Hello all!
I'm not coming today to share a new story but I'm coming with a problem that is a bit weird.
About a month ago, my car (2000 A6 2.7T 6sp Giac3) started getting the P0300 code which stands for Multiple Random Misfires. The misfires only happened soon after starting the car, mostly when the engine was warm. When the misfiring was happening, the check engine light was flashing. It never happened more than once. It would do it then stop. Today, on my way to a wedding (best time isn't it?) the issue started happening continuously. ONLY at part throttle (vacuum between -4 and -14) the car would seem to lose power for a few seconds then come back to normal, then lose power, etc. If I would let go of the accelerator or get on it, the issue would go away. I switched to the stock program and that didn't make any difference. I unplugged the MAF sensor to have that subsystem run open loop, but that didn't fix it. Since I'm at 110k I'm guessing the O2 sensors are on their way out. I know the sensors only affect fuel mixture at part throttle so that's the only correlation I see. Also, the check engine light flashes on each hesitation. Timing belt was done at 90k.
Any ideas or pointers on what could be wrong? I'll hook up VAG COM tomorrow and monitor more stuff but maybe somebody knows something about the symptoms I described.
Honestly, this is the first time in 2 years that this car gave me any trouble so I guess it was time
Thank you in advance!

 

See what happens. It is not the O2 sensors.

 

iMHO U needs to scan and see what the current codes are before throwing parts at it. OTOH, if you're due for plugs (what, 30K miles? .028"?) it wouldn't hurt to do them.

 

...inlikely to cause a misfire.

 

It was the coolant temperature sensor.
I actually bought the revised green one from ECS tuning from about 2 yrs ago when I did the TB job but forgot to replace it. Good thing I remembered where I kept it My original sensor was blue, not black as ECS says. So, even if you have a blue sensor, replace it. Seems to be a high failure rate item.
The plugs were done about 10k ago with iridium ones so I ruled that out form start.
I noticed the new style coolant sensor actually has a prong that sticks out into the coolant flow versus the old style which was just flat.
Oh well, replacing the sensor was quite simple because I did not open the coolant reservoir cap and I did it with the car cold. Barely any coolant came out.
Car runs great now and the idle seems to be even smoother, it used to have a barely noticeable lump that I could not make out... now I know what it was.
Thanks all for the help!
This forum rulez!

 

Car started doing the same old thing later in the day.
I also got a new code for the oxygen sensor on bank 2 sensor 1.
Using vag com I could see that the oxygen sensor in question was not returning any reading.
Time to replace it.

 

I would consider replacing the plugs with OEM as well.

 

The problem was that the driver side primary O2 sensor died. The secondary O2 sensor was pulled a bit out of the flow so was currently not seeing any fuel. The the primary died, the ecu tried using the secondary to judge the mixture. Due to that, it would run too rich, causing misfires at part throttle.
Replaced both O2 sensors with the bosch universal type and everything is back to great.

Thanks for all suggestions.

 

Honestly, the 02 sensor seems unlikely. Misfires would usually be caused by a problem with spark, air or fuel. Since you're getting 0300- this is a bit of a pain, since I believe that is the Random Misfire code and doesn't say that a specific cylinder is misfiring.

Normally, I would look at the coil packs for this if the spark plugs are fine. Since it is random, I may look at something like a fuel pump or air leak. The air leak could be tested with some throttle body cleaner being sprayed around the engine while it is running. If the engine revs up when you spray it, you know that the cleaner got sucked in through something in that area...

I hope it was the O2 sensor- but if not then I would look at these items next. Good luck.

 

What i forgot to mention is that when i used the vag tool i got a lot more codes including misfires on cylinders 4,5 and 6 which are the ones on the same side with the bad O2 sensor. So, yeah, now it's running great.