P0440 P0300 Back firing

[JoeySS]

1998 5.0 1500 with 180,000 miles. It has had a SES for 2 years dued to low compression in cylinder number 6. But you'd never know, ran 17 mpg.

The other day it started back firing and losing power. It threw a 440 and 300 code. It acts exactly as if the engine temp sensor was bad. I checked the engine temp sensor with my scanner and was able to watch it rise steadily... so I know it's good.

The distributor has 2 years on it with 15,000 miles. I replaced ignition coil, plugs & wires, fuel filter, PCV and did an oil change today. The problem is persisting.

I'll check cylinder compression tomorrow. Please feel free to offer any suggestions. Thanks in advance

 

[Supercharged111]

Backfire could be lean or ****** up ass timing, but timing likely isn't it with a Vortec.

 

[GarrettGmc]

I recently had the po300, swapped plugs wires cap rotor an coil and it made it better but still has times where it feeling like a miss but no codes. Next guess will be the injectors but if u have a compression problem that could be it

 

[JoeySS]

Just had the coolant flushed and cleaned 2 months ago. Opened it up and it was all gone. I'm wondering if I should do the intake manifold gasket... Or go for a rebuilt motor.

 

[Ttoyota]

Did you figure it outl?