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Discussion Starter #1
Already posted a while back but am starting a new one with some updates.

99 GMC Sierra with the 5.3, 130,000 miles. The engine idles rough and the cel is on and its getting bad mileage. The obd2 code showed a misfire in the #4 cylinder (P0304). So I replaced all the plugs, plug wires, new fuel filter, and new coil pack and injector in the number 4 cylinder. Checked the compression in the #2 (130) and 4 (122) cylinders. Checked the air filter which is fairly new and clean and the pcv rattles and is sucking so I figure they’re alright. None of that helped so I got a cheap laptop off of eBay and an Elmscan USB scan tool so I could check and clear the codes myself. I cleared the cel and then swapped the new coil pack with another cylinder to see if the code would move, thinking maybe the new one was defective, it didn’t, same code as before. Then I got a fuel injector noid light to see if the injector wiring was working right, it flashed as (I guess) it should. Checked the injector plug with my multitester and it is within specs (according to the Chilton manual.) Then I removed the EGR valve and blew it out. I don’t know if that did any good but I figured why not. I would assume that if it was bad it would throw a code anyway. Ran a whole can of seafoam intake cleaner directly into the intake to clean it up some….didn’t help. I understand it could be a bad valve so I got a leak down tester, pulled all the plugs and put the number four cylinder to tdc and checked it, it’s in the green at around 30% loss. I took off the Idle Air Control Valve and it looks brand new which probably doesn’t mean it’s good but I would think if it was bad it would throw a code and/or would affect more than one cylinder. All the plugs are kind of black and sooty so I know its running rich. The new plugs I’m using are the Bosch platinum+2 which you don’t gap….but even if they aren’t exactly right the motor idled rough and had the P0304 code before I changed out the original AC plugs. I was thinking of swapping out the new injector with the number 2 or 6 to see if maybe it’s bad. Other than maybe a bad ECU I don’t know what else to do except to start it on fire just for the joy of watching it burn.
Ideas??????????????????????????????????????????????????????????????????????
 

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Discussion Starter #2
I cleared the check engine light then swapped the new number 4 injector with the number 6 and reran the obd scan.
Still shows code P0304. Anyone have any guesses?
 

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I had and did all the same stuff on my suburban as you did (I spent alot of money just guessing at it too). It turned out to be a bad valve, so I had both heads rebuilt (I wish i would have just got a new motor), problem solved. My shop had to run a scope through the motor to find the problem as the leak test didnt show anything. The heads were rebuilt at a different shop in town and they did the same test, with the same results to verify. 95% will attest as well dont use anything but the OEM plugs for a vortec motor,
 

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Discussion Starter #4
Yep, I've gotten to the point where I'm going to have to take it to a shop............damn it.
 

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Pull those crap Bosch plugs out to start with. You want AC Delco Platinum/Iridium gapped at .040"
 

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X2, get rid of the bosch plugs and go with A/C Delco's.
 
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