2000 Zr2 Blazer auto 135k. Last Saturday afternoon I filled my gas tank at a different gas station than usual drove about twenty miles then parked. Monday morning on the way to work it started misfiring, it was 5 degrees below zero outside. It kind of felt like the transmission was slipping. The check engine light came on and started flashing. I drove it home Monday after work and was showing a P0300, P0301 and P0302. I noticed that the lights were flickering with the misfire. I checked out the wires, cap and rotor first which all seemed fine. Eventually P0300 was the only code showing. I pulled the spark plugs to check them. Swapped out the ignition coil. Cleaned MAF sensor. checked crank sensor, camshaft sensor, all four O2 sensors, coolant temp sensor, MAP sensor, Idle air control valve, throttle position sensor, Intake air sensor, all vacuum lines and even used an unlit propane torch to check for vacuum leaks at the upper intake and throttle body. all with no luck.. I couldn't understand why the PCM couldn't identify the problem so I bought a fuel pressure gauge. my pressure was 60lbs and it held. I dumped the fuel out of the gage in the snow and it looked green. so thinking bad gas... I drained my fuel tank and refilled it with fresh higher octane gas. same problem. I was stumped because it only misfired at idle or when I drove it and the transmission up shifted and the RPM's dropped low. now a week later I'm still stumped. so I decided to check my battery 12.3 volts! so I removed it and charged it up to 14.4 volts as soon as charging stopped it dropped back down to 12.3 volts within a few seconds. so I thought about how the cold kills batteries, and how car batteries have low voltage(pressure) and high amperage(current). The Haynes manual says a fully charged battery should hold at least 12.5 volts. this lead me back to my cap and rotor which looked fine to me. and I realized my low battery is not providing enough pressure to force the arc from my very slightly worn rotor to my very slightly worn cap, except when I step on it and force the current to jump. so I ordered a new acdelco set and she purrs like a kitten! and when I get the money the battery is going next!




I hope this can help someone else. I searched through hundreds of threads on different sites and most people don't ever return to say what their problem actually was.


also at one point the motor wouldn't start at all and I flooded it eventually by trying to start it. I had to pull the fuel pump relay and use jumper cables and my wife's car to turn the motor over long enough till it stopped farting then I knew the excess fuel was out. I didn't bother with a compression test because I just did my head gaskets six months ago, and I knew the gas had washed the oil out of my cylinders because I pulled the dipstick and smelled the gas. so I knew I wasn't going to get a good compression test. I had also ruled out a mechanical timing problem by turning the crankshaft with a 16mm socket and breaker bar while my cell phone was recoding the movement of the distributor rotor.(there was no slack or play) I checked my distributor shaft for any side to side play.


I read somewhere that if the catalytic converter starts getting plugged up it will cause this problem too.. so if it comes back after I replace my battery I'll have to cut the cat out and have a look!

 

De ja Vu for me.....however I have something to add. I tried aftermarket caps and rotors, 3 different ones, still got P0300. Replaced with AC Delco brand and is perfect now.

 

That is why it is absolutely necessary to use AC Delco ingnition parts on these engines. Aftermarket parts have slight differences in them that just prevent them from working properly. They are i believe not allowed to make them exact copies of the OEM parts. That is why there are differences.
when i bought my new V20 pick up truck back in 87 it took a year before you could even by ignition parts on the open market without going to a dealership. It even took Standard Ignition that long to get licensed to make the parts.