need help diagnosing misfires
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From: Missouri
need help diagnosing misfires I am working on a 2000 4.8 truck for a buddy. It is bone stock and has 90,000 miles on it. Has been running perfect then all of a sudden it started missing pretty bad at idle and set a code 300. I hooked up the 'ol A-tap and set it up to watch current misfires on all 8 cyls at the same time and #2 and #7 were missing bad.
I have switched plug wires around, put in a new set of spark plugs, swapped the #2 and #4 injectors (the easiest to get to), and cleaned the MAF to no avail. The misfires are still showing up on #2 and #7. Is there some common thread that ties those 2 cyls together? I plan on doing a compression test next, but I dont expect to find anything wrong there. It runs fine at all times other than idle-no misfires indicated while driving, just at idle.
What am I overlooking?
I have switched plug wires around, put in a new set of spark plugs, swapped the #2 and #4 injectors (the easiest to get to), and cleaned the MAF to no avail. The misfires are still showing up on #2 and #7. Is there some common thread that ties those 2 cyls together? I plan on doing a compression test next, but I dont expect to find anything wrong there. It runs fine at all times other than idle-no misfires indicated while driving, just at idle.
What am I overlooking?
TECH Senior Member
Joined: Nov 2001
Posts: 7,036
Likes: 1
From: Lake Zurich, IL
hmmm... only thing I can think of is... since 7 is at the back of the engine, and 2 is at the front... I wonder if they're both insynch w/the stroke (meaning both going up for compression stroke, down for combustion, up for exhaust, down for intake at the same time) ...
If that's the case... it could be a misfire on either 2 or 7 but the PCM wouldn't know maybe?
You've changed plugs, wires, and tried different injectors... only thing left is coil packs
I would put #7 coil pack on #1 and #2 coil pack on #8 and see if hte problem follows the coil packs
That doesn't rule out a valve issue though ... I think a bent valve or bad valve seal could cause a misfire? though if that was the case you'd notice some good oil consumption as well
If that's the case... it could be a misfire on either 2 or 7 but the PCM wouldn't know maybe?
You've changed plugs, wires, and tried different injectors... only thing left is coil packs
I would put #7 coil pack on #1 and #2 coil pack on #8 and see if hte problem follows the coil packs
That doesn't rule out a valve issue though ... I think a bent valve or bad valve seal could cause a misfire? though if that was the case you'd notice some good oil consumption as well
