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Jerky when accelerating

Tiny
MOER SKONT
  • MEMBER
  • 2011 AUDI S3
  • 2.0L
  • 4 CYL
  • TURBO
  • 4WD
  • AUTOMATIC
  • 300,000 MILES
Hi,

My car acceleration from standstill is not the smoothest. Occasionally, I also get a cold misfire. It feels as if it intermittently cuts fuels. When on partial throttle, it jerks but when I floor it, it drives better. I changed the following: O2 sensors (both), MAF sensor, Fuel pump, fuel filter, plugs and coil pack. But I still experience the same issue.

However, each time I disconnect the MAF sensor, it drives far better (although I later get a system rich code). And when I reconnect the MAF, it starts again. MAF is new and OEM. No error codes on scanning using OBDII.


Please help.
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Sunday, March 20th, 2022 AT 10:25 AM

21 Replies

Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,885 POSTS
When you disconnect the MAF, the computer goes into a manufacturer-provided backup. So, the issue is somewhere in the fuel/air system.

You indicated there were no codes. Were there codes originally that led you to part replacement? If so, what were the codes?

Also, when you scanned the computer, was the entire CAN scanned or only the powertrain?

Let me know.

Joe
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Sunday, March 20th, 2022 AT 8:52 PM
Tiny
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Hi Joe,

Thanks for the reply. I have not had any codes at all before I started replacing any parts. I was pretty much guessing what the issue could have been. Part of the reason I suspected the parts I replaced were because I had not changed them in 11 years (except for plugs which I changed 5 years ago).

I'm assuming the scanner scanned the entire CAN as I check marked every option before proceeding to scan. And the result showed no errors.

By the way, I had carbon cleaned and injectors cleaned about 3 years ago, new air filter few months ago. Intake valves looked so much better after carbon cleaning.
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Monday, March 21st, 2022 AT 8:36 AM
Tiny
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Okay, since it ran better with the new MAF disconnected, I need you to check a few things for me.

First, if the scanner you have can read live data, I need you to let me know what the short-term fuel trims are. That can help me determine if there is an air/fuel mixture issue.

Next, the engine coolant temperature sensor can also be a possible issue. With the engine cold (before starting for the day), hook up the scanner and check what signal is being sent by the ECT. If you, do it before starting, the signal temperature should be very close to the ambient temperature.

Let me know what you find. Also, let me know if you have checked for an engine vacuum leak and if you have checked fuel pressure. If you haven't, here are two links explaining how it's done. You will need a fuel pressure gauge, but most parts stores will lend one to you.

https://www.2carpros.com/articles/how-to-check-fuel-system-pressure-and-regulator

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

Let me know what you find or if you have questions.

Take care,

Joe
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Monday, March 21st, 2022 AT 7:24 PM
Tiny
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Hi Joe,

When I last checked; the Short Term fuel trims are flatulating between +-6%. LTFT is close to 0%. The upstream lambda probe usually fluctuates between +0.05mA and -0.05mA on the scan tool live data, while the downstream oxygen sensor is pretty steady at around 0.5V.

I will check the engine coolant temperature before startup and update you on what I find it to be.

Thanks for the links. I certainly need to know how to check for a vacuum leak. My fuel pressure on the scan reads about 5000kpa at idle and increases as I open throttle (not sure if I still need to get a fuel pressure gauge).

Regards,

Gerald
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Tuesday, March 22nd, 2022 AT 4:07 AM
Tiny
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Hi,

Trims sound fine. I do have a question. You indicated 5000kpa fuel pressure. Are you sure that is correct? That equals 725 PSI. The manual indicates 3.5 bar to 6 bar which is between 55 PSI and 87 PSI. I would think around 400 kpa. Please confirm that for me.

I'm questioning that because we could have a kinked line, or something is happening that shouldn't be. However, I don't believe the system could handle that amount of pressure.

Next, you indicated the O2 sensor voltage in mA. Was it recorded in mA or mV? Take a look through this link. On average, the sensor should be around 150mV.

https://www.2carpros.com/articles/can-scan-controller-area-network-easy

Also, is there any variation at the sensors when you hit the throttle quickly?

Let me know.

Joe
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Tuesday, March 22nd, 2022 AT 6:27 PM
Tiny
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Hi Joe,

The scan tool indicates an average of 5000kPa. I suspect I might have incorrect settings and it is probably 500kPa. The upstream (pre-cat) O2 sensor shows a reading in mA and the post cat shows them. I'm not sure if this is could also be a results of settings on my scan tool.

I will record a video of live data of the MAF sensor airflow rate, both O2 sensors, fuel rail pressure as well as fuel trim (STFT), which I will share with you.

Can a bad diverter valve cause jerky/hesitation when taking off or when cruising at 120km/h (foot lightly on throttle)?
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Wednesday, March 23rd, 2022 AT 3:01 AM
Tiny
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Hi,

Anything is possible. The interesting thing to always remember when working on an Audi is this. They are extremely touchy. If there is a voltage issue, it will cause problems. If torque isn't correct, it will affect the operation, and if there is corrosion on any connector, it too will likely cause issues. However, when they are running properly, they are absolutely a great vehicle to drive. I have an A4 Convertible which now has 160,000. It still feels new when driven.

Regardless, let me know what you find, and I'll watch for your reply. Also, please reconfirm the fuel pressure. I'm hoping it's wrong. For it to be that high, I would suspect there is a kink or restriction before the injectors which could be starving the engine of the needed fuel. However, no matter how I look at it, it's hard to believe it's producing that much pressure.

Take care,

Joe
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Wednesday, March 23rd, 2022 AT 4:26 PM
Tiny
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Hi Joe

I have recorded a couple of videos which I have attached below. I have examined the diverter valve and it seems to be in good working order. Plus it's a D revision which has a piston as opposed to the G revisions which had a diaphragm.

I have also made a recording of some live data which includes MAF airflow rate, OS sensors current and voltage, fuel rail pressure as well as STFT(No audio though). I added a video that shows hunting revs at idle (this is intermittent)

I have also relaized that the evap cannister hose had cracked and resealed using some adhesive like Q-bond ( I read that the Evap could cause the issues i have). I plan to replace the hose. Do you reccomend i also replace the cannister?

In the event that the attached videos' quality is poor, please view them on the following youtube links: https://youtu.be/g38nYo3YIfY , https://youtu.be/f9JW-sZTBPY , https://youtu.be/vMUJqsun4Mk

Regrds

Gerald
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Thursday, March 24th, 2022 AT 12:04 PM
Tiny
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In case the video quality is not great, please follow the youtube links below for each short video
1. Live data video : https://youtu.be/f9JW-sZTBPY
2. Hunting Revs at ide (interminntent) https://youtu.be/vMUJqsun4Mk
3. Diveter valve https://youtu.be/g38nYo3YIfY

Regards

Gerald
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Thursday, March 24th, 2022 AT 12:26 PM
Tiny
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Hi Joe

I have sent a reply with video attachments as well as some YouTube links. I cannot seem to find the reply in the communication thread we've since had. Please let me know if you have received a response prior to this one.
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Thursday, March 24th, 2022 AT 12:34 PM
Tiny
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Hi,

No, I haven't received anything since my last reply until this. Did you send it to this thread?

Let me know.

Joe
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Thursday, March 24th, 2022 AT 8:02 PM
Tiny
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Hi Joe,

I have recorded some vides which I have attached below. These include a video of live data for MAF airflow rate, O2 sensors' current (Pre-cat) and voltage (post cat), fuel rail pressure as well as STFT.
Another video shows revs hunting a bit at idle (this is intermittent). I have inspected the diverter valve and it seems to be in good working order (plus it's revision D with a piston and not a diaphragm).

I also found that the EVAP cannister hose has some cracks on it. I have already ordered a replacement (OEM).

Please let me know if the video quality is good enough. I can email you YouTube links of the same videos if need be.

Regards,

Gerald
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Friday, March 25th, 2022 AT 2:43 AM
Tiny
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Hi Joe,

The air intake temperature read 23 degrees Celsius and the ambient was measuring about 18 degrees Celsius. All this on cold start.

As soon as I get hold of WD-40 or similar solvent, I will do the vacuum test and relay feedback to you.

Regards,

Gerald
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Friday, March 25th, 2022 AT 10:34 AM
Tiny
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Hi,

That is a little variation, but I don't think that is causing the symptoms. When you check for vacuum leaks, use carburetor cleaner. WD 40 won't do the trick. LOL

Let me know how things turn out for you or if you have questions.

Take care,

Joe
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Friday, March 25th, 2022 AT 12:41 PM
Tiny
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Hi Joe,

I hope you managed to view the videos I have attached/uploaded in the previous reply on March 25.

I did a vacuum leak check as per instructions in the link you had sent earlier. The test does not seem to yield any results that suggest the existence of a vacuum leak.

When I inspected the diverter valve, I noticed that it has two parts that fit into each other and on removal, the two parts came apart. I was wondering if the valve should be one solid unit when in good condition?
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Monday, March 28th, 2022 AT 10:49 AM
Tiny
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Hi,

The diverter valve shouldn't be two parts. It is a one-piece unit. Also, the updated one usually has a blue ring at the end.

Also, I did see the videos. In the first one, I noted the jumping of the tachometer. The way that acted appeared to be more electrical than mechanical.

By chance, have you reconfirmed fuel pressure?

Joe
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Monday, March 28th, 2022 AT 6:08 PM
Tiny
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Hi Joe

After a couple of months of back and forth with the car issue that I initially sought assistance with, I found the problem. Neeldess to say, I resolved it.

Few weeks after the last text to you, I took the can in for diagnostics with a better scan tool than mine. The first error code pointed to the N249 (N75) valve. After changing that, the car still had the same problem. I changed the diverter/dump valve too after a P2261 error code.

Since both N75 and diveter valves are simply just valves, I thought I should check out the actual sensor that sends boost pressure info to the PCM. I changed the boost pressure valve, and voila! The problem is now resolved.

The drive now is as smooth as an egg. And I thought I should share with you this development

Regards

Gerald
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Monday, July 25th, 2022 AT 9:55 AM
Tiny
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Gerald:

Thank you for the update. It's appreciated. Also, I'm glad to know you got it fixed. Isn't it interesting the sensor never set a code? They can drive us crazy. LOL

Regardless, again thank's for the update. Also, please feel free to come back anytime in the future. You are always welcome here.

One last thought. I'm not sure what type of scan tool you have, but for personal use, I purchased one online specifically for VAG. I only paid about 40.00 for it, and it has been extremely helpful. It provides the P code or the VAG code, whichever you want. If you don't have one specific for VAG, spend a few bucks and get one. You'll love it.

Let me know if you want the brand or anything that I got. It's in my toolbox, and although it isn't far away, I'm exhausted and it feels like it's a mile away. LOL

Take good care of yourself.

Joe
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Monday, July 25th, 2022 AT 7:57 PM
Tiny
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Hi Joe,

I definitely will be getting the VAG scan tool. Coincidentally, I was browsing the OBDeleven VW Bluetooth Diagnostic Tool web looking for a VAG scan tool. It’s in my cart and I’ll be paying for it soon. The brand name for the o e I intend on buying is OBDeleven VW Bluetooth Diagnostic Tool.

The one and only minor drawback is that it works with Android operating system, and I’ve only ever used iOS in my life. I hope to find one that’s compatible with iOS.

Once again, thank you.

Regards,

Gerald
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Tuesday, July 26th, 2022 AT 2:58 AM
Tiny
JACOBANDNICKOLAS
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You may want to get one that plugs into the DLC. It is quicker and easier to use. Plus, there is no issue with blue toothing it to your phone.

It's just a thought.

Take care,

Joe
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Tuesday, July 26th, 2022 AT 7:50 PM

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