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2003 GMC Sonoma 4.3l 
93k miles 
P0300 random misfire nightmare 
Runs a little better after new spider injector assembly but still has random misfire code.
Blinking check engine light goes off at higher rpm’s. 
Runs real smooth over 1500 rpm’s. 
Lower rpm’s get random misfire code and CAT starts stinking, motor bounces a little and can feel misfire but it’s a lot less now. 
Get a backfire when putting in gear (reverse or drive) and even in park when rpm’s go below 1000 and motor shakes and hesitates a second then holds idle. Doesn’t die below 1500 rpm’s now and holds idle. 
Slight erratic, intermittent idle due to misfire. 
New LIM, UIM, spider assembly with fuel pressure regulator, PCV, oil pressure sending unit, fuel filter, NGK plugs, wires, Lucas in tank, oil and filter, radiator hoses, oil coolant lines, battery, air filter, alternator, starter, ignition coil, cleaned MAF sensor with MAF cleaner. 
Unplugged MAF while running. Made no difference.
MAP sensor connector has appropriate voltage. 
Took out 02 sensors first and post Cat and it ran a little better. Couldn’t get middle on off. Tried for an hour, used PB Blaster. Friends scanner showed 02 sensors okay. 
Will checked fuel pressure again since new spider. Before fuel pressure would drop 5 psi when rev up rpm’s and increase at idle. 
Pressure didn’t drop when I turned it off. 
Something drains my (brand new) battery if I leave it plugged in so I recharged it before doing any work/tests on it. 
Had friend bring Snapon scanner... showed #2 spider injector not working. So replaced assembly. 
Crank or cam sensors?? 
Any help is appreciated! ?
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  • 3 weeks later...

Just thinking if you removed the o2 sensors and ran a bit better, could be plugged exhaust i.e. catalytic converter. I am assuming you ran the with the o2 sensor ports open. Had similar issues with my 03 Sonoma 4.3 vin x.  I performed a tune up plugs, wires, rotor and cap and still had no power.  pulled one o2 sensor and truck idled and ran much better.  then replaced cat and truck runs very well but still have leaking spider.  started with codes P0101,P300,P420 AND P0151.  All codes now cleared but o2 sensors are indicating constant rich condition.  performed Fuel pressure check and will not hold pressure after power shut down.  and also occasionally hard to start after hot soak.  so my next step is spider replacement. my truck has 112k miles

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