P0300 is almost always caused by a fault in secondary ignition. Spark plug wires, plugs, cap & rotor, and ignition coil are the common items, but worn distributor bushings and or drive gear can also play a part, along with low fuel pressure. However, crankshaft position sensor relearn data and camshaft retard must be correct for the PCM to accurately identify which cylinder(s) are misfiring. If the wires haven't been replaced: without using expensive diagnostic equipment to test them under a load, it's far less expensive just to replace them.

 

I went ahead an bought the plug wires you recommended this afternoon. After installing them I cleared the codes to see what happens next. My slight rough idle did not improve but I don't have a misfire code, yet.

My next step is to do a fuel pressure test. Hopefully this will confirm my bad spider theory. I just can't wait to get this truck running right.

 

FINALLY!

I was able to run a fuel pressure test a few times today (thanks to Captain Hook's guidance) and here are my results: initial test results showed 58 psi. Five minutes later, I did the same test and it showed me 58 psi, again. Next, I did an excessive leakdown test. First results gave me 14 psi after ten minutes! So, I did it again five minutes later and I got 12 psi this time around! In fact, is was only about 2 seconds into each test when I noticed the needle dropping quickly.

Well, I got to thinking about this and removed spark plug #5 to see if I would get another strong fuel smell after removing the plug. Guess what?! The plug was clean and no gas smell this time. I checked a few other plugs and they were in the same shape as #5 - clean.

So, do you guys think it's my fuel pump or spider assembly?

Next, I'm going to try an diagnose the excessive leakdown by installing a shut off valve between the intake port and the fuel tank (thank again to the Captain's help on this one).

 

You need to pressurize the system and immediately isolate the pump from the plenum, (with the shut off valve) and watch for leakdown. If the pressure tester is connected to the service port, the pressure will drop if the leak is in the plenum. If it does not drop, the leak is in the tank. If the shut off tool is set up like this one:



The pressure will drop if the leak is in the tank.

 

Okay, I haven't isolated the leak yet, but I was looking into fuel pumps this morning (I want to have the right info, if I find out it's the pump). I came across this unit on Amazon.


Again, I have a 2 door model and just about every unit they sell is for a 4 door Blazer. This appears to be the right unit I would need but I wanted everyone's input. It also looks like it has the new flat plug design - which is nice! What do you guys think?

 

That's the correct pump for a 2 door. Confirm what's leaking before you replace it though. If the leak is in the tank, it could be a leaky check valve, which means replace the pump. Or it could just be a leaky connection from the pump to the module, which is repairable without replacing the pump. The point of DIY is to save money, correct diagnosis is absolutely necessary, if you want to save money

 

I understand what you are say. But I just wanted somone to confirm my findings. I still plan on checking the two possible scenarios, either the leak is in the fuel tank or in the intake manifold.

 

Update:

This afternoon, I was able to isolate my problem a bit further. After connecting my fuel pressure gauge directly to the fuel filter, I was able to conduct another leak down test on the pump itself. Initial pump priming indicated 58 (+/-1) psi. And, within 5 minutes, the pressure had dropped to 45 psi.

So, at least I know there is something wrong within the fuel tank itself. There could also be a leak in the intake plenum but without having the right fittings (i.e., a shut off valve) I could not test it. But, I pulled the spark plugs in question, again, and they still look pretty clean. All of this and still no spark plug misfire code - which is good!

If it's just the fuel pump, I'm speculating here, would this explain the slightly rough idle but smooth acceleration? The truck drives fine and my mileage is averaging 16.5 mpg (typically city driving) which is about normal.

 

Fuel pump maximum output pressure, and pump leakdown, (which is what you just tested), must be 73psi to 108psi while the pump is running. Pressure must remain above 55psi for at least 10 minutes after the pump shuts off. Both parts of the test failed, which means it's time for a new AC Delco or Delphi fuel pump.

EDIT: Low fuel pressure and excessive leakdown will cause all kinds of driveability problems.

 

Yes! You don't know how good it feels to finally get to this point (then again, you probably do).

Thanks again Captain!