Lq4 p0300
12-16-2023, 03:33 PM
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Lq4 p0300 Hello, I'd like to start with I'm not sure if this is the appropriate spot for my post or not. I apologize if this isn't the correct spot I'm just trying to find some help.
I apologize for the long read, I wanted to give as much info as I could on this subject.
My current issue is I have a p0300 code on my 05 gmc yukon with the lq4. I have a brand new set of 317 heads, Melling timing gear set and a Melling high pressure oil pump. My wife and I were driving to town and it started misfiring pretty bad. Check engine light started flashing I pulled her over shut her down and towed her home. I installed a new cam afterwards, why? Well because when I installed the new heads it didn't look the greatest and was pretty worn. I skipped out on replacement as I didn't want to wait on a camshaft and have the engine sitting torn apart. ( I do all my work outside in the gravel ) We went a good solid 2 months up until the driving to town story occurred. The cam I bought was an ls6 cam ( I couldn't find a stock cam anywhere online ) it was the closest to the lq4 cam I could find. I installed ls6 valve springs, ls7 lifters and hardened pushrods. I really thought the cam was the reason the misfire occurred. Cam was ok, some grooving and a couple lifters lost a little bit of metal but I don't believe this is what caused my misfire. Problem is now this pesky p0300 code. It's only at idle. There are no vacuum leaks, fuel pressure is at 55 psi. ( I even did a fuel balance test just to double check the fuel ) O2 sensors are switching just fine. Plugs, wires and coils are all brand new. I did them all with the head install. This is our only winter vehicle, our truck does terrible in the snow and my wife absolutely will not make the trek to town in anything but this vehicle. We live 25 miles outside of the closest town and need this rig. If anyone could point me in the right direction I'd be very grateful.
I apologize for the long read, I wanted to give as much info as I could on this subject.
My current issue is I have a p0300 code on my 05 gmc yukon with the lq4. I have a brand new set of 317 heads, Melling timing gear set and a Melling high pressure oil pump. My wife and I were driving to town and it started misfiring pretty bad. Check engine light started flashing I pulled her over shut her down and towed her home. I installed a new cam afterwards, why? Well because when I installed the new heads it didn't look the greatest and was pretty worn. I skipped out on replacement as I didn't want to wait on a camshaft and have the engine sitting torn apart. ( I do all my work outside in the gravel ) We went a good solid 2 months up until the driving to town story occurred. The cam I bought was an ls6 cam ( I couldn't find a stock cam anywhere online ) it was the closest to the lq4 cam I could find. I installed ls6 valve springs, ls7 lifters and hardened pushrods. I really thought the cam was the reason the misfire occurred. Cam was ok, some grooving and a couple lifters lost a little bit of metal but I don't believe this is what caused my misfire. Problem is now this pesky p0300 code. It's only at idle. There are no vacuum leaks, fuel pressure is at 55 psi. ( I even did a fuel balance test just to double check the fuel ) O2 sensors are switching just fine. Plugs, wires and coils are all brand new. I did them all with the head install. This is our only winter vehicle, our truck does terrible in the snow and my wife absolutely will not make the trek to town in anything but this vehicle. We live 25 miles outside of the closest town and need this rig. If anyone could point me in the right direction I'd be very grateful.
12-16-2023, 05:00 PM
#2
TECH Regular
Said you replaced coils . Have you tried running the old coils that USED to work okay ?
Possible you pinched part of the coil wire harness, or one of them is rubbed through touching the coil bracket ?
Possible that you missed one of the GROUND wires connected at back of engine on passenger side ?
are you sure your NEW spark plugs are not defective Xhinese knock-offs ?
.
How's cam position sensor connector ?
Can you probe and assure good Ground ( ZERO volts ) and source ( +5 volts )
If you have a wire pinched somewhere, and ( intemittently ) lose the 5 volt bus, everything goes to ****.
Possible you pinched part of the coil wire harness, or one of them is rubbed through touching the coil bracket ?
Possible that you missed one of the GROUND wires connected at back of engine on passenger side ?
are you sure your NEW spark plugs are not defective Xhinese knock-offs ?
.
How's cam position sensor connector ?
Can you probe and assure good Ground ( ZERO volts ) and source ( +5 volts )
If you have a wire pinched somewhere, and ( intemittently ) lose the 5 volt bus, everything goes to ****.
12-16-2023, 06:15 PM
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I did try my old coils, no change there. Harnesses look good on both sides. They haven't rubbed through. Grounds are all tightened down in good shape. Plugs are NGKs gapped at .045 I'll post again with info on the cam sensor. My misfire from the data is on all eight cylinders. Highest count misfires are cylinders 4 & 5.