Ok we have a 1997 chevy blazer 2dr and have gotten the dreaded p0300 the truck runs has great fuel pressure and it holds the list of whats been replaced is very long it has a flashing ses lite and is missing you can hear and feel it. It gets horrible gas milage you can smell gas after it is run a while. Ok here is the parts list so far.

New MAP sensor
new FPR
cleaned maf
rebuilt spider
new Ac delco Cap and rotor
new Ac delco plugs
new pcv valve
fuel pressure is 63 key on engine off dosn't loose and pressure
new distributor
new CAT
new plug wires
Vacuum holds 20lbs steady and snaps to close to 0 if you snap the throttle.
coil has been tested and is good also good spark at all plugs
ignition control module has been tested and found to be good
swapped PCM

Only other codes that show up after running for a while are p0134 and a couple other o2 codes i cant remember them right now ill add them after i scan it again i don't have a very good code scanner or access to one at the moment.

I cannot find any vacuum leaks or any thing like that.

the compression is about 163-181 across all cylinders the old plugs had a bit of gas on them in cyl 3, 4, 5, and 6 if i remember right been a few days and this truck is making me crazy. any thing else you need to know just ask I'm really at my wits end with this thing about to blow it up or burn it down LOL
thanks for any and all help ya'll can give as this has been a real pita thanks again Tom.

 

In what order did you do all this?

The keys are NOW, IMO, the distributor and the MAF. Pending oyur posting of other codes. You need to see what your actual timing is and if it needs to be set by 'factory authorized full featore scan tool'
{dont get me started!}

Meantime:
Get it to operating temp. Disconnect the MAF connector and the battery neg terminal.

Wait 10 minutes. Drive it and see if it performs better. It will be running off the default fuel/air matrix in limp-in mode.
If it doesnt run better, remove and check your EGR valve.
In fact do that anyway.

 

Ok well i feel like a complete moron now as i took it to a shop that was very nice and reasonable only to find out that some how myself and 3 other (mechanics) didn't relize that the #3 and #5 spark plug wires were swapped not sure when this happened as too many hands been under that hood latley but its now fixed ses is off and runs like never b4 not bad for a $400 dollar truck.

thanks for the help and ill b lurking around if i need help again!!!