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Gary, Chevy Technician
Category: Chevy
Satisfied Customers: 16744
Experience:  15 years GM master tech
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04 chev tahoe 5.3 p0300 shows missfires on #1 and 6 cyls

Customer Question

04 chev tahoe 5.3 p0300 shows missfires on #1 and 6 cyls
Submitted: 6 years ago.
Category: Chevy
Expert:  Gary replied 6 years ago.
ok and it is skipping those 2 cylinders you can feel it skipping?and you are sure you have spark and injector pulse to those 2 cylinders?all 8 flowed fuel?how did you do the injector purge test to check for PSI fuel drop on the injectors?
Customer: replied 6 years ago.
At Idle you can feel one cylinder miss very lightly once in a while, and reving it up or driving it you can not feel a miss, but the missfire counter on scanner reads them. yes all 8 injectors flowed also never feel more than one miss at a time.yes fuel drop on the injectors.
Expert:  Gary replied 6 years ago.
and you have a new coil and spark plugs in both #1 and # XXXXX ?and it made no difference?
Customer: replied 6 years ago.
Expert:  Gary replied 6 years ago.
ok swap #1 plug and coil and wire with # XXXXX then swap plug coil and wire from #6 to #8 see if the misfired cylinders move to other cylinders.
Customer: replied 6 years ago.

just got done switching coils,plugs,wires,1to3 and 6to8 does samething. when you start car and the idle finally settles does it starts lightly missing and you can feel it, and the miss fire counter states about 10-20 counts per cycle then when reved to 2300rpm the miss fire counter goes right with the cycles and sets p0300 on #1 cyl. and #6 misses alot too but not as much and you can not feel the miss in or out of gear when step on throttle?

Expert:  Gary replied 6 years ago.
ok so it DID NOT MOVE THE SKIPPING this correct # XXXXX and # XXXXX are still showing skipping?
Customer: replied 6 years ago.
Expert:  Gary replied 6 years ago.
ok do you have a vacuum guage that read vacuum?we need to run the truck at idle and put a vacuum guage on any manifold vacuum line and see what the needle is reading.and also note if the needle is jumping when the engine is running poorly
Customer: replied 6 years ago.
HAS 17 inch lbs at 650 idle when acting up,needle is steady,even when reved needle is steady.
Expert:  Gary replied 6 years ago.

and you are 100% sure you have no vacuum leaks at all from the intake manifold? do you know how to flow the fuel injectors with a fuel guage hooked up to the fuel line test port and fire each injector and see if they all evenly drop the same PSI? but remember to only fire the injectors after you key it to get the fuel PSI at the same starting point to get accurate injector drop readings.and try this bulletin too and see if this will fix it.

Information on Engine Misfire MIL/SES Light Illuminated or Flashing DTC P0300, P0301, P0302, P0303, P0304, P0305, P0306, P0307, P0308, P0420 or P0430


1999-2007 Cadillac, Chevrolet, GMC Full-Size Pickup and/or Utility Trucks


with 4.8L, 5.3L, 5.7L, 6.0L or 6.2L VORTEC GEN III, GEN IV, V-8 Engine (VINs V, C, T, Z, B, 3, M, 0, J, R, U, N, Y, K, 8 - RPOs LR4, LY2, LM7, L59, L33, LC9, LH6, LMG, LY5, L31, LQ4, LQ9, L76, LY6, L92)

with Active Fuel Management(TM) and E85 Flex Fuel

If you encounter vehicles that exhibit the above conditions, refer to SI for the appropriate DTC(s) set. If no trouble is found, the cause may be due to an ECM ground terminal that has corroded with rust over time. Inspect the main engine wiring harness ground terminal (G103) for this condition.

Attachments are only available to registered users.

Register Here

The wire terminal (G103) attaches either to the front or to the rear of the right side cylinder head, depending on the model year of the Full Size Pickup and/or Utility Trucks. If the ECM ground terminal has been found to be corroded, then follow the service procedure outlined in this bulletin to correct the corrosion issue.

Attachments are only available to registered users.

Register Here

Remove either the nut or bolt securing the main engine wiring harness ground terminal (G103) to the right cylinder head. Refer to the above illustration to determine where the ground is located on the vehicle (1).

Remove all rust from the ground terminal, the cylinder head and the retaining nut or bolt.

Position the main engine wiring harness ground terminal and install the nut or bolt.


Tighten the retaining nut or bolt to 16 N.m (12 lb ft).

Apply some type of electrical moisture sealant to protect the harness terminal from further corrosion.

Customer: replied 6 years ago.

Yes we did a fuel drop injector flow test and all passed, has no intake leak,we had already cleaned a bunch of grounds but went crazy with them again,and even added some,and that did we hardly get any missfire counts except #1 it acts as follows. at idle in nuetral maybe gets 10 to 20 missfires per cycle,accelarate to almost 1750 with no missfire and from 2000to 3000rpm counter goes with cycles and sets code p0300 after 3000rpm no missfires, in gear under load or driving no missfires no code set?

Expert:  Gary replied 6 years ago.
the next step is to have the fuel injectors cleaned with a pressure purge kit.any shop can do this.they will clean those injectors right out so they are spotless and clean.
Customer: replied 6 years ago.
Cleaned fuel injectors with otc fuel injector cleaner pressured at 120psi did not help. took fuel injectors out and switched # XXXXX an 3 around,and no change? I'll give you a nice tip you help me threw this man. this things getting to me.
Expert:  Gary replied 6 years ago.
well we have eliminated injectors and eliminated spark plugs/coils/ can you do a compression test on all 8 cylinders?thats the only other thing other than the last bulletin from GM says to update the computer program in the computer with the latest and greatest programs have fixed a bunch
Expert:  Gary replied 6 years ago.

here is that bulletin

LM7, L59 or L33 Engines with Cold Start Rough or Incorrect Idle, Misfire, Malfunction Indicator Lamp (MIL) or Flashing Requiring 02 Sensor Replacement (Reprogram PCM)


2004-2005 Cadillac Escalade
2004-2005 Chevrolet Avalanche, Silverado 1500, Suburban, Tahoe
2005 Chevrolet Silverado Hybrid (HP2)
2004-2005 GMC Sierra 1500, Yukon, Yukon XL
2005 GMC Sierra Hybrid (PH2)


with Vortec 5300 5.3L V-8 Engine (VINs T, Z, B - RPOs LM7, L59, L33)


Some customers may comment on a Malfunction Indicator Lamp (MIL) On or that the MIL is flashing. An actual engine misfire may or may not be felt along with an incorrect idle speed. Typically this occurs during cold winter months, on cold engine starts. Some vehicles may have been previously serviced for the same condition. Technicians may also find the following DTCs stored with the Tech 2(R) P0131, P0133, P0134, P0137, P0138, P0140, P0151, P0153, P0154, P0157, P1133, which may be in conjunction with P0171, P0174, P0300 and P1138.


Water shock occurs when water droplets are present in the exhaust. The water in the exhaust contacts the sensor element inside the H02 sensor, causing the H02 sensor element to crack.


Upon diagnostic investigation, the technician may find that the H02 sensor requires replacement. In addition to replacing the affected H02 sensor, these vehicles should also be reflashed with the latest PCM calibration. An updated PCM calibration will be available to dealerships as part of TIS 2000 incremental satellite update 11.0 (NAO version 25.010) or greater. Always, make sure your Tech 2 is updated with the latest software version.

Important: If a DTC P0420 or P0430 is stored along with DTC(s) P0300, the catalytic converter diagnostics in Service Information (SI) must be followed to determine if the catalytic converters are operating properly.

Gary and 3 other Chevy Specialists are ready to help you
Customer: replied 6 years ago.

Your not going to believe this,just had a 2002 suburban with 6.0 in doing the samething.

at 2300 rpm check engine light flashes and sets p0300 code you can watch the missfire counter missing on # XXXXX and 6 cylinders but no drivabilty issues?have not figured out either one.

Customer: replied 6 years ago.
Thanks for what you did help with??
Expert:  Gary replied 6 years ago.
no at a loss with your other vehicle so i opened it up to other experts