My 98 blazer suddenly started idling very rough , I checked the pcv valve and noticed someone had replaced the rubber on the tube with a boot from a "Spark Plug Wire" ! I'm not sure that's the problem but may check engine light flashes as I accelerate. What's going on? HELP!

 

you have a misfire most of the time this is it, on one of your cylinders Could be a bad wire, arcing spark from wire to metal. Bad plug. other possibilities, cap and rotor. bad valve or bad injector. Check your plugs first, make sure wires are on correctly. To check for arcing run truck at night or in dark area with no underhood light and look for a spark around the plug or wire. Do this stuff first, Cheap and easy

 

You have a misfire causing the roughness and check engine light to flash. Is this the same Blazer that has a new engine? Probably an ignition problem (plug wire, spark plug, etc,) If someone just did major work on your vehicle and you paid them - then they should fix it gratis. Hope this helps. If you need more detail to work on it yourself, please advise and give much more details about what was done to it - but be warned that I'm going to insist that you get a OBD2 scan tool to read live data.