I am completely guessing here but I am beginning to suspect a bad BCM or ECM. Something related to them internally that is not diagnosable from the diagnostics query. I suspect hardware related like a cracked trace in a PCB or maybe a bad ground plain or something wrong with a ground plain trace.
In reply to Snrub :
I doubt the would just send a short block- dealers don't rebuild engines anymore- they would do the entire unit all at once. Much cheaper for GM to do that than pay the price to complete the motor. And it would change a bunch of stuff that may also be the problem.
alfadriver said:In reply to Snrub :
I doubt the would just send a short block- dealers don't rebuild engines anymore- they would do the entire unit all at once. Much cheaper for GM to do that than pay the price to complete the motor. And it would change a bunch of stuff that may also be the problem.
AKA, "Load the parts cannon!"
When CORPORATE goes for automotive grapeshot, you know they either want to ensure there is no comeback, or they want to get their hands on everything so they can tear into it and find out what happened. (Which rarely happens)
alfadriver said:In reply to Snrub :
I doubt the would just send a short block- dealers don't rebuild engines anymore- they would do the entire unit all at once. Much cheaper for GM to do that than pay the price to complete the motor. And it would change a bunch of stuff that may also be the problem.
I don’t know about GM, but Hyundai initially spec’d a short block on my Elantra (to fix the piston noise) until the dealer provided them the quotes for getting the head done locally.
The price of the short block and head work was more than the long block cost so they authorized and sent a long block.
After 3 weeks, the dealership gave the car back. The car still misfires, albeit at a lower level than before (for now). Long term fuel trims might be generally improved (for now), but they are beyond -20 at idle. They replaced catalytic converters on both sides and all O2 sensors (4). They also replaced all of the plugs again. Sigh.
How do you spell "lemon law" in Canadian?
Hello Ford Mustang dealership!
^ Miata or 370Z dealership maybe.
Lemon law is spelled CAMVAP in Canadian and it's not a law and it's not as straightforward as what you guys have. In case 8 pages of thread aren't enough, I can't begin to tell you how upset I am with this this situation. :)
I spoke to the service manager and asked him if he'd help me up to get out of the car if the problems persist (explained later that they are). I pointed out that GM is ripping us both off, him in unfair payments for diagnostic works, etc., me in that I don't have a viable car. He said there's a set process that has to be followed to convince GM of a buyout and we haven't gone far enough yet to convince them. He said when we get there he will do the right thing. I have to wait for the CEL to come back on.
GM customer care called today and explained that they are confident the issue is resolved, etc. I'm trying to be as objective as I can, but if a dealership is generally making semi-attempts to fix stuff, they do nothing. They provide a person you can speak to, but they cannot do anything actionable if you point out issues, etc. They appear to have a script they work off. All technical and process decisions are made by the dealership.
In reply to Snrub :
id be checking to see if th clluster still contans a CEL.
Datsun310Guy said:Hello Ford Mustang dealership!
Jalopnik had two stories today: GM denying the warranty claim of a Camaro ZL1 whose side airbag deployed from hitting a pothole, and Ford Mustang GTs suffering from mysterious ticking (which may or may not result in severe damage) for which Ford's fix is a block replacement. Neither one is great, although GM's customer service seems par for Snrub's experience.
YouTube + typewriter tick
Dave M said:Datsun310Guy said:Hello Ford Mustang dealership!
Jalopnik had two stories today: GM denying the warranty claim of a Camaro ZL1 whose side airbag deployed from hitting a pothole, and Ford Mustang GTs suffering from mysterious ticking (which may or may not result in severe damage) for which Ford's fix is a block replacement. Neither one is great, although GM's customer service seems par for Snrub's experience.
Hello Hellcat?
In reply to Snrub :
PM’d you.
edit: since you’re Canadian, I feel I should clarify. I private messaged you, I did not Prime Minister you.
Thanks AngryCorvair - wrote you back.
General update:
I've had the car back for a week. It's just a matter of time before the CEL comes back on, but it's amazing how high the thresholds are for generalize poor engine operation. This morning, while idling the car there were ~1400 misfires on one cylinder, plus smaller amounts on other cylinders over a few minutes. The PCM seemed to store this count in the long term misfire count data. I had another incident with ~900 earlier in the week where it also stored the value. Yesterday the long term fuel trim hit -30% at idle for several minutes, again no CEL. I stopped/started the engine hoping I would help the PCM hit the needed thresholds for a CEL (I read a tech sheet which suggests you need to be over the threshold for 12 "cells", 2 startups), but no such luck. :)
I had started following this thread, then lost track of it for a few weeks. I figured that surely it would have been resolved. I'm so sorry that I was wrong.
At least the service department that you are working with now seems to be truly trying to help, albeit within the corporate and dealership restrictions.
Good luck!
Sometimes the CEL thing isn't just about hitting a threshold, it's about hitting it on 2 or 3 consecutive drive cycles. So if the fault isn't super super severe and is intermittent, sometimes you'll get a pending code but no CEL.
New metal will not solve the problem
It's in the electronics.
I agree the CEL thresholds often have multiple conditions to satisfy, including duration and occurrence on multiple consecutive starts. I'm truly surprised how poorly this car ran run without a CEL. If you search this for P0300 (multiple, or random cylinder misfire), or P0172/0175 (rich on bank 1/2), it's rather complicated to determine all of the requirements. https://gsi.ext.gm.com/gmspo/mode6/pdf/2017/17OBDG03%20Engine%20Diagnostics.pdf eg. misfires for 0.7%, but there are all sorts of enabling conditions and it states misfires on 1 cylinder may not be enough at low rpm. Two starts ago freeze frame long term misfires per cylinder hit 20/13/1368/37/58/36/52/35. Last start they hit 77/35/1586/121/93/57/61/100. Yesterday it was 2/7/967/3/5/6/0/5.
I also agree that electronics are a likely cause of my issues. How else can the same operating conditions sometimes cause horrible misfires sometimes and other times very few? At operating temperature at idle sometimes it runs steadily-10 LTFT, other times, -20-30, most of the time -14-17. It's all just enough that the plugs foul over time and eventually throw a code.
https://tenor.com/view/headache-migraine-ugh-sick-cronenberg-gif-4610846
enough already.
there is nothing grassroots about this. from what I can discern on this board carburetors are as complicated as most of us want to get and not many of us at that. 
Could it be something as benign as an errant fuel octane reading affecting timing advance?
In reply to JmfnB :
Not likely. That would not show up as misfires. GM is really good about knock control.
The more I hear about what gets tried and fails to fix it, the more I think it's dribbly injectors.
Knurled - would the AFIT test not have identified leaky injectors? (https://srs20assets.service-solutions.com/Media/OEM/GM/en-US/Product/CH-47976-220/Images/AFIT%20User_guide.pdf). After they replaced injectors 2, 4, 7, the backfires and exhaust popping significantly reduced. It seems to be becoming worse again. I suppose it's possible the injector on cyl 3 became out of spec since the test (?).
There are two conditions where things currently act up with respect to misfires.
1. Stone cold engine starts, after a tiny bit of heat gets into the engine, at low load or idle. If the engine has a tiny bit of heat into it when it starts, it will misfire, but it doesn't seem to get into the terrible state (intermittent, so difficult to say).
2. If I apply throttle there are misfires. Often when I remove throttle as well. Constant misfires can be observed by bouncing on/off throttle.
The LTFT is all over the map, I can't find a correlation to different conditions. Previously it seemed somewhat correlated to the conditions causing misfires, now not so much.
I'm trying not to get too deep in this. It just adds to the stress and there's not much I can do to influence the process. I don't feel I can risk doing the work to figure out the cause.
Where's that crappy mechanic that takes a customers car out for a "test drive" and totals the car when you need him?
Sorry to hear this is still dragging on.
Adam
The LTFT is why I asked if there could be an issue with the fuel sensor. If the sensor randomly reads that you are putting in E85 when you are running 87 octane (or vice versa) won't that affect your fuel trims and cause for unintended ignition advance or retard?
In reply to JmfnB :
It could but doesn't explain in this case why he has one cylinder mising 100x more than the others
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