Tydriver

iTrader: (2)

OK, guys I want to start off saying, I've searched and I've read everything I can find about the P0300 code and I am still having issues after all the work I've done. I'm hoping someone here has an idea of what else to try, I'd certainly like to get this resolved.

Here's what I am working on:

2003 Silverado LS 4.8L 4WD with 148k miles.

Here's what I have done so far trying to fix this problem:

*Installed New Plugs (NGK Iridiums, gapped to .040")
*Installed New Wires
*Installed New Walbro 450 Fuel Pump
*Using EFILive, I've individually cycled each injector OFF and ON with the engine running and was able to notice an appreciable change at idle which tells me the injectors are all working.
*Also using EFILive I've datalogged the truck, and received nothing out of the ordinary, the MAF and MAP reading seem normal.
*I've disassembled the intake setup and verified no obstructions exist, it's clear as it came from GM.
*I've completed a compression test on every cylinder, with a reading achieved of 174-180 psi on every cylinder (#8 is a real pita).

Here's my symptoms:

At idle, the truck runs fine, no codes, no misfires, cant even tell it apart from any other vehicle.

At WOT, the engine is slow to accelerate above 3000 RPM and wont ever reach redline, it has audible pinging and misfiring is evident. The Check Engine light illuminates and goes out after a minute or so.

The P0300 code is pending and drilling down further into EFILive the most misfires are detected on cylinders 3 and 6 (opposite banks).

I really need to use this truck to move soon, and at this time I dont have the confidence in this truck to trust it on a 500 mile move.

I need to get this fixed and I'm hopeful that someone on here has something that they were able to fix OUT OF THE BOX from what's been suggested in other threads.

At this point, I'm still leaving things on the table like :

*Wiped Camshaft Lobes
*Failing/Bad Crank/Cam sensor
*Collapsed lifter ???

Please offer up any suggestions you have.. I'm all ears..

skolman91

iTrader: (28)

This is a complete guess, but have you checked the knock sensors at all?

Wolftrk99

iTrader: (5)

Have you by chance checked to make sure your intake manifold isnt leaking, they are kinda known for it... And what about the fuel pressure and regulator?? Is the engine stock or have you done anything to it?? Have you tried swapping coils around? I have seen them break down at higher rpm.

1FastBrick

iTrader: (17)

Is the truck Bone stock?

Have you verified fuel pressure under load?

Did the truck sit for a long period of time???

Tydriver

iTrader: (2)

No I haven't curious why this would matter. Can you please elaborate ? I mean I can hear audible pinging when it begins misfiring.

I haven't I assume that I can do the standard carb cleaner spray trick around the intake while its running ??

As far as the coils are concerned, I'll get at it next weekend and swap a few around to see what happens..

Engine wise, the truck is 100% stock. I haven't verified fuel pressure, I assumed it was going to be good when I changed the fuel pump to a Walbro 450.

The truck has sat for a bit once this problem came about, prior to that it was driven pretty regularly. Once this problem came up, I was soo busy I didnt have time to address it so it sat.. Are you thinking the fuel is bad or ?

Wolftrk99

iTrader: (5)

The pinging should be picked up by knock sensors as spark knock, yes spray around base of the intake with carb or brake cleaner something volatile enough to change mixture.... Need verify fuel pressure and not assume, just because pump is new doesnt mean you are getting proper power to it or output from it and you should have a pressure regulator on the fuel rail is it leaking?? Also check your maf... If its not reading properly it could play hell with everything else, look at BARO vs MAP pids with koeo and make sure theyre the same
Also make sure the maf g/sec is going up as more air pulled through... P0300 can be a pita!!!!

Tydriver

iTrader: (2)

I have a fuel pressure gauge that screws into the fuel rail schraeder valve, I'll try that out next weekend.

I had a chance this morning to start the truck up and run it, I sprayed carb cleaner all around the base of the manifold where it attaches to the heads, around the back of the manifold and at the front where the throttle body attaches. I did this with the engine running (cold) and the engine running (warm) with no changes.. I'm ruling out a leaking manifold. Also, I noted no leaks at the FPR or fuel rails, ruling this out temporarily until I can put FP gauge on and try it again.

I also thought about the issue with the MAF sensor being dirty, I'll be sure to spray it down next weekend when I get a chance to work on this truck.

I forgot to look if the knock sensors were picking up knock or not when it was misfiring, I mean they should be, but I didn't verify this, another project for next weekend. I'll check the PIDS at this time as well that you mentioned.

Thanks for all the help/suggestions guys... I'm sort of stuck here since I am working in Memphis Area and I live in Indy.. It's a bit of a bitch of a commute and I only make it back to Indy where the truck is at on the weekends.

1FastBrick

iTrader: (17)

Spraying around the intake does not always guarantee you will find a leak if there is one.

Sometimes the leak occurs on the bottom side of the intake and there is really no way to get under there effectively.

If you have an Audible Ping the knock sensors should be picking that up and pull timing.

As mentioned, Verifying pressure will insure your not losing it under load. Loosing pressure can cause it to go lean, it could also be why you're losing power over 3000 RPM. You also want to verify the Fuel pressure regulator is working and that the Vacuum hose isn't cracked or soft. If you remove the line, make sure the valve isn't leaking fuel.

Now the reason I asked about the truck sitting is because Gas breaks down over time. This can cause deposits to form on and around the injectors. They may perform Fine at at idle but under load the may not be up to the task. In some cases I have to Physically remove the injectors and clean them. How ever, those usually cause a specific misfire on the affected cylinder.

If the cam Had a wiped lobe, You would probably hear the noise at idle.
If the lifter was collapsed you would have a similar issue at idle.
Typically if you have a cam or crank sensor issue you would get a code or you would have issues with it randomly shutting down. If your login data you would see the signal cutting out as well.

Wolftrk99

iTrader: (5)

Did this issue start before the truck sat or was this all after it sat??
If it was before it could possibly rule out fuel breaking down or whatever, not to say it wasn't contaminated to begin with.... Or that the injectors couldnt stand to be cleaned!

Ive seen quite a few of the fuel pressure regulators leak fuel into the vacuum line and probably just as many with at least one of the rubber ends cracked or torn...

dewmanshu

iTrader: (4)

<p>&nbsp;I definitely know where you are with frustration and needing the truck but can't trust it. Don't we all. </p><p>o2 sensors can give false negatives just like a MAF or leaky intake. Make sure they aren't junked up with rust or something.</p><p>You replaced the fuel pump, but I didn't see where you stated you replaced the fuel filter as well. If you haven't replaced the filter yet, get your *** to the store right now and get one and get that f'er replaced. THEN check FP at &lt;3000RPM, while&nbsp;Under load, driving.</p>