OK so, I've read through every P0300 thread the search will bring up on this forum and I haven't found any one that's showing the same symptoms as mine, so I turn it over to you fine gentlemen.

I have a 2001 Blazer 4dr. 4x4 LS, on Saturdays I go to work early (4 am est.) For the past two Saturdays the SES light has come on while cruising down the highway, 70-75 mph. But it's gone off on the way home in the afternoons.

There are absolutely no symptoms of a misfire, while the light is on, it runs just as smooth as the day I bought it, no stumbling, no lurching, no jerking, no rough idle, there's absolutely no difference in how it runs between when the SES light is on and when it's off.

The plugs and wires are all fairly new, I know that doesn't completely rule them out, but I'm thinking it has something to do with the cooler air in the mornings maybe? The early morning air is maybe in high 50's while the afternoons are in the mid to high 70's, maybe a vacuum leak?

I know this is getting long but would you guys even worry about it if it keeps going off and shows no symptoms, or where would you start?

 

You can of course leave a misfire code forever if you want, but depending on the issue you might be hurting performance or fuel mileage.

You are on the right track. With a misfire you always start with the easy stuff. Cap, Rotor, Plugs, Wires. Don't trust these because they are "fairly new", sometimes you just get some that aren't very good.

It's also easy enough to clean the EGR valve, but you'll want to pickup a <$10 gasket for it. 2nd Gen S-Series EGR Valve Cleaning

You can also pull the intake off and clean any carbon build up in the throttle body and of course make sure your air filter is clean.

It fairly easy to check to see if you have a vacuum leak. You might be able to borrow a vacuum tester from your local parts store, or in a pinch you can buy one, they aren't too expensive. Harbor Fright probably has a cheapo Chinese one that will work in a pinch.

Beyond that, you'll want to do a leak down test. Again, you can probably borrow a tester, or buy one, Harbor Freight probably has a cheapo one of those too I'm sure.

 

I'll definetely check what you mentioned, thanks for the tutorial on cleaning the EGR valve

From what I've read this is a pretty persistent code, is there any reason why it would come and go? And am I actually misfiring if I can't technically feel it and it's not showing the tell-tale symptoms of a misfire?

 

if its during cooler air, maybe a condensation issue...crack dist cap?
double check wires for good connection..

 

Have a read through the OBD2 diagnostic thread linked to below as well as the thread which is linked to at the bottom of the diagnostic thread. It'll take you a while, but until you have gone through all of the suggestions found in those two threads and let us know the outcome of each of them, there are a ton of issues that could cause your condition.

P030x - Random/Multiple Cylinder Misfire Detected