Hi Guys,

I am having some major issues regarding my 1998 blazer 4.3L 4x4.

Here are the engine codes I have gotten recently.

PO300 - Engine misfire random or multiple
PO304 - Misfire cylinder 4
PO305 - Misfire Cylinder 5
PO101 - MAF or VAF sensor performance problem
PO442 - EVAP leak detected (small)
PO446 - Evap emission control system vent control circuit
PO453 - Evap emission control system high input

While I drive on the highway after 5 minutes or so at about 110km/hr in 4th gear (i assume) at 2100RPM I can feel my truck studder and it is hard to maintain that speed and my engine light is flashing. It only happens when I am on the highway.

The other codes have come and gone all while doing normal city driving or in conjunction with the highway speeds. Usually I will get a mix of them.

Recently I have changed my cat, replaced my plugs, wires, cap and rotor and the O2 sensor just after the cat as well as a new gas cap. I took out the MAF sensor and cleaned it out as well as looked at the EGR valve and it looks good.
I have also recently changed my fuel pump but replaced it what seems to be correct (had to change the wiring harness due to the new and improved ones that NAPA sells.

Ya its been a good year for my truck.

Any ideas guys what could be causing this. Is it all caused by one simple problem (i really hope so cause my gas milage is horrible).

Thanks a mil

Mike