Weird miss fire??
03-20-2008, 10:50 PM
#1
Staging Lane
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Weird miss fire?? Hey guys, i havent really posted here before but i am a long time reader to, i am from LS1truck.com
ok so here is my deal. i have a 98 chevy RCSB with a LQ9 in it that was installed at Nelson perofmance. allen finally finished everything up and was driving it around town and he started to notice a miss fire in the #4 cylinder. its not big, or noticeable when driving, only when idle. it kinda sounds like a loap from a big cam.
the motor has never been opened up before, it had less then 17xxx miles on it. allen changed the pcm, coil, all the coils, injector, all the injectors, spark plugs, sp wires, checked the compression, it all came out a little above 10 to 1 in each cylinder. he did a compression leak down test, it all came out good. so after idling for a few minutes u can hear it miss fire, and u pull the plug out, its black from soot not oil, which to me seems like it is running rich. well the truck runs awesome except for this one little thing. anyone ever had this problem before??
we dont want to open the engine up because my budget is over as it is and that will defantly break my wallet
here is allens post over at ls1truck..
any ideas would help greatly
i have posted this in a few other sections i dont know which one to post in srry haha
thanks
ok so here is my deal. i have a 98 chevy RCSB with a LQ9 in it that was installed at Nelson perofmance. allen finally finished everything up and was driving it around town and he started to notice a miss fire in the #4 cylinder. its not big, or noticeable when driving, only when idle. it kinda sounds like a loap from a big cam.
the motor has never been opened up before, it had less then 17xxx miles on it. allen changed the pcm, coil, all the coils, injector, all the injectors, spark plugs, sp wires, checked the compression, it all came out a little above 10 to 1 in each cylinder. he did a compression leak down test, it all came out good. so after idling for a few minutes u can hear it miss fire, and u pull the plug out, its black from soot not oil, which to me seems like it is running rich. well the truck runs awesome except for this one little thing. anyone ever had this problem before??
we dont want to open the engine up because my budget is over as it is and that will defantly break my wallet
here is allens post over at ls1truck..
Originally Posted by NP
#4 plug, second one on the passenger side, keeps fouling during idle.
I can detect a misfire once in a while during cruising with the laptop.
If the truck idles long enough, it will start to rock like the engine has a small cam.
But during acceleration in fourth gear, everything is smooth and I don't feel a misfire or hesitation, like you would if a bad plug or wire exists.
Here's what I've changed and done to troubleshoot this:
replaced plug
replaced injector
replaced wire
replaced coil
replaced entire coil set
confirmed harness is correct. Measured from pcm to coil and all terminals.
replace pcm
all sensors look good and are working.
If I take the truck out for a spin, it seems the plug will clear up a bit. What I mean is, if the truck idles, the plug will get real black, if I go drive it, the plug starts to get cleaner. So that's why I say an idle misfire.
I shook stuff while the truck was running too to see if the cause was a bad connection somewhere.
Every once in a while, the engine seems like it clears up for a minute or two.
The only cylinder giving me this issue is #4.
I pulled the #4 injector connector and plugged in another injector, you can feel the injector firing, so I know the wiring is good and that injector is firing.
I pull the plug wire, put a spare plug in it and laid it on the valvecover to see if I was actually getting fire, it is. The plug fires just like all the other ones, as far as spark intensity.
Does anyone else have any suggestions on what to look for or replace? This is driving me nuts!
I did a compression test and it looked ok, I haven't done a leak down test, which would show a valve seat, bent valve, blown head gaskets, etc.
I wanted to add, I drove it some more tonight and it's definitely missing during cruising and semi aggressive acceleration, so pretty much all the time.
keep them coming, just need to get this last issue taken care of.
I can detect a misfire once in a while during cruising with the laptop.
If the truck idles long enough, it will start to rock like the engine has a small cam.
But during acceleration in fourth gear, everything is smooth and I don't feel a misfire or hesitation, like you would if a bad plug or wire exists.
Here's what I've changed and done to troubleshoot this:
replaced plug
replaced injector
replaced wire
replaced coil
replaced entire coil set
confirmed harness is correct. Measured from pcm to coil and all terminals.
replace pcm
all sensors look good and are working.
If I take the truck out for a spin, it seems the plug will clear up a bit. What I mean is, if the truck idles, the plug will get real black, if I go drive it, the plug starts to get cleaner. So that's why I say an idle misfire.
I shook stuff while the truck was running too to see if the cause was a bad connection somewhere.
Every once in a while, the engine seems like it clears up for a minute or two.
The only cylinder giving me this issue is #4.
I pulled the #4 injector connector and plugged in another injector, you can feel the injector firing, so I know the wiring is good and that injector is firing.
I pull the plug wire, put a spare plug in it and laid it on the valvecover to see if I was actually getting fire, it is. The plug fires just like all the other ones, as far as spark intensity.
Does anyone else have any suggestions on what to look for or replace? This is driving me nuts!
I did a compression test and it looked ok, I haven't done a leak down test, which would show a valve seat, bent valve, blown head gaskets, etc.
I wanted to add, I drove it some more tonight and it's definitely missing during cruising and semi aggressive acceleration, so pretty much all the time.
keep them coming, just need to get this last issue taken care of.
i have posted this in a few other sections i dont know which one to post in srry haha
thanks
03-21-2008, 09:12 AM
#3
Staging Lane
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both o2s are brand new, ill take a look at them an double check the temp switches. the motor as been running for about a week now an has been doing it since it was put it.
thanks for the input
thanks for the input