Need help with weird misfire in closed loop!
Thread Starter
Heel & Toe
Joined: Sep 2021
Posts: 22
Likes: 11
From: Arizona
Need help with weird misfire in closed loop!
Hello! around a month or so ago i went out to start my 99 c5 one morning after driving it fine the previous day and i did just fill up on gas but i'm not sure if that matters but it had a weird idle which would only happen around 30 seconds or so in to a cold start, it drops down in rpms and starts stumbling then will jump up a couple hundred rpms by itself. in non idle conditions or higher rpm i do not get this stumbling, occasionally i also will get hesitation when trying to accelerate usually from a dead stop. it's now getting worse, i've narrowed it down to when the car goes into closed loop at idle it start's misfiring, im going to attach a log file for HP Tuners that i logged this on. I have already moved around some plug wires and the misfire did not follow that. I do have a CAI, Long tube headers with an offroad x-pipe so i am catless and turned off O2 sensor codes with hp tuners but didn't mess with anything else and i am not tuned for these mods otherwise. Do you guys think that it might be my front O2 sensors or spark plugs? or do i just need a tune? If anyone could provide any insight or help it would be greatly appreciated !
Moderator, Tech Contributor
Joined: Sep 2013
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From: Cape Coral, Florida
I’d do the plugs first if you are doing “swaptronics”…if the front O2’s are either stuck rich or lean the whole bank would be misfiring…you probably need a tune with your CAI and LT’s…without specialized diagnostic equipment you may as well swap around or change everything in the ignition or fuel injection system…good luck !!
Thread Starter
Heel & Toe
Joined: Sep 2021
Posts: 22
Likes: 11
From: Arizona
i appreciate the response and well it kinda is almost the whole bank (1) that is misfiring it's just 3 & 7 that are doing it more than 1 & 5, the voltage for that bank in the o2 sensor also is barely fluctuating like how it should it's basically just floating around 500ish and barely moving compared to the other side which is going from 300 to 800, is that normally a sign that it's dead or dying? i've been running this setup for like 2 years with no tune so i wouldn't be surprised if it has fouled out and i'm about to hit 100k miles as well and im pretty sure they are the original ones
Moderator, Tech Contributor
Joined: Sep 2013
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From: Cape Coral, Florida
If it’s floating around it’s bias voltage of 450mv’s you can either introduce a vacuum leak to make the O2 go lean or spray some Brake Kleen into the intake to make it go rich…if it responds the sensor is working but you may actually have a lean or rich condition with your mods…what do your fuel trims look like ??…with LT headers the O2 sensors are placed far back so the sensors will not read correctly…they will most likely be reading a lean condition…I would expect both banks to be misfiring.
Thread Starter
Heel & Toe
Joined: Sep 2021
Posts: 22
Likes: 11
From: Arizona
If it’s floating around it’s bias voltage of 450mv’s you can either introduce a vacuum leak to make the O2 go lean or spray some Brake Kleen into the intake to make it go rich…if it responds the sensor is working but you may actually have a lean or rich condition with your mods…what do your fuel trims look like ??…with LT headers the O2 sensors are placed far back so the sensors will not read correctly…they will most likely be reading a lean condition…I would expect both banks to be misfiring.
https://youtu.be/5WnM_NsOtd8?si=ZSoX6zSVl3suV94w
https://youtu.be/5WnM_NsOtd8?si=ZSoX6zSVl3suV94w
Moderator, Tech Contributor
Joined: Sep 2013
Posts: 15,600
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From: Cape Coral, Florida
With long terms at -18% I would expect the O2 to be well above 450mv’s…you said it’s only about 500 and steady ??…what does your MAF read at HOT IDLE in grams/second…also what does your MAP sensor read key ON engine OFF ??…at -18% do you have a trouble code ??
Thread Starter
Heel & Toe
Joined: Sep 2021
Posts: 22
Likes: 11
From: Arizona
Yeah the O2 sensor on bank 1 fluctuates very slowly from 450 to around 570 at idle and hot idle was around 7 grams a second for MAF i will have to check the MAP sensor once I'm home
Moderator, Tech Contributor
Joined: Sep 2013
Posts: 15,600
Likes: 4,060
From: Cape Coral, Florida
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Thread Starter
Heel & Toe
Joined: Sep 2021
Posts: 22
Likes: 11
From: Arizona
Ok so sorry for the late reply i wasn't able to check anything last night but this morning i did some testing, MAP with engine off and key on was reading at 98, I also changed the spark plug out on Cylinder 3 which is my lead misfiring cylinder and it changed nothing, i took off the brake booster line to introduce a vacuum leak and the bank 1 O2 did respond higher almost up to 700mv peak and a 340mv low however it was delayed to catch up when it when it started climbing back up and took a few seconds before it finally did so. bank 2 went to 900mv and down to 130mv. On hptuners scanner tool i also turned on an option to check ignition coils and it said all 8 passed as well.
Thread Starter
Heel & Toe
Joined: Sep 2021
Posts: 22
Likes: 11
From: Arizona
Sorry about the late response, Bought a new O2 Sensor and replaced it (was not fun with long tube headers) but was immediately sounding better as soon as it went into closed loop, virtually no misfiring on the driver side bank then noticed on the mv graph how slow the 2nd bank was responding as well bought a sensor for that side and all is well now! This can now be closed my problem has been solved , Thanks for the suggestions.