OK, a little history.
I bought this 96 blazer on the cheap, knew it would need some work.
Ended up having the head machined and put it all back together and now I'm getting a P0300. Replaced the EGR, the spider, new plugs (E3), new wires, new cap and rotor, still getting the code.
Checked vacuumed lines for leaks.
I'm at a loss, my other truck is a 59, so this is kinda new to me.
Any help would be appreciated.

 

Several things can cause a P0300. Most of the time it's caused by a fault in the secondary ignition system, but it can also be caused by incorrect camshaft retard, worn distributor gear and or bushings, low fuel pressure and or volume, or a vacuum leak. The plugs, distributor cap, & rotor, all need to be AC Delco, (aftermarket stuff doesn't cut it on these engines). Wires need to be a good name brand, not a store brand. Fuel pressure and leakdown need to be within specs both at the service port, and at the fuel filter. These are all basic things that must be correct before chasing a P0300.

 

checked the fuel pressure and it's good, You're not the first person to tell me about the AC Delco cap and rotor, and I was skeptical because BWD and Accel have always worked but I guess it's worth a shot. Guess I can pull the coil and have it bench checked too.
Thanks for the help, looks like I have some things to check.

 

https://blazerforum.com/forum/2nd-ge...eakdown-88305/ What were the 4 readings? Bench check on coil is not always conclusive. Better to use a squirt bottle with water and spray a mist over the coil while the engine is running.

 

I only tested at the service port and it was @ 58 PSI.
Never heard of the mist over the coil method, how's that work?

 

Fuel pressure at the service port, key on, engine off, fuel pump running, needs to be 60-66psi and it must remain above 55psi for at least 10 minutes after the pump shuts off. Without checking at the fuel filter outlet also, those numbers are not 100% conclusive for an accurate diagnosis. For example, your reading of 58psi is a problem, but without the other test, you have no idea where or what the problem is.


With the engine running, spray a mist of water over the coil, jazz the throttle a few times and watch for stray sparks. Obviously easiest to see sparks if it dark out.

 

If you have disturbed the crankshaft sensor by removing the crank sensor or the timing cover during repairs, then you almost certainly need to do a crank sensor relearn. That in itself can give you a PO300. But you don't want to do a crank relearn on an engine with a felt miss or a knock. Also know that a doing a crank sensor relearn will not fix a misfire - it just calibrates the misfire detection system.

I have what I consider to be a good way to test the misfire detection function, but it requires a scanner that can read the individual misfire counters as live data. If it seems your engine runs well, but you are getting a PO300 (or any single cylnder misfire code), just induce a dead miss into the engine and read the misfire counters (or even do this for each cylinder in turn). If the PCM can correctly detect a steady miss, then misfire detection appears to be working fine and chances are that a crank sensor relearn will do little for you and that your PO300 is most likely real. If it can't detect the steady induced miss, then know that you need to fix the misfire detection system.

But first things first..... How does the vehicle run? Is there an apparent miss at any speed or a roughness at idle? Are there any knocks in the engine at any speed or temperature? How is the oil pressure at idle hot and with what weight of oil? In other words, does the PO300 appear to be real, or is there a possible problem in the misfire detection system? Excessive bearing clearance can also cause misfire detection issues (sadly I know this very well from my own vehicle). These things are not clear in your original post and the answers are needed to know an initial diagnostic direction.

If miss is a felt one in addition to the PO300, it could be something very simple and easy to fix - or it could be very deep and/or difficult to diagnose. Need to approach in a logical manner such as Captain has described.

 

It runs rough at idle and is idling at around 700 rpm, and once I up the rpm it evens out, with no real miss, but the code stays. Haven't removed the timing cover or touched the crank. Oil pressure is good using 5W30 because of the temp ranges here (single digit in winter to triple digit in summer)

 

OK so misfire and PO300 is real. That is progress.

Try spraying carb cleaner (flammable kind) around the plastic intake plenum while running to ensure the seal is 100%. If idle smooths out, you have found the problem. Seal for spider needs lubricated with engine oil when installing.

How is the distributor gear? This is common problem on the Vortecs. Take cap off and see if you can turn the rotor back and forth (should be zero slop back and forth when holding shaft down so it doesn't ride up on the gear). Do you have the factory non-adjustable distributor hold down? If any felt slop, remove the distributor for inspection and post closeup of the bottom of the gear.

 

inspected the distributor when removed to pull head, gears in remarkably good shape, yes on factory non-adjustable hold down. Need to get more carb cleaner.