dtrader
Newbie
I had a 96 c1500 with the same P0300 code for over 2 years at about 144k miles. Due to bi-annual smog checks in California I needed to get this fixed if truck was to stay on road. So, during this time in preparation for the smog test, I replaced all ignition parts, yes all, all fuel parts, yes all including rubber fuel hoses, all drive-ability issue sensors, yes all including smog, O2, vapor recovery and position sensors. I then replaced the motor with a Chevy Performance L31 crate engine. My local dealer spent a month and $1000 of my money to diagnose the P0300 problem after new engine was installed that included, crank position resets, camshaft offset resets and GM tech support recommending adding ground wires to the PCM, heads and block from frame, increase ground wire gauge from battery to frame and firewall and finally testing back pressure of exhaust claiming I had too little and replaced my muffler with a stock version. Nothing fixed the problem.
So, I decided to open the engine compartment harness, isolate the injector wires from PCM and disconnect each wire at PCM to do a balance test. This is when I discovers cylinders 5,6, and sometimes 8 offered intermittent differences in idle rpm or during 0 to 60 time trials when pulled individually from the PCM connector. The variances were like my golf game, all over the range of possibilities.
I checked all the injector wires and found them to be similar to the others in resistance, about +/- 2 ohms for average at 18 ohms when isolated (injector distribution cap to PCM connector).
I pulled the PCM and sent it to an electrical shop for test. It was returned with no issues found in the bench test, although I found out during inquiries, many of the functions during the test were shorted or crossed with resistance jumpers to "bench test" the PCM so a true full function test is not possible. I then for the sake of commitment and stubborn determination to find the problem, I ordered a "rebuilt" PCM that was subject to same short comings on testing as my original PCM had, and got 2 used PCM's from a junk yard with no change in P0300 code behavior .............
On my return to the junkyard to lament my problem, the counter sales guy suggested I use a PCM from a 2002 Chevrolet Express Van (411). The shop had a guy for $150 that would burn my VIN into PCM ROM, delete the VAT (Vehicle Anti Theft), gas tank vapor recovery pressure switch and alternator codes. I paid $200 for the 411 PCM and $50 for a wiring conversion spreadsheet. During install I completed an EGR work around for power to the EGR valve and finally added wires behind the dash cluster to tell the new PCM what position the shifter was in - P,R,N,D,OD.
Finally the P0300 code was fixed, the truck ran like it was when I bought it new in 1996, and passed California smog with better numbers than the first smog test in 1998!
My only regret is not having the Tow/Haul option on new PCM turned on before the VIN was burned, since it seems the PCM is now "locked" and the original tuner does not know how or does not have tools to unlock PCM for further tuning.
It is a real statement of California's intention of "smog" mandates when if the smog tech wanted to or knew the PCM was changed he could fail my truck and C.A.R.B. (California Air Resources Board) does not offer exceptions for PCM year model replacements, even though the truck is running cleaner than when sold by GM; a constant fear of smog test failure I will endure every two years.
Sorry for the long thread, I tried to be as concise as possible.
So, I decided to open the engine compartment harness, isolate the injector wires from PCM and disconnect each wire at PCM to do a balance test. This is when I discovers cylinders 5,6, and sometimes 8 offered intermittent differences in idle rpm or during 0 to 60 time trials when pulled individually from the PCM connector. The variances were like my golf game, all over the range of possibilities.
I checked all the injector wires and found them to be similar to the others in resistance, about +/- 2 ohms for average at 18 ohms when isolated (injector distribution cap to PCM connector).
I pulled the PCM and sent it to an electrical shop for test. It was returned with no issues found in the bench test, although I found out during inquiries, many of the functions during the test were shorted or crossed with resistance jumpers to "bench test" the PCM so a true full function test is not possible. I then for the sake of commitment and stubborn determination to find the problem, I ordered a "rebuilt" PCM that was subject to same short comings on testing as my original PCM had, and got 2 used PCM's from a junk yard with no change in P0300 code behavior .............
On my return to the junkyard to lament my problem, the counter sales guy suggested I use a PCM from a 2002 Chevrolet Express Van (411). The shop had a guy for $150 that would burn my VIN into PCM ROM, delete the VAT (Vehicle Anti Theft), gas tank vapor recovery pressure switch and alternator codes. I paid $200 for the 411 PCM and $50 for a wiring conversion spreadsheet. During install I completed an EGR work around for power to the EGR valve and finally added wires behind the dash cluster to tell the new PCM what position the shifter was in - P,R,N,D,OD.
Finally the P0300 code was fixed, the truck ran like it was when I bought it new in 1996, and passed California smog with better numbers than the first smog test in 1998!
My only regret is not having the Tow/Haul option on new PCM turned on before the VIN was burned, since it seems the PCM is now "locked" and the original tuner does not know how or does not have tools to unlock PCM for further tuning.
It is a real statement of California's intention of "smog" mandates when if the smog tech wanted to or knew the PCM was changed he could fail my truck and C.A.R.B. (California Air Resources Board) does not offer exceptions for PCM year model replacements, even though the truck is running cleaner than when sold by GM; a constant fear of smog test failure I will endure every two years.
Sorry for the long thread, I tried to be as concise as possible.
