Help with a P0300 code,

[nzachman]

Hello all,
I have a 2000 "old body style" Z71 Chevy Tahoe. In Oct 2008 I had
a new 5.7L long block, plug wires, dist cap, rotor and crank case sensor
put in. Didn't change any O2 sensors.

It ran fine, but they reused the old timing chain cover, it states right on it not to reuse so I bought a new one and they replaced it.

Well now the Check Engine light is on, some time it flashes for two minutes then it will flash 4-12 times. My mechanic is stumped, any ideas on what is throwing the code?
Thanks for any help,
Nathan Z.

 

[nzachman]

Also my mpg dropped from over 16mpg to around 13mpg since the code came up.
Thanks,
Nathan

 

[peterey2]

check the routing of your wires. could be crossfiring going on. check for vacuum leaks, plugged fuel filter...any misfire codes? check the O2's as well. with MPG's dropping, its likely the O2's could be bad.

 

[Sepiroth]

I would also say fuel injector.

 

[peterey2]

dirty MAF sensor. throws the wrong air/fuel ratio, makes the O2s think theres a problem and could cause multiple misfires bc of it. lots to check out.

 

[nzachman]

Thanks for the tips, I will start sorting through them tomorrow.
Nathan Z.