1996 C1500 5.7l P0300 code

[jmach96]

I have a p0300 code above 2000 RPM's with all driving conditions including towing.  I encountered the problem 2 years ago during California smog test.  I have searched DIY "fixes" and have diagnosed problems, completed fixes and still have had problems.  The truck, with 149000 miles runs well and gets 13.4 mpg city and 15.6 highway.  it has been taken to two independent shops, with numerous repairs, and no fix, until finally, it is with a Chevrolet dealer in Santa Ana, California.  My biannual smog test is this July and I need some help in understanding this problem and process.  Last smog test I cleared the code and drove the vehicle ~30 miles below 2000rpm and talked with smog tech that kept truck at 1900rpm on dyno for test that met the + or - 100 rpm California requirement.  As a Note compression tests completed at least 2x a year since 7-16 with less than 3lbs difference from first to last with base results of:
CYL#
1=180 3=170 5=168 7=170
2=180 4=170 6=170 8=175
 

The DIY fixes performed 7-16 thru 9-17:
1) Spark balance and spark gap test - replaced cap,rotor,wires,plugs,coil utilizing dialectric grease - with ACDelco brand
2) All 4 O2 sensors based on exhaust analysis per smog tech i.e. higher than average NOx and CO2 - with ACDelco brand from local dealer
3) MAF sensor based on trim codes for injectors which showed lean trim above 1600rpm - with ACDelco brand
4) Fuel pump replacement due to low pressure, less than 55lbs at idle (~53lbs) - with ACDelco brand from local dealer
5) All Engine compartment harness connectors removed, inspected, cleaned with wd40 electrical contact cleaner and all relays tested with ohm meter, cleaned and re-secured
Independent shop repairs 11-16 thru 7-17:
1) Lower/upper intake gasket, spider injectors and fuel pressure regulator due to failed leak down test and wet spark plug #5 cylinder

2) Distributor, spark exciter and spark knock sensor as a "could only be the problem left" explanation by shop
3) Throttle position sensor and egr valve and egr engine temp sensor as a "could only be the problem left" explanation by shop
Chevrolet Dealer 03-18 to present: Note: truck developed an engine knock (diagnosed as piston, piston pin or rod bearing failure), excessive rear main oil leak and check engine light "on" at 2000rpm - customer stated concerns on original service estimat/order
1) New crate long block engine with harmonic balancer and tins (12530283), new spider injectors, crankshaft timing sensor, motor mounts, water pump and thermostat, cap, rotor, wires and plugs.

Good news the knock and oil leak are gone!  Bad news check engine light on above 2000 rpm.  Have taken truck back twice to dealer, first repair was timing adjust, with no fix, now I'm told problem may not be repairable, my option is for Chevrolet Performance to look at truck, diagnose at an expense to me, credited towards cost of any repairs I approve.

What are your thoughts: impression, advice, process, guidance?
 

[Doug_Scott]

This may sound stupid, but you never mentioned it, did you change fuel filter?

You may want try unplugging the brake booster vacuum line and plugging it.  Take truck for drive to see if the light still comes on.  Before driving it on road, try to stop to make yourself aware of how hard it is to stop without vacuum assist.  This is best done without traffic around.  That will test the booster for vacuum leak. 

A partially plugged exhaust can create issues as well. 

When pump was replaced was the inside of the tank checked for crap sitting on the bottom of the tank? 

 

You have replaced everything right up to the wiring harness.  The first error code is random misfire and that usually means the error is in a global system.  Like gross vacuum leak, fuel supply issue, exhaust issue etc.  Something that impacts all cylinders. Check all vacuum lines for decay or oil damage.  I would think replacing the engine would find those though.  Do you have a relative in another state that can buy your truck, then forget to take the truck home with them? After 6 months take a trip to Vegas, spend the night, then have your relative come for a visit again.  You know, wash, rinse, repeat. 

[Jsdirt]

Wow, that thing took a full frontal assault by the parts cannon!

 

Your first problem is your going to a dealer. A vehicle that age has no business in ANY dealership, unless you're related to GM's CEO. The way GM pays techs today, they have no choice but to blast the parts cannon at the vehicle. They don't get paid for any kind of lengthy troubleshooting. They follow the factory checklists (which SUCKS) and don't deviate from that. Had you brought that to a competent shop, it would've been fixed 2 years ago with 1/10th the amount of money you've spent. There's some competent techs at dealerships everywhere ... but GM's stingy flat-rate system doesn't let them shine. It brings out the worst in everyone.

 

You've thrown so many parts at that thing, you now have to suspect EVERYTHING. New parts today are just as bad as 20 year old used parts these days. To fix that, you're going to need someone who knows their stuff. Ask around your neighborhood - find out who the go-to guy is, and go there! STAY AWAY from dealerships!! 

 

 

[Yjmoose]

Have you checked for play in the distributor drive gear?  My 98 k1500 with 77,000 miles was worn out. When I pulled the distributor you could slice your finger in the drive gear it was worn down so bad.