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!

mountainmanjoe

Sounds to me like you're on the right track. Insufficient fuel pressure or vacuum leak, that order, are the most likely culprits.

CitationX

Thank you mountainmanjoe.
I don't have the pressure gauge, but perhaps replace the fuel filter and report back the findings. It needs a new one based on mileage anyway.

mountainmanjoe

you won't regret getting a pressure gauge it's a useful thing to have. You can use it for testing the fuel pump, compression testing the cylinders , and other things. I made one for $15.

Another quick test you can try is spraying starting fluid around the intake and manifolds. If the RPMs pick up you have a vacuum leak.

hanky

If you don't have any success pinning down what is going on, there is another way to go.
A capable scan tool using mode 6 will point out which cyls are producing the misfires and how often. This will provide some direction where to start checking.

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

mountainmanjoe

I thought P0300 meant a general misfire condition, not specific to any cylinder

hanky

A lot can be learned from Mode 6 info, you just need to know what you are looking at and what to look for.

pomerlo

Yep P0300 is not specific to a particular cylinder. It means it happen randomly.
here is some cause :

P0300 Causes

(* = Most Common)

• Faulty or worn spark plugs *
• Faulty spark plug wires or coils *
• Distributor failure *
• Faulty fuel injector
• Vacuum leak
• Low fuel pressure
• Camshaft sensor defective
• Crankshaft sensor defective
• Engine timing off
• Leaking head gasket
• Low engine compression
• Poor quality fuel

In addition to fuel delivery, crank and cam sensors must not be overlooked.

mountainmanjoe

The "most common" causes don't really apply here
For example, 2004 vans don't have a distributor.
It's unlikely that all spark plugs/wires/coils/injectors would fail at the same time, so you can skip right to the bottom of the list.