2003 gmc w4500 gas engine with p0300
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CF Beginner
Joined: Oct 2021
Posts: 3
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2003 gmc w4500 gas engine with p0300
Hello. I'm new here. I recently got a 2003 gmc w4500 with the lq4 I think is how it's spelt. I got it with a random cylinder misfire p0300. I took it to a garage and they did a crank shaft positions sensor relearn on it and it seemed to be fixed although still had a very slight but obvious rough idle, tons of power and good get up and go.... I did the exhaust manifold gasket on drivers side yesterday. In the process I snapped a bolt off (like they always do) so I unhooked the ground from the battery and welded a nut to the bolt and got it out. After reinstalling new gasket and bolts and manifold, it now has a very obvious misfire and flashing cel and p0300 is back.
I have replaced all injectors with new and can hear them firing, I have replaced all plugs and wires (not coils), fuel system is completely redone. If I unplug an injector it is obvious in sound that the cylinder isn't firing and is that way on any of the 8 cylinders.
The coils are all delaminating, but was told it wasn't that. I've sprayed them with water at night with truck running to see if theres sparks but don't see anything. So I'm lost as to what is next. Compression seems to be in the good area. No knocking... no smell of unburned fuel.. I'm leaning towards electrical issue..
If anyone has any input I'm all ears!
I have replaced all injectors with new and can hear them firing, I have replaced all plugs and wires (not coils), fuel system is completely redone. If I unplug an injector it is obvious in sound that the cylinder isn't firing and is that way on any of the 8 cylinders.
The coils are all delaminating, but was told it wasn't that. I've sprayed them with water at night with truck running to see if theres sparks but don't see anything. So I'm lost as to what is next. Compression seems to be in the good area. No knocking... no smell of unburned fuel.. I'm leaning towards electrical issue..
If anyone has any input I'm all ears!
CF Monarch
Joined: Oct 2017
Posts: 4,605
Likes: 269
From: USA
Hello. I'm new here. I recently got a 2003 gmc w4500 with the lq4 I think is how it's spelt. I got it with a random cylinder misfire p0300. I took it to a garage and they did a crank shaft positions sensor relearn on it and it seemed to be fixed although still had a very slight but obvious rough idle, tons of power and good get up and go.... I did the exhaust manifold gasket on drivers side yesterday. In the process I snapped a bolt off (like they always do) so I unhooked the ground from the battery and welded a nut to the bolt and got it out. After reinstalling new gasket and bolts and manifold, it now has a very obvious misfire and flashing cel and p0300 is back.
I have replaced all injectors with new and can hear them firing, I have replaced all plugs and wires (not coils), fuel system is completely redone. If I unplug an injector it is obvious in sound that the cylinder isn't firing and is that way on any of the 8 cylinders.
The coils are all delaminating, but was told it wasn't that. I've sprayed them with water at night with truck running to see if theres sparks but don't see anything. So I'm lost as to what is next. Compression seems to be in the good area. No knocking... no smell of unburned fuel.. I'm leaning towards electrical issue..
If anyone has any input I'm all ears!
I have replaced all injectors with new and can hear them firing, I have replaced all plugs and wires (not coils), fuel system is completely redone. If I unplug an injector it is obvious in sound that the cylinder isn't firing and is that way on any of the 8 cylinders.
The coils are all delaminating, but was told it wasn't that. I've sprayed them with water at night with truck running to see if theres sparks but don't see anything. So I'm lost as to what is next. Compression seems to be in the good area. No knocking... no smell of unburned fuel.. I'm leaning towards electrical issue..
If anyone has any input I'm all ears!
- Spark plugs that are damaged or worn
- Spark plug wires and/or coils that are damaged or worn
- Distributor cap that is damaged or worn (only if applicable to the vehicle)
- Rotor button that is damaged or worn (only if applicable to the vehicle)
- Fuel injectors that are defective
- EGR valves or tubes that are clogged
- Ignition timing that is off
- Vaccum leaks
- Fuel pressure that is low
- Head gasket(s) that are leaking
- Distributor cap that is cracked
- Camshaft sensor that is faulty
- Crankshaft sensor that is faulty
- Mass air flow sensor that is faulty
- Oxygen sensor that is faulty
- Throttle position sensor that is faulty
- Catalytic converter that is faulty
- PCM is faulty
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