Code Po300 question

Rebel Biker

Code Po300 question

Unread post by Rebel Biker » Tue Dec 15, 2009 6:11 pm

i have a 2003 Trailblazer, was running fine , but coding for O2 sensors,, all of a sudden, "check engine light" started flashing when excelerating, had codes checked, only coding Po300 , does run rough now, can feel the misfire any suggestions as to where to start the repair project would be greatly appreciated

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TerryAuto
Posts: 313
Joined: Fri Dec 11, 2009 8:48 pm

Re: Code Po300 question

Unread post by TerryAuto » Tue Dec 15, 2009 11:58 pm

What engine in this Trailblazer?
4.2 inline 6, or 5.3 V8 ?
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4Walt2

Trailblazer P0300 Misfire Repair

Unread post by 4Walt2 » Fri Oct 15, 2010 8:16 pm

If this is the 4.2 L straight six cylinder engine, a very common cause of misfire is a bad ignition coil, and/or spark plug. They usually break down under heavy engine load. Start the truck and put in gear with your foot on the brake very hard, then give the gas pedal a press all away down and let up, then all the way down that let up. You should feel the misfire as you're hitting the gas pedal. This procedure is called a brake torque. This is good for putting a heavy electrical load on the ignition system. If it were fuel injector, this brake torque would not have much effect. To replace ignition call on the straight six engine, you need to remove the aircleaner assembly. You will see the six ignition coils on top of the valve cover. The spark plugs are under those ignition coils. Each coil is held on by a 10 mm bolt.

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TerryAuto
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Joined: Fri Dec 11, 2009 8:48 pm

Re: Code Po300 question

Unread post by TerryAuto » Fri Nov 02, 2012 2:53 pm

If the misfire and code P0300 is only at idle, then your Trailblazer could also have a bad cylinder head. These had problem of the valve seats getting damaged and causing a misfire at idle only. Carbon gets in the cylinder head valves and beats up the seats so they will not seal. The cylinder runs lean at idle and causes a misfire.
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4.2L Valves.jpg
4.2L cylinder head valves.
4.2L Valves.jpg (13.08 KiB) Viewed 9960 times
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