Originally Posted by CitationX

Thanks hanky. I will check mode 6 data as well. Looks like I should look at A2-A9(TID 0C)

Found this GM site containing Mode 6 specs and diagnostic parameters that might be helpful for others.
https://gsi.ext.gm.com/gmspo/mode6/index.html

Originally Posted by CitationX

Hello everyone,

New member here. I shamelessly signed up to get some help with an engine problem that stumped me. I wanted to see anyone with experience could give me some advice on this.

2004 Chevy Express Van 6L V8. I can feel misfires, and P0300 DTC. Just P0300 without specific cylinder number. I know this can be a tricky one.. I guess I am lucky. The vehicle was unregistered and sat for a year - shame. Cleared out old fuels and fresh fuel in.

Work done so far-
Identified bad spark plugs for two cylinders and replaced.(ACDelco) All fuel injectors(GM) replaced. After reset, for 250+ miles no issues, no pending codes, but Catalyst and EVAP Monitor wouldn't set after several drive cycles. Then, one morning, P0300 DTC showed pending, then triggered the P0300 and Engine Check light.

Observations at idle

• Upstream O2 Sensors voltages look good and fluctuating. i.e. 0.285, 0.085
• MAF sensor reads 0.02 lb/s
• Short Term Fuel Trim looks good fluctuating between +-3% on bank 1, -4% on bank 2
• Long Term Fuel Trim high : 12.5% on bank 1, 16.5 on bank 2
• Haven't done vacuum test yet.
• Haven't done fuel pressure test yet
• Hard start sometimes
• P0300 DTC set and MIL lit

I was leaning toward the fuel system.. Fuel pressure low due to maybe the fuel filter being obstructed by sitting for a year, ECM compensating with trim. I guess it could be a vacuum leak too considering trim? If anyone has any pointers or ideas, I would appreciate it very much! Thanks!

Originally Posted by CitationX

Hello everyone,

New member here. I shamelessly signed up to get some help with an engine problem that stumped me. I wanted to see anyone with experience could give me some advice on this.

2004 Chevy Express Van 6L V8. I can feel misfires, and P0300 DTC. Just P0300 without specific cylinder number. I know this can be a tricky one.. I guess I am lucky. The vehicle was unregistered and sat for a year - shame. Cleared out old fuels and fresh fuel in.

Work done so far-
Identified bad spark plugs for two cylinders and replaced.(ACDelco) All fuel injectors(GM) replaced. After reset, for 250+ miles no issues, no pending codes, but Catalyst and EVAP Monitor wouldn't set after several drive cycles. Then, one morning, P0300 DTC showed pending, then triggered the P0300 and Engine Check light.

Observations at idle

• Upstream O2 Sensors voltages look good and fluctuating. i.e. 0.285, 0.085
• MAF sensor reads 0.02 lb/s
• Short Term Fuel Trim looks good fluctuating between +-3% on bank 1, -4% on bank 2
• Long Term Fuel Trim high : 12.5% on bank 1, 16.5 on bank 2
• Haven't done vacuum test yet.
• Haven't done fuel pressure test yet
• Hard start sometimes

1. P0300 DTC set and MIL lit

I was leaning toward the fuel system.. Fuel pressure low due to maybe the fuel filter being obstructed by sitting for a year, ECM compensating with trim. I guess it could be a vacuum leak too considering trim? If anyone has any pointers or ideas, I would appreciate it very much! Thanks!