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I got a 2006 chevy 4.8L. I get random cyl misfire code. #7 cyl, misfireing the most. steady, but no code for # 7 misfire, we changed the plugs, wires, switched coil and injector with another cyl. replace intake gasket, and still # 7 misfire, checked wiring for the circuit coil. no brakes in the wiring, checked the ground splice and all ok. checked compression, good 145 across the board. removed valve cover and checked for bent push rod or any other mechanical failer. performed a injestor balance test and all injectors passed, what am I missing??

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I got a 2006 chevy 4.8L. I get random cyl misfire code. #7 cyl, misfireing the most. steady, but no code for # 7 misfire, we changed the plugs, wires, switched coil and injector with another cyl. replace intake gasket, and still # 7 misfire, checked wiring for the circuit coil. no brakes in the wiring, checked the ground splice and all ok. checked compression, good 145 across the board. removed valve cover and checked for bent push rod or any other mechanical failer. performed a injestor balance test and all injectors passed, what am I missing??

 

Three Basic reasons for a misfire:

1: loss of spark

2: the air/fuel mixture is too far out of balance to ignite

3: loss of compression

 

looks like you have checked for almost everything. Look at the EGR valve, check your fuel pressure (fuel pump sometimes will not up to par). Aside from what you have already done, I am not sure that there is anything you may have missed. By the way, what is the error code that you are getting???

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also check #8 it fires just before #7 - look over your valve train and make sure you cat is not getting pluged up.

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1st thing i do is swap the coil with another and see if it changes cylinder! easy to do. done it many times at the shop to help diagnose where its from

Edited by sic lsx

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dude i dont know what it is but if you figure it out you need to post the results cuz theres tons of us with similar problems

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Heard something the other day on a truck program when they were doing a tune for mis-firings. They said to do a antifreeze coolant "volt" check. They claimed that old antifreeze creates electrolysis and will throw P300 codes and random misfires. Volt check consists of using a volt meter black wire on ground and red wire dipped into the antifreeze. I don't remember what the bad voltage was, but I think it was over .5 volts.

Edited by redsled2007

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Well found the problem. it was a broken valve spring , don't know how I missed it the first time, anyway problem fixed. runs like a top now.

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Well found the problem. it was a broken valve spring , don't know how I missed it the first time, anyway problem fixed. runs like a top now.

 

 

check your pushrod too - the truck we had that had a broken spring had a bent one.

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FWIW I just had a broken valve spring on my 04 Silverado 4.8l. Had a P0300 code. Changed the plugs, all looked good except #8 was black/sooty. Tried a different fuel injector in 8 but no help. Finally threw in the towell and took it to a shop and they confirmed good fuel, spark and compression. They pulled the valve cover and sure enough a valve spring was broken.

 

I'm 54 years old, I thought broken valve springs were ancient history.

 

Thanks for the Forum

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