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Misfire help!

mike99

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mike99 Enthusiast

mike99

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P0300 random

1999 5.3 new body Sierra

Misfire, extended crank intermittent, while driving it lacks throttle response like it's starved. Engine has new gaskets throughout, just finished head gasket job on it. New timing chain kit, fuel filter, plugs, wires. Maybe a failing fuel pump?? Do these engines need any ecm flash when timing redone? I did delete EGR with block offs, but I've read several things that it doesn't make it run any different without it. Any suggestions, tests to run? going to check fuel pressure tonight. 

No MAF, MAP, injector or throttle body codes. No vacuum leaks present.

Thanks.

 

no HD Enthusiast

no HD

Posted
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Sometimes it takes a long time for a code to set. If your fuel pressure checks out OK, you might try unhooking the MAF and driving it. That will set a code but the ECM  will use preset parameters and if an improvement is seen in driveability the MAF may be throwing things off. Also with a Tech or Supertech type scanner you could continuous monitor and take a big look at things. I had one recently acting similar to yours and it was the MAF. It took forever to finally set a code.

mike99 Enthusiast

mike99

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No luck with MAF unplugged. Everything is the same as it was when I tore the heads off, I strongly doubt a sensor has gone bad. I'm thinking the timing may be off a tooth...

mike99 Enthusiast

mike99

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Can anyone tell me what the number should be for ignition timing advance for #1 cylinder? On my scanner is shows 21-25° fluctuations. And fuel trim numbers. @ idle 

riverbanks Senior Enthusiast

riverbanks

Posted
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Certain perameters, cause a change of advance,21 to 25,seems fine, check fuel pressure, very important,load percentage is a  clue to see if it's struggling or not, should be able to see that on the scanner

mike99 Enthusiast

mike99

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Checked fuel pressure, 55 key on engine off, 48-50 at idle. Spec says 55-62 for non flex fuel 5.3. when disconnect vacuum from regulator, psi goes to 55. From what I've read, the fuel pump is the culprit.. what do y'all think?

riverbanks Senior Enthusiast

riverbanks

Posted
Posted

48 psi,it needed to be changed, with the 155,swap it with other side?see if code follows? sometimes when it rains,it pours

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