lmm fuel knock misfire

mamu

mamu

Member
Mar 14, 2010
162
4
18
36
arkansas
hey guys,

i had a lmm come in with a fuel knock and bucking and jumping on the road. reduced engine power and barely would pull it self. showed 3 and 5 cylinder misfire on the computer but that was the only codes. heres what I've checked.

crank ring good and tight.
load tested and ohm tested all injector wires from ecm to injectors
swapped ecm and didn't change.
ran compression (all 375-400ish)
pulled injectors and had them tested...replaced 3

put it all back together and now 4 and 6 show misfire codes. at idle its smooth and fine but when you crack the throttle at about 925 rpm it starts knocking and missing. i can cut out #1 injector with the scanner and the fuel knock goes away. but it still misses.

all balance rates are -2.5 to +2.5 are the 2 worse the rest are -1 to +1

truck has whiteish blue smoke. smells like fuel. but honestly i don't have a clue. im to the point where i need anybodys thoughts and see if we can't stumble across the issue.

truck is 08 lmm 2500 crew cab. deleted with ppe. has been that way for over a year. rite before christmas this issue started. no other mods that i know of.

thanks
 
Last edited:
Chevy1925

Chevy1925

don't know sh!t about IFS
Staff member
Oct 21, 2009
21,743
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Phoenix Az
put the stock tune in it and see if the knock stops. it will still start and rev without the emissions crap hooked up on a stock tune
 
mamu

mamu

Member
Mar 14, 2010
162
4
18
36
arkansas
put truck back to stock. same thing.

truck will rev but it misses and shakes all the way through the rpm range.

checked rail pressure and it shows to be matched pretty consistent with desired.
 
mamu

mamu

Member
Mar 14, 2010
162
4
18
36
arkansas
yea we reprogrammed all the injectors. had 2 that wouldn't program. figured out they where plugged in wrong (4 an 6 was swapped) but all programming is done now.

a broke crank? this don't act like any other broke crank I've seen but ill check today and see.
 
mamu

mamu

Member
Mar 14, 2010
162
4
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36
arkansas
checked crank. cant see any signs that its broke. turned it from front and read and theres not any play. it idles smooth now. but as soon as you get to the 925 rpm it goes crazy. I honestly have never seen this so I'm sure theres a sign somewhere I'm overlooking.
 
mamu

mamu

Member
Mar 14, 2010
162
4
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36
arkansas
thats the only thing i haven't watched specific. like the cam ring came loose. but the rpm doesn't change when its acting dumb at idle. hard to tell. ill check the cam counts missed in the morning tho.
 
mamu

mamu

Member
Mar 14, 2010
162
4
18
36
arkansas
with truck running and missing it doesn't count any on the cam reference missed. but when i crank it up it will count to 33-35 before it starts and runs. but never goes back to 0 while running. but never goes up either.

still showing #1 cylinder misfire.
 
M

mamumtsllc

Member
Sep 4, 2014
91
1
8
36
Burn Down said:
With the 1 misfire I’m still betting you have harness issue, maybe ecm... how did you test the wiring?
Click to expand...
I've changed ecm and it stays. I load tested all wires and checked resistance. Made sure they weren't rubbed together and jumping fire. I'll check them again to make sure.

Sent from my SAMSUNG-SM-G891A using Tapatalk
 
Burn Down

Burn Down

Hotrodder
Sep 14, 2008
7,092
28
48
Boise Idaho
Hmm, find all the ground wires and make sure there good and have good continuity to ground. My lly has a loose ground wire on the right front of the engine that caused mis-fires and fuel knock issues with no codes.
 
mamu

mamu

Member
Mar 14, 2010
162
4
18
36
arkansas
checked all grounds. wires are good. check good. cleaned and put back on. unplugged #1 and plugged in a different injector. seemed that the misfire went away. so i changed that injector with a different one. put it all back together. the misfire code didn't come back but it still acts up and misses. now the only cylinder that shows out of range on balance rates is #7. it goes to a 7 or 8.

im at a loss here. im wearing bolts out and patience and just waisting money on this rig. but im down to try anything that someone has seen or tried or ever heard of!

thanks guys
 
Burn Down

Burn Down

Hotrodder
Sep 14, 2008
7,092
28
48
Boise Idaho
How many miles on truck? What injectors are in the truck, stock or? Can you do a return rate test. Who tested the injectors?

Did you pull the crank sensor and ck the reluctor wheel?
 
M

mamumtsllc

Member
Sep 4, 2014
91
1
8
36
stroke250 said:
X2 on checking the reluctor.
Click to expand...
Checked reluctor ring on crank. No play. Checked cam ref missed with efi and doesn't count up once running or missing. I can do a return rate test. I'll just have to get them set up. A injection shop checked them and said 3 failed so we put 3 new ones in it but the others was "good" I'm not sure if they are good or not now. I'm starting to doubt everything about this rig. Miles are high. Like 250k plus. And the injectors are stock.

Sent from my SAMSUNG-SM-G891A using Tapatalk
 
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