuel system can cause a single cylinder to misfire as well as cause a random misfire. If there is a misfire in Cylinder #7 it is possible that water has collected in the fuel rail.

If there is a misfire detected in Cylinder #4or #6 it is possible that the fuel pressure regulator diaphragm has ruptured, causing fuel to be drawn in through the regulator vacuum line. Remove the vacuum line and inspect for fuel contamination.

A restricted fuel filter can cause sufficient misfire to set DTC P0300.

Excessive vibration from sources other than the engine could cause a misfire DTC. The following are possible sources of vibration:


Variable thickness brake rotor


Drive shaft not balanced


Certain rough road conditions

Observe, if more then one cylinder is misfiring, the scan tool may only display one cylinder misfiring. This will not be apparent until the repair is completed. Also, if an ignition coil ground circuit is open for one side of the engine, the scan tool may only display 2 or 3 cylinders misfiring. Inspect the ground circuit for the ignition coil on the cylinder bank of the engine that has more then one cylinder misfiring.


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