I have a 2002 blazer that just recently started throwing a P0300 code at me. I cleared the code to see if it was indeed just a random misfire and drove it all day around surface streets with no engine light. However, when I hit the freeway and passed 70mph the light came back on. I am aware of the multitude of problems that could be causing this, but I am pretty new at working on vehicles and was wondering where I should begin. What should I check first?

 

Just because the light appeared after you hit 70mph doesn't mean that's when the code only happens. CEL will only light when it fails certain criteria in a designated amount of time or revolutions, and it needs to fail this certain criteria 3 times in row in so much time. Pending code could've been in the system regardless if the CEL is on or not.

Point is, the only way to really tell when the code is being produced is to view the freeze data. A misfire can be caused by failing spark plugs, ignition wires, cap and rotor, ignition coil, or crankshaft position sensor.

My advice is to first start out with the most basic.... pulling out spark plugs, see what condition they're in. Cap and rotor is also easy to unscrew and look underneath checking the points on cap and on rotor. Good luck and let us know how you make out.

 

I too was throwing p0300 @70+mph.I did a full tune up (acdelco is a must) cap rotor wires plugs cleaned EGR and MAF sensors. I still throw a code ay highway speeds but its now only a cylinder 5 misfire. Try thise things first. It might narrow it down to specific cylinder or get rid of misfire all together. This forum is equiped with great threads search around. Good luck and let us know how youmake out
Shane