Hi,
I have a 2000 escalade with a 5.7 vortec. I have had a p0300 code for a few months now, and the misfire is making my truck barely driveable. It started as a slight misfire. Then, i lost top end power. Now i barely have power and it shakes horribly.

So far i have replaced:
Dizzy cap/rotor
Plugs
Wires
Cam/crank sensors
FPR
Plenum gasket

I did the test to check my spider injection, none of the poppets were stuck open. I see no leaks anywhere when fuel pump is primed (plenum off of course).
The misfire is always there, and worsens as it heats up. Im at a loss, and have thrown countless hours and hundreds of $$ at it trying to fix this... i really need help, im only 21 years old and this truck is my means to support my family. Any help is appreciated!!

 

first step would be getting a scanner and seeing the misfire data to narrow down which cylinder(s) you need to focus on. Have you checked fuel pressure or fuel trim with a scanner? Compression test?

 

I had the same problem on a 96' Vortec with spider injection. It may or may not be related but it was a tough one to track down. The distributor gear that meets the camshaft was chewed up. Not so much as "chewed up" but more like rounded off on the tip. A shop ran all the high end computer scanners and such, even pulled the distributor and never noticed the sheared edges because it was such a clean wear pattern.

 

Like previously mentioned, use a scanner. I use a $10 OBD2 USB adapter with software. I can track and log each engine function. It gives you an idea of what exactly the engine is doing during each stumble/misfire. If I recall, I believe the only way I was able to properly set the timing was through the PCM with software I do not have access to so I had to have a dealer or shop do that. If you pull the distributor be sure to put it back exactly where you left it.