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Discussion Starter #1
I purchased a used 2016 Impala LT 3.6L V6 a couple months ago and I've found a few issues with it that I'd like a little bit of help with diagnosing. A few things to note: The car is now out of it's factory warranty (just turned over 60k miles), it's clean, but I believe it was stored in a barn at one point with mice because I've found dog food in between the hood liner and hood, and some insulation underneath the plastic engine cover. Besides that, I have not found any mechanical damage due to mice.

1. Hesitation during acceleration: I've had this issue a couple times since owning the vehicle and I can't really replicate it on demand, but the issue typically occurs when attempting to accelerate onto the highway. This morning I got the worst hesitation I've felt yet. From turning onto the on-ramp at around 25mph, I increased throttle pressure steadily but not aggressively and it felt as if the engine was starting to miss. I'm leaning toward a bad or clogged injector or fuel filter. I just dumped in a bottle of Lucas Oil Fuel System cleaner at my last fuel fill up, so maybe that will help, however I'm not noticing any improvements yet. I'm also a little worried that there could be gunk on the back of the valves due to the PCV system and direct injection. I'm going to probably get an oil catch can installed on this car like I did with my 2014 Silverado. (I'm a firm believer in them as I've managed to filter out about 18 fl oz of oil and from the catch can in only about 12k miles. If I did not have the catch can, all of that would have been dumped right back into the intake and could have been building on the back of my valves.) Does anyone else have any ideas?

2. Slow 2-3 up-shift: This issue occurs about once a week, and typically occurs at low speeds and low load. When shifting from 2-3, transmission releases 2nd gear, engine RPM's raise slightly, then transmission engages into 3rd gear. This whole process takes about a full second. It doesn't slam into gear at all, but the disruption of power transfer causes a small jolt in the vehicle's shift. I have drained and replaced the transmission fluid twice now with no improvement. I realize that this is not a full fluid flush, and I do have plans to drain and replace the transmission fluid again at my next oil change. When I first drained my transmission fluid, it was darker than it should be, however it wasn't black. All of the pink color had gone away and it smelled slightly burnt. All other shifts are very smooth, though I would like to hook up HPTuners to the car and firm up the shifts a little bit, and if these transmissions have a learning feature, reset it.


3. Vibration from right front wheel on cold tires: When I bought the Impala it had a set of Firestone Firehawk A/T's on it and although they were soft and grippy, they were unbearably loud. I also noticed a small vibration from the right front of the vehicle with these tires at certain speeds. I've had issues with vibrations on my Silverado before, and it was narrowed down to junk tires (even though they were Bridgestones that cost $350 a piece!!). I swapped the right side tires front to back to see if the vibration followed the tires, however there was no significant change. So I decided to buy a new set of tires for the Impala, mostly to get rid of the terrible road noise. I got a set of Pirelli Cinturato P7 All Seasons and the road noise is dramatically reduced. However, I'm still getting a vibration from the right front wheel. It is especially prevalent when the tires are cold. However, after a few miles the vibration nearly goes away entirely. I can still feel a slight vibration when I turn slightly to change lanes on the highway, but when I'm driving straight, it is very smooth. The vibration doesn't get worse under hard acceleration (so I would believe that rules out an axle), and it doesn't make any sounds while turning (ruling out a wheel bearing). Maybe I've got another bad tire, and I will be rotating them at my next oil change as well and I will see if it follows this tire at all.

4. Ticking sound from engine bay when engine is at full temp and idle: I've traced the sound to a small canister at the back of the engine bay, however I cannot seem to find out exactly what the part is through the GM parts diagrams online. As you're looking at the engine bay from the front, the canister is between the firewall and throttle body (slightly to the right of the throttle body). It has a coupler and two wires running to the throttle body, and a stainless steel line running all the way back to the fuel tank. It is dark brown, about 1 1/2" in diameter and about 2" long laying sideways (left to right). There is a black hard line routed above it with corrugated plastic around it. I'm assuming that it is emissions related, however I can't seem to find the part online. The ticking is very fast, and audible when the hood is open, however when the hood is closed, it is difficult to hear. I cannot hear it when I am inside the car at all.

Any information that can be given is very much appreciated. Share your experiences if you've had similar ones and hopefully we can work together to help out anyone else having similar issues!
 

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Discussion Starter #2
Just gonna give this a quick bump to see if anyone will share their ideas.

Any help is appreciated!
 

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I have the same flex-fuel model and experience the same issues with acceleration and shifting. From my research, I'm led to believe that these transmissions are prone to these issues, regardless of storage conditions. There are chips out there that claim to improve throttle response and smooth out shifting but I can't imagine it's that easy of a fix. Good luck and please update if you find a solution. Going to dyno mine this weekend and go from there.
 

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The ticking and canister you're describing is the high-pressure fuel pump. The "ticking" is normal as it's building very high PSI (don't want to throw out a wrong figure). Near a building, like in a drive-thru, you may even hear the pressure release (it's pretty cool sounding to me).
 

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Discussion Starter #6
The ticking and canister you're describing is the high-pressure fuel pump. The "ticking" is normal as it's building very high PSI (don't want to throw out a wrong figure). Near a building, like in a drive-thru, you may even hear the pressure release (it's pretty cool sounding to me).
That's actually incorrect. It was the the EVAP Purge Solenoid. I have replaced that as I got a P0449 engine code, and it was the most suggested part to replace.

After I replaced that, the ticking went away. Unfortunately, the code did not go away. So my next part to replace is the EVAP Vent Valve Solenoid which is on the top EVAP Canister, requiring me to drop the fuel tank out of the car.

The ticking was caused by the solenoid actuating back and forth. During normal operation, the solenoid is supposed to be closed, and when the throttle body is open (i.e. you're hitting the gas) the solenoid opens. What this solenoid is doing is allowing fuel vapors from your fuel tank to enter your intake manifold to then be burned when it goes into your cylinders.

I have another forum for the P0449 code if you'd like to follow my progression through that issue.
 

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Macjubz, did you get any insight on your similar issues from the dyno run?
I apologize for the delayed response, had no idea there was a reply. The dyno I had done by a friend of a friend didn't yield any conclusive data. The problem didn't replicate itself as usual but more importantly the ability of the individual performing the test rose my suspicions so I only ran it once. I installed the Jet Throttle Xcelerator (part # 40108) which although worked very well, increased the issue substantially. The shift "hiccup" began occurring consistently on the higher Jet settings when I gave it too much throttle between 2nd & 3rd. I ordered a replacement through Summit Racing to confirm that it wasn't a faulty unit and I had the same issue. I have since removed the Xcelerator and the issue still arises at random intervals. Held on to the Jet unit so I can replicate the issue during the next dyno. Finding a reputable shop to run a dyno in the central valley of Cali has proved a little harder than I expected but when I have the results I will post them. Apparently machine calibration is a real issue out here,,
 

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From what I've read and experienced first hand, this "shift flare" is a very common problem with the 6T70 and 6T75 transmissions. I have both transmissions between my '17 Impala and '16 Traverse and both have the problem. The Traverse is different though in that it only does it under moderate to hard acceleration. Backing off the throttle right before the shift avoids it. In my Impala, it only happens under lower throttle. Regardless, it always pisses me off when it happens since my wife will only buy GM and this is our reward for brand loyalty.

Both started out happening only for the first shift of the day, but eventually began happening more often. Haven't tried it on the Traverse, but unhooking the battery for several hours on the Impala does seem to clear the learned TCM settings and results in better shifting for a while (1,000 miles?).

I've read that the shift flare is usually the result of the line pressure being too low when it comes time to shift. So to avoid damage from low pressure, the converter releases leaving 2nd and waits until pressure gets high enough then reengages once in 3rd. I don't really understand if this is an issue with the pump design or bad tuning. But, my Traverse has a tow button that you can engage when trailering. When tow mode is engaged, the line pressures and shift points are increased. When this is on, the shift flare does not happen regardless of throttle.

So, that said, I would think some clever tuning could remedy the shift flare in the Impala.

2. Slow 2-3 up-shift: This issue occurs about once a week, and typically occurs at low speeds and low load. When shifting from 2-3, transmission releases 2nd gear, engine RPM's raise slightly, then transmission engages into 3rd gear. This whole process takes about a full second. It doesn't slam into gear at all, but the disruption of power transfer causes a small jolt in the vehicle's shift. I have drained and replaced the transmission fluid twice now with no improvement. I realize that this is not a full fluid flush, and I do have plans to drain and replace the transmission fluid again at my next oil change. When I first drained my transmission fluid, it was darker than it should be, however it wasn't black. All of the pink color had gone away and it smelled slightly burnt. All other shifts are very smooth, though I would like to hook up HPTuners to the car and firm up the shifts a little bit, and if these transmissions have a learning feature, reset it.
 

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Discussion Starter #9
Yeah, I was hoping to use HPTuners and bump up the shift pressures a little bit in hopes that it would fix it.

Haven't gotten around to that yet as I've had other, bigger problems now come up.

I've had a code for random misfire which I cleared after it started running smoothly again (after 10-15 seconds), and recently I got a cylinder 3 misfire. I'm hoping to buy a new coil pack for that cylinder and replace all of the spark plugs. It's odd that the misfires only happen after leaving the car sit for a while (3 days - 2 weeks).
 

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I have a 2015 and at 71K miles they just replaced the Torque convertor and a solenoid in the tranny. I was experiancing similar issues as you are and finally got a check engine light which pointed to the Torque convertor . Within 500 mile of light going on it became undrivable.
 
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