2010 LLT - Intermittent ROUGH Accel. Potential P0306

[predatorftfw]

Long time lurker here. First off, thanks to everyone here. I've benefited SO MUCH over the years from info found in these forums. Ya'll rock.

I have a tendency to type too much so here's a TL;DR
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Vehicle -- 2010 2LT LLT with ~155k miles. Only major service was the timing chains several years ago. Otherwise, just oil changes, tires, etc. Primarily highway miles & I do about 30-40k/yr. For the past 4 years or so, it's had a CAI, ported throttle body, & a "mail-order" tune (from Trifecta iirc).

Car runs like garbage when accelerating in low RPMS. Idle is fine, flooring it seems fine. Car has a P0496 & my OBD reader "suspects" I have a P0306 but doesn't look like the car is reporting that. Has been SUBSTANTIALLY colder than normal here lately. Only real "Change" I can think of when I started to notice the issue. Planning on trying things in this order --> Install EVAP Purge to eliminate it as a problem. --> Check plugs/coilpack/wires --> Start looking at compression vacuum etc.

End TL;DR --> Excessive detail below.

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I say "potential" because from what I can tell. The ECU didn't throw the code. I use "FIXD" to scan my OBD II & it showed my P0496 (expected) as a "Detected problem", and the only code coming from the car. The app live-monitors the vehicle and shows a "Possible problem P0306 Cyl6 misfire". While I'm not confident in it's accuracy, my problem definitely feels like a misfire. The app logs freeze-frame data when it reads codes & it looks like the P0496 & the "possible P0306" were logged at the exact same time. 1902 seconds after engine start. I've been clearing the EVAP code for awhile now to keep remote start working.

I've had a P0496 for awhile now & haven't replaced the EVAP yet. Have one coming today just to make that code go away & rule it out as the cause. From what I've read though it doesn't seem like that would cause anything as severe as I'm feeling but I'm also not a professional by any means.

Symptoms: Rough acceleration. Best I can describe as "Wubbing" , maybe even a small surge up and down. I'm not sure exactly how much the car is fluctuating but probably something like a ~200-500 RPM "range". Kind of like what you'd expect if you just kept tapping the throttle 0%-20% repeatedly while going down the road. Happens primarily during low-RPM acceleration. If I notice it happening and don't adjust throttle, it'll get extreme enough I can feel the vehicle lurching because of it.

The weird part to me is it only starts to happen after the car gets to second gear (or higher) and hits about 2k RPMS. With "normal" acceleration, from park until the car decides to upshift, it runs like normal. Idle is also normal. Once the car decides to upshift, then it runs like hell. It started as a smaller problem & I thought nothing about it,( given the weather was -20F & roads were terrible, which isn't normal here.) It'd happen intermittently for awhile. Sometimes after the car warmed up, it'd stop. Sometimes it'd be fine for 30mins and then randomly start being a problem. Yesterday on my drive home from work it got very severe & that's when I saw the "potential" code. Drove it to the parts store today, still very severe.


If I give it ~50%+ throttle, or enough to get the car to downshift it'll start to accelerate fine. Holding highway speeds @65-75 with cruise on, it will have the issue while maintaining speed. However if I set cruse +/- a few MPH, I can usually get the car into a "sweet spot" where it'll run fine on cruise.

After some research I think working off of the "possible P0306", is my best lead for now. I cleared the P0496 thismorning before driving a mile or so to the parts store & back. Ran terribly but it wasn't long enough to throw any codes again, apparently.

A bunch of googling leads me to believe I should do things in this order.

1. I'm replacing the purge cannister later today when it arrives at the parts store, just so that's "out of the way". I don't think it'll fix anything.

2. Check plugs out & if they don't look bad, swap 8 & 6 coils/plugs/wires & see if maybe the APP would detect issues in CYL 6. I'm not confident in this one because it takes like 30mins to get the car to throw the EVAP code (I'm a bit hesitant to drive it further than necessary to go get parts), & since the car itself never threw the misfire code I don't know how much I can trust it's accuracy.

3. If plugs/wires aren't visibly bad & switching things doesn't change things, replace them anyways. I've had the car from 55k - 155k & I've never changed them so I assume these are the factory ones anyways.

4. If not fixed by step 4, cry. I have a friend coming up this weekend to look things over & help me do a compression check. I'm not mechanically inclined but I can usually handle simple stuff with a youtube video. Probably will go to a shop if we don't end up with a solution.

I've seen some stuff about the v6's hating winter gas? I did fill up at a different gas station than normal, about the same time as the issues started when it was -20F out. I'm down to almost empty & got some STP to dump in when I fill up later. I doubt that makes a difference, but worth a go? I usually don't go straight to "bad gas" since my bad luck is a bit more expensive than that.



Any thoughts in the meantime? Am I maybe taking the "possible" misfire code too factually & I should look elsewhere for an issue?

What about testing solutions? Thoughts on damage caused by further driving? It definitely feels "bad" but I'm not sure how exactly I'd get it to throw any codes when I swap cyl8 & cyl6 plugs/etc around without driving for ~30+mins.


If you made it this far, thanks for the read. Hopefully some excessive detail will be enough info to make up for my lack of mechanical knowledge.

EDIT -- Here's the "data" from the app if it's meaningful to anyone https://imgur.com/a/GWkJjjQ the data image is identical when looking at either of the codes, leads me to believe they pop up together but that seems more coincidental than anything?

Only thing I can see that my ignorance would find odd is 18% throttle for 650rpms? Seems a bit low on the RPMS let alone for 18% throttle. I'm not sure when it took the datalog & if "0mph" is accurate or just not recording. Pretty sure that's about idle RPM but I never really paid attention to what it idles at.

[predatorftfw]

Update --

Swapped plug/coil between 4&6 last night. Still ran like hell, but didn't throw a code afterwards. (I cleared before swapping them).

Drove it to/from work today to see if it'd throw a new code for cyl4 or if it stayed on 6. The dang thing ran just fine all the way to work (40 miles or so), no codes at all. Idled it for an hour on my lunch, same thing. No issues, no codes.

Got off of work tonight & it threw the EVAP code within a few minutes of starting the car & idling. Ran fine for probably 10 minutes & started hesitating on acceleration again, only much less severly. On a 1-10 scale, yesterday was severe @ a 9 & today would have been a 2, barely noticable unless I was paying attention for it.

Going to swap the EVAP out tomorrow & replace plugs & whatnot while I'm at it.

[predatorftfw]

Well, it never got really bad again nor did it give any misfire codes. Did 3 hours of driving & 2 days. Thismorning the driver window wouldn't work. It would would index but switch did nothing. Odd, fixed itself next time the car started. Replaced the EVAP purge & spark plugs since they've never been done. Found a bit of oil in #3 & 5. Drove another 2 hours, still light hesitation when it accelerates slowly, most notably in the 3rd-5th gear range.

[AlanRubin]

P0496 is a NON purge condition. Pressure is building up in the tank. That can be
for many reasons, from the canister to the purge valve on the intake manifold.

You need a GOOD scan tool to see what is going on with the missfires.

I have been having a #4 misfire issue, with P062B. I pulled the X2 connector off, and
tested each injector with test pins. Well, the test pins are thicker than the ECU
pins. Each injector tested 1.5 to 1.8 ohms, which is within spec. I plugged the X2
connector back in, and I had all kinds of misfires and injector codes. My guess was
the "T" pins spread the contacts in the female holes, so they were not tight holes
anymore. What I had to do was spread the pins apart in that first row. There is
another thread here on what I did, so I won't repeat it here. Needless to say,
I wold tweak the connector left and right, and it made a huge difference on
how the car ran. When I have it a slight twist to the left, it got so bad, I had
to turn the car off, with all the knocking and noise. One time it shut off by itself
it ran so bad. That's how I knew the ECU pins were losing contact inside the
holes of the X2 header.


I found the problem with mine, and it's the X2 connector.
https://www.camaro5.com/forums/showt...1#post10962891

[fhrex]

replace your purge valve

[predatorftfw]

Originally Posted by fhrex

replace your purge valve

That was done. Ran what I'd call 80% normal for 2 days and started to have issues today. Now I'm getting "tighten gas cap" (upon review it's a bit too easy feeling to take off) so maybe that? Guess I'll order one. Definitely not a tight "lock" when it clicks. Or the rear purge?

https://youtu.be/2JliulbfZqI

Took a video of the readings while driving. Wtf is going on with the pressure? Seems to like to be in that 18k range a lot but drops if I smash the gas. I'm pretty sure I heard a "woooshhh" sound from the rear of the car when I did it, but I was on the highway & may have heard something else.

Throttle position compared to rpms/speed (partway in the video I switch to a speedometer) seems a bit funky too?

[predatorftfw]

Originally Posted by AlanRubin

P0496 is a NON purge condition. Pressure is building up in the tank. That can be
for many reasons, from the canister to the purge valve on the intake manifold.

You need a GOOD scan tool to see what is going on with the missfires.

I have been having a #4 misfire issue, with P062B. I pulled the X2 connector off, and
tested each injector with test pins. Well, the test pins are thicker than the ECU
pins. Each injector tested 1.5 to 1.8 ohms, which is within spec. I plugged the X2
connector back in, and I had all kinds of misfires and injector codes. My guess was
the "T" pins spread the contacts in the female holes, so they were not tight holes
anymore. What I had to do was spread the pins apart in that first row. There is
another thread here on what I did, so I won't repeat it here. Needless to say,
I wold tweak the connector left and right, and it made a huge difference on
how the car ran. When I have it a slight twist to the left, it got so bad, I had
to turn the car off, with all the knocking and noise. One time it shut off by itself
it ran so bad. That's how I knew the ECU pins were losing contact inside the
holes of the X2 header.


I found the problem with mine, and it's the X2 connector.
https://www.camaro5.com/forums/showt...1#post10962891

Will take a look. Now looks like after replacing the purge valve I got a p0455 once but now just check gas cap on the screen

[predatorftfw]

Originally Posted by predatorftfw

Will take a look. Now looks like after replacing the purge valve I got a p0455 once but now just check gas cap on the screen

Thought I replied on my thread but jacked his. Oops.

Didn't make a difference. But had a weird issue yesterday where my AC didn't work until I had been driving like 10 mins. No light in the button and no cold air. Tried it down the road and it worked. Car was running rough before, ran fine after. That combined with it shifting oddly, the windows randomly not working last week and then working again. Maybe computer is the right place to look. Trying a battery since it's at least 5 years old and had a bad week over the winter dying and getting jumped. Otherwise, still no idea what's going on.

[AlanRubin]

If you have a bad ECU connector like I do, you will get all sorts of random issues. My
TPMS was not working, now all of the sudden, I got readings. Not sure if it will
stay that way. Started it up Thursday, it ran, no misfires. Have to put rear knuckles
on it before I can drive it. Also on my 4th throttle body, this car keeps burning them
up. Cam phasers are also starting to knock.

[predatorftfw]

Originally Posted by AlanRubin

If you have a bad ECU connector like I do, you will get all sorts of random issues. My
TPMS was not working, now all of the sudden, I got readings. Not sure if it will
stay that way. Started it up Thursday, it ran, no misfires. Have to put rear knuckles
on it before I can drive it. Also on my 4th throttle body, this car keeps burning them
up. Cam phasers are also starting to knock.

Any way to tell when wiggling doesn't make it worse?

Battery didn't fix things. Car seems to hate sitting at work all day but doesn't mind sitting overnight at home. 99% of my issues have been on my drive home from work (usually instantly after starting), and it runs mostly fine after sitting overnight. But, I'd assume sitting outdoors for several hours is the same regardless of location.

Checked o2 sensor voltages when idling after I got home.
Took a recording of it. Shouldn't 2&6 be fluctuating like 1&5? httpss://youtu.be/MU1FoysfQrI

[predatorftfw]

Twisting it both directions did not impact the vehicle.

Can hear what sounds like a small vaccum leak around that front passenger area. I can't tell for sure over the radiator fan but I'm thinking it's the vaccum line for the... Brake booster pump, I think?

Gonna see if I can hear it better in the morning while cold.

[AlanRubin]

I can hear the hiss through the PCV on mine, that is probably what you hear. Having
an LLT engine, I removed the PCV valve and drilled out the holes to the new sizes,
that made a big difference in the hissing sound through the hard-line to the intake
manifold; through a catch can first though. The PCV was updated by GM in 2012,
with a 7/64" top hole, and 5/64" bottom two holes. I drilled my top to 9/64". When
I pulled it, the bottom holes were clogged! This was the problem, so GM updated
the part later in 2012.

The brake booster pump only comes on when needed. Some have the vacuum
sensor switch right at the booster where the line goes in. If not, the vacuum pump
only comes on at hi-idle when you brake. I had a bad pump, and I had no brakes
when first starting the car and it was at hi-idle. I replaced the pump, and it was fine.

[predatorftfw]

Spent some time in the shop. They replaced front wheel bearings (needed anyways). Said they detected a misfire on 5, swapped that coil with 4 and never had a misfire again. Ran fine for them. Drove it 1.5hrs home and it was fine.


Took it to work today (hr drive each way) and on my way home... Go figure. Still "stuttering" in rpms but more rapidly (think shorter wavelength I guess) and less severly this time. Come to a stop sign and boom, check gas cap. P0455. Car starts to run a bit better after that, assumedly when it went into open loop?

Any thoughts before I send it back? Still seeing pretty negative fuel trims. Odor of gas in the exhaust but I have to basically huff the pipe to smell it

[AlanRubin]

Do you smell any gas fumes?

It should not go into OL with P0455.