All,
This is my first post here so please be gentle. I have searched around this forum and several Google searches to resolve my issue and have followed several recommendations to no avail. I have a 2000 LS Blazer 5speed 102K(Yes I know hard to find but in Colorado where I bought it that is all they make dealer said automatics were custom order). I have the dreaded P0300 code on and off for several years now. This all started after I had a tune-up and several other routine things done by a private shop. I refused then to take it back to them after I got the truck and they actually cut the plastic molding in the tire wells to get to the bottom plugs. I have taken it to 3 different Chevrolet dealers as well as 2 different private mechanic shops with no resolution other than an empty wallet!
The code only comes when it is humid or rainy. It will run horrible (like a bucking bronco not a blazer) for a good 20-30min between 1200 to 2500 rpm’s under normal load. Hard acceleration it is fine till I go to shift. Nice sunny days I have no issues.
The 2nd dealer was dead set on the injector being the culprit and I replaced it with the new spider MFI. Both private shops said that they can’t get the computer to sync with their Snap-On equipment to read correctly. Seems odd that 2 different mechanics said the same thing without me saying anything to them, while dealer never said a word.
I have recently done a complete tune up:
· Plugs
· Wires
· Cap
· Rotor
· Ignition coil and module
There was a crack on the coil so I replaced them both (solved my starting troubles in the rain!). Plugs are gaped to specs as well. I found that the original shop didn’t use any dielectric grease when they did everything so I replaced it all and put grease where it was needed. I found that the rotor was getting moisture and that the rotor was actually rusted. I have also noticed that there is a small hole in catalytic converter before the tail pipe on the top as there was a good amount of water leaking from it after startup the other day.
I don’t have all the high tech mechanics tools but consider myself pretty handy (just completed heater core replacement myself) and am looking for some options as where to go from here before I do more damage to it from driving those first 30 minutes. I wish I could take a video of how it bucks feels like the whole top of the truck is shifting on the frame.

 

as far as your code p0300 is a misfire but the rest of the problem i cant help you on have you searched gen2 tec help in mechanical and maintenance section

 

Remove the distributor cap and rotor. On the bottom plate of the distributor there should be a small vent area that sometimes gets debris over it allowing moisture to get trapped in the distributor. Make sure it is cleaned off and perhaps take a hair dryer to dry out any residual moisture. Ascertain that the cap is put back on tightly and see if that helps your problem.