Reman engine with 65000, truck with 318000 miles.
Starting around 40000 +/- miles, when towing a trailer through the hills it occasionally would show the check engine lite (CEL) and I would slow down and the light would go out, only happened a few times. When I got home
I checked the dtc and it showed p0300 random misfire.
It also took a P1641 Malfunction Indicator Lamp (MIL) Control Circuit and there were times when the CEL didn't come on when starting the truck but I didn't think that effected the misfire. (I later changed the PCM with this in mind)
Then over time it would start showing the CEL more often and reset itself.
Fast forward 20k miles and maybe 6 months ago I got a scan tool that could read the misfire live data. It was mainly showing thousands of misfires on cylinders 4 & 7 with most of the others occasionally with hundreds. I started troubleshooting, I have either checked
Voltage and/or changed all the sensors, ie: map, maf, cam, crank, etc, checked my 02 readings and eventually changed all four 02 sensors, the 2 on the right were not reading correctly.
Fuel trim short term both sides seemed to flux correctly and long term both sides were running steady around -5 but within specs?
I checked back pressure on the catalytic converters (cats) which was 1.5 psi or lower, checked the temp before and after the cats I think they were slightly hotter on the downstream side. I checked fuel pressure, it was a solid 60 but when I checked my Haynes Manuel it showed for 2005 5.3 Vin Z, the pressure should be like 46- 54 so I thought maybe because the fuel pressure was too high the engine was getting more fuel than the PCM figured and that was causing the misfires. But after 2 new fuel pumps putting out 60 psi and discoverd my Haynes manual was wrong so I reinstalled my original fuel pump. I did a smoke test both through the intake before the maf sensor and through the brake boost pump vacuum line with no vacuum leaks and smoke coming out the oil dip stick tube which I assumed was normal. I performed a cylinder compression test at TDC, throttle body open, 1=175, 2=165, 3=185, 4=190, 5=155, 6=180, 7=155 and 8=180. I also did a cylinder leak down test at 80 psi with 1=78 psi, 2=76 psi, 3=76 psi, 4=78 psi, 5=76 psi, 6=77 psi, 7=74 psi, and 8=78 psi. Only on # 7 did I heard air coming out of the intake throttle body, the rest I couldn't hear where it was going. I believe with # 7 the loss was 7.50% which I thought was questionable but acceptable and the difference between cylinders figures to a 5.41% which I thought was good.
I changed the intake gasket and didn't see evidence of leaking. I did have to drill, tap and install four helicoils for the intake.
I then decided to change the PCM cause I couldn't prove anything to be bad and it is the original pcm in the truck. Of course that didn't fix anything. So after conferring with a local shop I decided to pull the heads to inspect cylinders for ring wear.
Checked cylinders 7 & 1 for wear:
#7 cylinder bore measures-
Top fnt to back 3.7965 =.015 wear
Top Top to btm 3.7960
Center fnt to back 3.7995
Center Top to btm 3.7960
Btm fnt to back 3.7965
Btm Top to btm 3.7960
#1 cylinder
Top fnt to back 3.7990
Top top to btm 3.7990
Center fnt to back 3.800
Center Top to btm 3.7990
Btm fnt to back 3.7960
Btm top to btm 3.7940
Even though these are within specs
I then decided to pull the engine to check piston ring wear.
Upon pulling the oil pan off I found a sliver of metal on the drain plug magnet and in the bottom of the drain pan and in the pump screen was some aluminum shavings which I now believe was from drilling/ tapping the intake bolt holes. Even if metal past the filter and into the engine it didn't cause the misfire sense I didn't put more than maybe 20 miles on it before teardown.
I removed cylinder 7 piston and lightly ran the hone in it and checked the top ring at the top of the cylinder for the ring gap which is.020 which I believe is slightly over specs so I checked the ring gap in # 5 cylinder which is.013 which is good.
So I expected to find bad rings in # 7 and it does seem a little low but not low enough to cause a misfire.
I am stumped again.
What else can I check to prove what is causing the misfires?
Sunday, July 6th, 2025 AT 2:58 PM
