I've got a 99 Blazer, 4Dr with about 240k on it. I have had an ongoing problem for months that I'm trying to figure out. When I'm at highway speeds and going up and sort of incline causing the motor to have more of a load than normal, I'll start misfiring and the motor will kinda bog out like it's running out of gas. Chekc engine light flashes as well. Its never died from it and usually goes away if I pull over and sit a few minutes but it's my only vehicle and worries me that it might get worse. I take very good care of my Blazer and my father is a mechanic so it's always got the work it needs on time. I also use full synthetic

So far I've replaced the following:
1. Crank shaft sensor
2. Ignition Module
3. Plugs
4. Distributor, Cap and Rotor
5. Fuel injection system
6. Mass airflow sensor is getting done today
7. Oil change and synthetic

Anyone think they've had this problem and solved? The blazer runs amazing otherwise. And only has the problem around 55+ mph with a larger than normal load on the motor

 

I assume the fuel pump and filter was included in "fuel injection system" replacement?

What all was changed with that?

It sounds like you have a barely adequate fuel supply for normal cruising but it reaches it limits of fuel delivery once an extra load is put on the engine.

I had a still functioning but weak fuel pump cause all kinds of drive-ability problems that would come and go depending on conditions.

Also, a leaking spider injector will starve some cylinders and richen up others depending on where the leaking fuel is going.

 

It was the Spider Injectors I did last week. Didn't just do the fuel pump or filter but they've been done in the past year for sure, I'll have to check them out. I'll also mention I inky get 15mpg highway so it really seems like it's something fuel related. I use to get 20 highway

 

Check fuel pressure and leak down.
Next check cam retard.
There is also a GM TSB for P0300 under load and it has to do with a sticking valve.