Thread: Cylinder 3 misfire Tried everything I can think of

so replaced all my wires and the check engine light is no longer popping on still idling a little rough but got much better after idling for about 5-8 min while doing a pcm relearn due to the battery dieing. my idle LTFTs were around 9-10 ish fluctuating a little bit but never running over 10.4 is this a sign im running rich or is that normal for a parked prix idling? seemed a little high to me but not as bad as when I had a vacuum leak and it just pegged at 16.4 uploading a video of me making sure it was not a vacuum leak causing the rough idle and to give you guys a sound bite of what im dealing with
1st video

second video 5 min after 1st

both were made after wire change to brand new acdelco LT warranty

the noise in the back round is coming out of my intake assuming its my rotors dont know why its the most dominant sound on the video but the first cpl seconds of first video are me turning phone to the intake so you can hear it and try to block it out as you hear the idle

gonna take it out for a drive a little later and see if the misfire kicks in after some higher rpms but seems to have completely dissipated just idling like I got a small vac leak now but my LTFTs arent pegged at 16.4 confirming it and the vac leak search via carb cleaner was completely inconclusive

so after talking some more in chat and checking the plugs I was sold it turns out I ordered tr5s and got tr 6s which are two heat ranges cooler I went back to the stock and will just have to swap plugs when I do pulley is it possible the 2 heat ranges cooler could have been causing my engine to stutter cause even though it was idling better en route to parts store I was still boggin and got a shot a code the only time I brought it over 2k rpms

Swapped plugs last night for acdelco copper cores idled alot better so tok it into the shop en route boggin and misfires continued.

Had my glass pack and installed and a professional scan done he told me the injector was getting constant voltage so I'm most likely looking at either a wire harness issue or my pcm could be going

heading to junk yard now to go grab a pcm to swap in a check it out if that doesn't work im heading in tomorrow to have them look over my wires

does the pcm sound logical to anyone it strike me as a strong possibility the way it reset after getting wet the other day

PCM swapped and the problem persists think the only thing left would be a break in my injector line but with the way it backfires I'm assuming I'm getting fuel just not spark
gonna take it in tomorrow to have them look over all my wires

if anybody has anything else I might be missing let me know so I can have them check it at the shop for me

thanx for all the help guys sorry about the never ending diagnose me thread

Rob

You'll know your getting fuel if the spark plug is wet when you take it out after it misfires.

Have you replaced the wiring harness that goes to the icm? I bet a wire in there is broken.

no I have not how hard is it to replace that is it something I can just grab from the yard or is it one big ass hairball of wires I would have to rip out?

i'm also gonna have to ask the shop what they were talking about as far as it's getting constant voltage cause I figured they meant my icm and o2 sensor but gonna need some clarification i guess

What they're saying is your injectors are getting fire, the wires going to the injectors are hot. That's one of the first things my dad checks when he diagnoses a car.

It could be that the injector itself is bad, which you could probably use a stethascope to hear the damn things, or just move em around or whatever. But this sounds more like ignition.
I think it's quite rare if ever that I've seen a pcm just go bad. But I guess anything's possible. Good luck.

Ive found small vacuum leaks cause misfires and can be hard to track down. Best way I've heard to find or rule out vacuum leaks is a smoke test.

Some things I do when dealing with problems similar to yours: I eliminate mechanical causes...lifters, valves, etc. Perform a relative compression test. When I am satisfied the engine is mechanically sound I move on to the fuel system...fuel pressure both at idle and when driving down the road, specifically when duplicating problem. Scope injectors. Perform a injector balance test. A faulty injector will quickly become apparent during this test. It is important to note that after performing an injector balance test that the vehicle be started and run to burn off the fuel that will accumulate in the cylinders during the test. I also like to run a injector kill test from time to time. Once I have eliminated fuel delivery and injector function as the cause I move on to secondary ignition. Visually check for arcing or faulty components. Simply put, current is alot like water...it will flow to the path of least resistance. If your secondary ignition can find a path to ground thats easier than the path it takes from coil to spark plug, it will follow that path. This is more pronounced under load. I like to monitor fuel trims when looking for secondary ignition issues. STFT should remain relatively low, +/- 5% up to 8%. LTFT should show a relatively steady reading of around 10% up to 14%. Try and keep the PID list down to 4 selected items to ensure a quicker data refresh rate. A weak or intermittent secondary ignition fault will show up as higher fuel trims. Once you have pinpointed your problem and correct it, I highly recommend clearing your KAM. The PCM has learned a strategy based on the conditions you have been driving your car under with the misfire. Dump that garbage and let your PCM relearn a strategy based on a properly performing engine.

thanx alot for the walk through at least I know what I've done up to this point is right on task my FTs are right where you saying they should be along with my boost staying at -10psi

I had gone over alot of the connections last night ground from icm and harnesses trying to find anything lose or unstable everything seemed solid

Im the exact same way when it comes to allowing the pcm to relearn tony not sure if its out of concern for my car or just an excuse to go open up the throttle afterwards

one one question would be my fuel trims for instance if they were spiking high indicating a failure in the secondary ignition system how would I go about tracking it down

Do you have the metal boot isolators on the plug wires? That was my problem, I had misfire under boost. After doing mulitple tests I gave it a try since it was my last resort, i took off the metal isolators and that fixed my problem. Maybe this could be your problem too? Its worth a shot

tracking down ignition problems can be a little frustrating if you don't have the proper equipment. Some ways to do it without the equipment: move components around. It seems you have your plugs eliminated but I have seen(and done this myself) where a plug can be cracked on installation. I battled a Ford driveability problem for days. Turns out the socket I was using to install the plugs was causing the insulator to crack. Just wanted to share that with ya...just in case. I have made some trips to the salvage yard when business is slow and picked up some common failure and test items. On the 3800 I grab some ICM's, coils, MAF sensors, TP sensors, EGR valves, just to name a few. I also look for recently installed plug wires as these can come in handy from time to time. Heres a little advise: If you plan on keeping with the GP 3800, get to know what GOOD sensor values are. Familiarize yourself with what good scan data looks like so you can pick up on a failing component. Over the years I have compiled a lot of data on a lot of cars. Notebooks full of notes and wave form patterns. Spent hundreds of hours driving cars with no problems just to record scan data. Most times I can spot a junk sensor input, faulty component value or engine mechanical problem simply by watching scan data. I'll dig around and see if I can find some scan data snapshots of a good running 3800 and post em for ya.

ok thanx Tony yeah I had pretty much the same idea on the pare parts for checking right now I also have an icm pcm 2 sets of wires one new one used and a few extra wires Im starting to wonder if it might just be my fuel pump taking a **** on me not sure if it would kick a light or not bu it seems to do well right off the bat but after driving a lil it becomes worse and worse I know that cylinders 3 and 5 tend to run the leanest so if for some reason I was getting shorts in fuel pressure it would probably throw a misfire in those cylinders first and its the only thing I havn't really checked yet I tried to familiarize myself with as much of the common good running numbers as far as LTFT boost and vac pressures intake temps and transmission temps aside for those my torque pro still leaves a little wanting

just to dbl check though what would be the parameter on a scanner or my app to search for to view the o2 sensor voltage and the fuel pressure if possible

if its not my fuel pressure imma just end up having the mechanics do it for me I'll probably have them check it sometime this week for me as its really driving me crazy and dont wanna risk doing any mechanical dmg just starting it up to troubleshoot at this point shes probably ran for 2-3 hours just resetting pcm or trying to trigger it to fault

all my plugs were in good condition I have probably checked those more than anything else at this point
also all have the metal boots on them.

my LTFTs would normally jump up to 16.4 on vac leaks which is why I was leaning away fro that as the LTFTs im seeing are all in order it almost starting to point me more towards the fuel pump going out at higher pressures because the PCm is commanding the ranges it should get.

Thanx for all the help guys I really appreciate it anytime someoen gives me something to check I get a little bit of my sanity back

unless you have a transducer, you cant really check fuel pressure short of having a guage attached to the service port. i dont normally worry to much about O2 sensor voltage as the pcm will generally kick out a dtc for them if they are failing. i do monitor the downstream O2 if i get a cat efficiency code, just to verify the cat is functioning or not functioning.

keep us posted on your progress.

yeah looked into a fuel tester test today gonna go rent one tomorrow and see how it goes the service valve seems like it makes it pretty easy debating on whether or not to have them cut an o2 bung out in my crossover for it Ive heard of it being done just wasnt sure on the effectiveness as it would pretty much only be getting the front bank af right?

hopefully its my fuel pump going out service records were all in order when I bought the car aside for brakes which the previous owner had done himself so seems like 150k is a round about number for it to start weezing on me but im gonna grab a new fuel filter and the test tomorrow and verify my fuel system is in proper running order after that its going in to the shop to have them trace the broken connection as it seems like the only thing left

the things that are just really bothering me is that cylinder 3 runs the leanest so I feel like the consistent misfire after swapping new components to it mean the engine might be running leaner than it should be

headers and intake and egr delete dont seem like they should be effecting that to the point of misfires though

the fact that it idled fine for a while and than as soon as I bring it up over 2k after the relearn it wants to sputter like a vac leak consistently thereafter and just completely mis in the higher rpms

fuel pressure test: as I understand it I should be looking a around 60psi with throttle and about 30 primed and turned off

is that goes fine is a short my last possibility?
I wanna say I have ruled out any other possible failure but for a recap

Plugs
wires
ICm
PCM
coil packs
grounds
injector voltage
compression test

to be tested
Fuel Pressure

even holding pressure is it possible my cylinder 3 valve could be an issue even not making any racket(since glasspack its been a lot easier to hear whats going on)

if it is the fuel pump does anybody have any suggestions I see the stock one around 200+ but some aftermarket 255lph for around 100

Originally Posted by Beelzeboul

thanx alot for the walk through at least I know what I've done up to this point is right on task my FTs are right where you saying they should be along with my boost staying at -10psi

VACUUM, should be at -19 to -20 at idle. And thats -19 in/hgs, because boost cant read negative numbers.

Originally Posted by Fivefingerdeathpunch

VACUUM, should be at -19 to -20 at idle. And thats -19 in/hgs, because boost cant read negative numbers.

19-20 hgs is about 10psi aint it?

You cant have negative psi. Boost is above 0 vacuum.

So i dont know how that app works to figure that info out.

hmm alright Ill have to check it out see if I have another pid to monitor after I pick car up I finally caved and took it to the shop to have them track it down im leaning toward my fuel pump being on its way out ot a break in the connection from rocking the engine to install headers but with school starting I ran out of time Ill be sure to let you guys know what they find out for me and appreciate all the help

so the verdict is in even though my ses light was only shooting me a cylinder 3 misfire I had misfire on 3 5 and 4 so apparently I had swapped bad components for other bad components that just weren't tripping the ses but were showing up on the gm scanner end result was 2 bad injectors and 1 bad coil pack . the diagnostic fee 60 bucks and the fee for the installation was 350$ they seemed a little pissy when I said I would come pick the car up and do it myself just to make sure on my invoice it gave me a list of which components on which cylinders were bad after saying that they told me I would have to stop in tomorrow so he could remove the parts he already installed to diagnose I could see the frustration but 350 for an hour worth of work maybe seemed irrate gonna head to the JY tomorrow to grab injectors and a coil pack. The coil pack was corrosion they said but the reason they had told me the injectors went out was most likely caused by watery gas so I just finished filing my claim with BP hopefully FTMFW

I know for coil packs checking for breaks in plastic and erosion around the metal plug connectors is there anything else I should worry about?
for spotting good fuel injectors what am I looking for just no discoloration on the tips no corrosion and clear 4 holes?