I have done just about everything under the sun to try and fix this issue. I took it to a mechanic earlier today and apparently he didn't like it after looking it over and basically told me to come pick it up at no charge. He noticed (I have yet to see it) a crack in the intake, also, I just ordered it, the PVC elbow had a decent crack in it.

If the p0300 were to be caused by leaking air, wouldn't there be a code set indicating a lean mixture? Or could it just be minor enough to not set any codes?

Here is a list of everything that I have done thus far:
Replaced Head Gasket
Replaced Lower and Upper Intake Gasket
Replaced Wires
Replaced Plugs
Replaced Cap
Replaced Rotor
Replaced Spider (MFI Upgrade)
Replaced fuel pump
Replaced fuel filter almost 1 year ago

Any ideas would be very helpful, Thanks in advance

 

Did you use an AC Delco distributor cap & rotor?
Did you put silicone dielectric grease on each terminal,
inside and out, of the distributor cap?
Did you ever get camshaft retard within specs, (zero, plus or minus 2 degrees)?
Take care of these things and post your results.

 

The cap and rotor I am not sure if they are ACDelco or not, I borrowed them from a buddy of mine, the ones I had on there before were Duralast. Can't tell for sure since the caps or rotors dont have any markings. Only thing I noticed that was different was the cap and rotor are both kind of grey in color. The duralast cap and rotor are black and tan/white (respectively).

When I initially had the cap installed I used dielectric grease on each terminal and on the plugs. Basically almost anything that had to do with the spark. Then as time has gone by I've only applied it to the spark plug boot.

I have not been able to actually check the camshaft retard. I'm not 100% if the copy of ScanXL is a legit copy or not. Been using someone else's laptop. From what I can tell its not legit since its been giving me erratic readings. Especially with the RPM, goes from 650 up to 1800 even when the RPM in the dash shows 650.

I just replaced my PCV elbow earlier and have yet to take it for a test drive. The elbow I had on before just disintegrated in my hands. When I got the blazer back from the mechanic I noticed that the lower portion of the manifold has a crack along where one of the bolts go. I'm looking in to going and picking up a junk yard piece. I wasn't able to find them on rockauto, any place I can find one?

 

The issue with aftermarket caps & rotors is crossfire inside the cap. It may have something to do with how they insulate them, the precise placement of the terminals, or the materials they are made out of. Whatever it is, they don't perform as well, or last as long, as AC Delco. Among other things, camshaft retard perfectly aligns the rotor with the terminals inside the cap to eliminate crossfire. The farther off camshaft retard is, the larger the gap between the rotor tip and the cap terminal when the ignition coil fires, which increases the possibility of crossfire. Duralast stuff is "bottom of the barrel", entry level, low quality. It "looks" good, but that's as far as it goes. Avoid it if you want the engine to run properly. Get an AC Delco cap & rotor and make sure camshaft retard is within specs.

 

I second Captain's comment on AC Delco parts! I just went through this myself. After trying to use other brands for my distributor cap and rotor, I finally spent the money on the AC Delco brand parts. It made a HUGE different in my idle and I haven't had a P0300 misfire since the change.

If you get a chance sometime, take a few minutes to look at the difference between distributor caps and rotors from various brands (you can do this online). AC Delco's distributor cap does appear to have "thicker" insulating runners and I think they use a different type of material in the molding process to eliminate the potential for crossfiring. And, their rotor has a plastic isolator molded to the metal strap, which, I'm assuming, also helps with negating crossfire.