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cylinder misfire, gm 5.3 engine
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shawnd1
New User
Apr 3, 2011, 10:47 AM
Post #1 of 3
(3760 views)
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cylinder misfire, gm 5.3 engine
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Posted to another site but no help yet.
99 GMC Sierra with the 5.3, 130,000 miles. The engine idles rough and is getting bad mileage and the cel is on. 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 multimeter 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.
Ideas??????????????????????????????????????????????????????????????????????
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Timthecarguy719
User
Apr 6, 2011, 5:33 PM
Post #2 of 3
(3733 views)
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Re: cylinder misfire, gm 5.3 engine
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Did you do a vacuum test on it?
Your repair manual should tell you how to do it.
You'll be looking for a nice steady reading.
If the injector is good and the spark in good and compression only other thing of the top of my head is a vacuum leak.
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shawnd1
New User
Apr 6, 2011, 8:33 PM
Post #3 of 3
(3731 views)
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Re: cylinder misfire, gm 5.3 engine
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Haven't checked for a vacuum leak but I'll try that. I'll try about anything at this point. Thanks for the suggestion.
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