Chevrolet Repair: 1995 Blazer, fuel pressure gauge, mass air flow

Question
I have a 1995 Chevy Blazer 4.3 vortec motor.125000 miles.I have been having a hard start and a rough idle.I did a tune up consisting of plugs,wires,cap,pcv,fuel filter and air filter. Did not fix. Had a Dio. check done codes were p0300 and 172. Said random miss fire,vac leak and others. I changed my spider injectors gasket and o2 sensors. before and after the cat.It still runs rough at Idle and hard to start. I am at a loss I dont know what to actually do next.I doubled checked firing order.Also had a GM mech. look at everything I have done and said what I did was right. But still did not fix .(also fuel pressure is 53lbs but drops. should it drop truck not running) Please give me some Ideas

Answer
HI DAVID, THE FUEL PRESSURE WILL DROP SLIGHTLY WHEN YOU TURN IT OFF. HOWEVER IT SHOULD NOT DROP DOWN TO ZERO..IF SO ONE OR MORE INJECTORS ARE LEAKING OR THE FUEL PUMP REGULATOR IS FAULTY..PRESSURE SHOULD BE IN THE MAIN LINE TO LET THE ENG START UP AFTER SITTING. IF NO PRESSURE IS THERE, THIS WILL CREATE LONG CRANK TIMES BEFORE IT STARTS AND MORE THEN LIKELY THE RICH CONDITION AND SKIP...PO 300 CODE IS THE RANDOW MISFIRE WHICH COULD BE CAUSED BY MANY THINGS.. ONE IS A STICKY INJ,NOT SEATING AND CLOSING AS IT SHOULD..THE WAY YOU HAVE EXPLAINED THIS THIS SOUNDS LIKE YOU ARE LEAKING FUEL..LONG CRANK AND ROUGH IDLE. THIS TYPE OF INJECTORS HAVE HAD MANY PROBLEMS !!!! GET A FUEL PRESSURE GAUGE AND WATCH WHAT HAPPENS WHEN YOU TURN OFF THE ENG..I SUSPECT THE PRESSURE WILL DROP OFF. IF SO TIME FOR NEW INJECTOR UNIT..STOP HERE IF YOU FOUND YOUR PROBLEM ABOVE.... THE CODE PO 172 IS A RICH CONDITION WHICH MAY CAUSE THE MISFIRE CODE..GET YOURSELF SOME CARBURETOR CLEANER AND SPARY ALL AROUND THE INTAKE AREA AND THROTTLE BODY AREA.SEE IF THE ENG SMOOTHES OUT.IF SO, WHERE YOU SPRAYED HAS A VACUUM LEAK THIS WILL TRY TO RICHEN THE MIXTURE UP..FIX THIS  AREA, COULD BE A BAD INTAKE GASKET, THROTTLE BODY GASKET OR A VACUUM LINE SPLIT...BUT LETS SAY YOU DO NOT HAVE ANY VACUUM LEAKS..CK YOUR MASS AIR FLOW FOR ANY DIRT ON IT OR ANYTHING FOREIGN PAST THE MASS AIR FLOW IN THE DUCT WORK..IF ANYTHING THERE REMOVE IT..ALSO GET SOME MASS AIR FLOW CLEANER AND CLEAN THE MASS AIR FLOW OUT...IF THIS DOES NOT WORK YOU MAY HAVE A FAULTY MASS AIR FLOW SENSOR...YOU COULD DO A LIVE DATA TEST WITH A SCAN TOOL TO CK THE OPERATION OF THE MASS AIR FLOW IF YOU DO NOT WANT TO GUESS AT THIS..GOOD LUCK TO YOU AND LET ME KNOW HOW THIS GOES,,,TOM..