Mechanics

WHAT IS CAUSING MY TRUCK TO RUN SO ROUGH AND BACKFIRE?

1999 Chevrolet Other • 142,000 miles

99 Chevy k1500 5.7 4x4. Truck is running rough doesn't seem like it has all the power it should and is backfiring when I am driving slow and not giving it allot of throttle. I have already replaced the whole exhaust including o2 sensors from the manifold back. New maf sensor, new map sensor, new timing chain, new computer, and new plugs, wires rotor and cap only code I get is p0300. No mechanic can seem to fix it. Any help is appreciated.
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Dmolby
November 25, 2011.




P0300 random misfires is normally caused by vacuum leak and contaminated fuel-check fuel pressure with a fuel pressure tester designed for your vehicle fuel system

Tiny
Rasmataz
Nov 25, 2011.
Fuel injectors were tested. Compression tests were good as well and I have also ran cleaner through gas and done fuel filter. Only things on the list that I Havnt done is iac valve, manifold absolute pressure sensor, throttle position sensor, and the egr valve. Would any of these 4 things cause it to backfire? It backfires when I let off gas or when Im driving slow like backing into my driveway. Doesn't have the power and throttle response that it should. It almost seems kind of****** when im driving

Tiny
Dmolby
Nov 25, 2011.
An engine running to rich or lean will cause a bacfire Ignition wires arethey wired correctly, wrong type of plugs, check and test the rest I've mentioned-

Tiny
Rasmataz
Nov 25, 2011.
My neighbor has the same truck so I double checked all te wires. All are correct double checked plugs they are the right ones. Cleaned the egr valve and replaced the throttle position sensor still no change. I've tried everything on that list now except the manifold absolute pressure sensor and the iac valve. Someone old me it could be camshaft sensor or crankshaft sensor also? Wouldn't these sensors throw their own code other than the p0300 random misfire? So right now I know the coil and spark is good plugs and wires rotor and cap are good egr, tps, maf, and map sensors all okay. Fuel pump working new fuel filter. Good fuel pressure and good compression, no vacuum leaks and timing is correct with new timing chain. So my only possible options are the manifold absolute pressure sensor, cam sensor or crank sensor? I would think one of those would give off their own code. I apologize for long posts just trying to give as much info as possible

Thanks again for all the help.

Tiny
Dmolby
Nov 25, 2011.
What's the actual fuel pressure readings
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Tiny
Rasmataz
Nov 25, 2011.
55 at slight incline was around 65

Tiny
Dmolby
Nov 25, 2011.
Your fuel pressure is on the low side-should be 60-66psi-Also check the fuel filter if its clogged-

Tiny
Rasmataz
Nov 25, 2011.

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