96 5.7 vortec

[Caleb Stoken]

Po300 code. All ready Replaced the following so far, spark plugs, wires, cap and rotor, spider injectors, both upstream o2 sensors. Runs 20 times better but still misfires just not as random anymore. Idles fine but when put into gear the misfire gets 100 times worse.  

[tlaw91]

MAP sensor failing thinking the engine is under a heavier load then it actually is? Does it get better at high rpm?


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[Jsdirt]

If the misfires get better with RPM, you probably have failing intake gaskets.

[2018GMC]

Have you tried cleaning or swapping in another Mass Air Flow sensor? Does it still misfire if you disconnect MAF?

[Doug_Scott]

Have you done the basic trouble shooting procedures prior to throwing money at it?  Did you check the mechanical condition of the engine? 

Do a compression test, if it is good, take a small bottle of water and with the engine running at operating temp pour water over the connection of the intake manifold to cylinder head.  Listen for a change in engine running.  You can also use a bottle of propane with the torch attachment.  Run the unlit tip around the mating point of intake/cylinder head, listening for a change in rpm.