I have a 2003 chevy blazer with a 4.3L. The car drives perfectly fine. Oil pressure and coolant temp have always been within spec and there's plenty of power. The car has had a p0300 code for some time now. In the process of trying to fix this I've replaced plugs, wires, coil, cap and rotor with brand name parts. Checked for vacuum leaks and replaced some worn vacuum lines. A compression test was done and all cylinders ranged between 170 and 185 psi. Fuel injector cleaner was put in the tank but no change. I am confident that this is not a true misfire. You cannot feel any hesitation or jerking or anything I would associate with a true misfire. Even took the vehicle to a certified mechanic for a cam relearn. First, the car would not take a relearn even when revving up to 5000 rpm. The mechanic then proceeded to take the car for a drive and reported that he also could not feel a misfire. Trust me, this vehicle drives perfectly fine and gets the factory reported fuel mileage. A misfire counter was used and there was roughly 11,000 misfires on #4 and roughly 1400 on #1 after only a few hours of driving. Fuel trim was also looked at and the mechanic said it was not reminiscent of a bad injector or something. I don't know the specifics of fuel trim but I was told overall trim was well within an acceptable range of plus or minus 10. I'm trying to get this car to pass inspection and the light is my only real issue. Any help you can provide would be much appreciated.

 

First you should make sure that CMP retard is within spec (+/- 2 degrees) to reduce the chances of false misfire reporting.

George

 

Finally got the bluetooth obd scanner in today. I checked the cmp on dash command and it was showing a reading of -3.4 degrees. I understand that this is outside of that + or - 2 but is that extra 1.4 degrees a big deal? It seems like the only means of adjustment are to elongate the hole in the distributor clamp.

 

That's pretty close to the spec, I doubt that has anything to do with your problem. That said, if you want to get it within spec then you have two choices: elongate the hold down hole or replace the hold down with an adjustable one which I believe is around $5 on Amazon.

George

 

Elongated the hole this morning and got it within spec. SES light initially went out after making that change but came back on after about an hour of driving. My next guess is checking the fuel injectors.

 

OK, that takes this factor off the table. A P300 code with no felt misfire can be an adventure. The question is: real or falsely reported? The engine computer is perceiving a slight change in crankshaft rotation speed and concluding a misfire condition. The process is actually fairly complicated. Some data is filtered, the TCC is manipulated and the ABS is involved as well as the crank and cam sensors, the powers, grounds and wiring for all of these systems. Especially if the misfire cannot be felt then intermittent operation of any of these areas is possible, especially on an older truck. If the misfire is real, even if not perceived the possibilities are endless:

1. Ignition circuit powers, grounds, wiring
2. ICM, coil, distr cap, wires, plugs, wire routing, plugs, ignition leakage
3. Fuel pressure, fuel quality, injectors, injector power, grounds, wiring
4. Engine mechanical problems
5. O2 sensors
6. Vacuum leaks
7. PCM power, ground
8. Exhaust restrictions
9. .......

The question is where to begin? Do you have any other codes or any repair history preceeding the issue? If not then I would start by ruling out the easier stuff first before digging into fuel injectors and other labor and cost intensive steps. Lets start with:

1. All connectors to the coil and ICM clean and tight. Power and grounds good.
2. All ignition wires routed properly and firmly connected at both ends
3. Distr cap on straight and tight with no carbon tracing under the cap
4. Rotor in good shape
5. All ignition components AC Delco
6. Mist the ignition in the dark while running and look for arcing

If all of that is good then swap the ignition wires and plugs between the worse cylinder and the best cylinder and see if the problem follows. If not, check the resistance of each ignition wire.

Lets stop there for now. If this brings no joy then we move on.

George

 

When you checked the CMP retard, did you bring the engine up to about 1200 rpm? If not, suggest doing it again. If you can look at the CMP retard in real-time with the engine at around 1200 rpm.
I have found that if over by +/- 1° will start the misfire counts and set the CEL after a few minutes. At least that is what my '02 will do.

 

Had same EXACT problem. P0301. '02 LS 4.3 4X4. I finally gave up and just lived with the Service Engine Soon light.

 

Recently I addressed a P0455 and P0449. Swapped out the purge valve by the gas tank and replaced some lines going to the vapor canister. I haven't seen these codes since making those changes. I've swapped out the plugs and wires multiple times yet I still see steady misfire counts on cylinders 1 and 4 (4 having a considerably larger amount of them).The ignition components have been checked a few times already (cap, rotor, coil, wires, plugs) and are all quality brands. This past winter I replaced the muffler and rear portion of exhaust pipe. There was a pretty large exhaust leak prior to doing this. Idk if that could impact the O2 sensors or not. I've checked for vacuum leaks pretty thoroughly by the base of the throttle body, intake manifold, and the vacuum lines about the engine bay. I even replaced the purge solenoid above the #4 cylinder just in case. I had checked the cmp at idle after bringing rpms up to 2000 and even during driving scenarios. It never once moved outside of that + or - 2 degrees. I can work on getting it closer to 0. And I can live with the ses light but I need to get it inspected in New Jersey. I have also recently replaced the fuel filter on the frame and have run lucas oil fuel system cleaner through it a few times now. By accident I left the gas cap off a few weeks ago and during this time the SES light would flash occasionally but would turn off. My scanner didn't always pick up the P0300 while driving like this either. After replacing the gas cap the light became steady again. Could this indicate something?

 

It is best to use AC Delco for all ignition parts and fuel pumps. For your next steps:

1. Mist the ignition in the dark and look for arcing
2. Measure the resistance of the plug wires
3. Measure the quality of the power and ground to the coil and ICM

Can you graph O2 sensors and measure fuel trims?

Has the crank sensor been replaced without a relearn?

How about the cam sensor?


George