I'm driving a 2006 Jetta TDI. I bought it a few months ago.
Within a few weeks of buying it, the engine started to stutter every so often. Usually after two consecutive drives of about 20 kilometers or more. The car would get better for a while, then the problem would be back.
At first I thought it might be bad diesel, something I haven't completely ruled out, but I don't think that's the problem.
I've since taken it to the mechanics and paid over $2000 having the fuel filter changed, the EGR valve changed, various carbon deposits cleaned, various valves changed, all to no avail.
After the first service, the car ran good for a day, then the same problem came back.
After a second mechanic visit, the car ran fine for several hundred kilometers, then started running rough and stuttering again, from time to time, with increasing frequency.
The car would run fine for a while, then start stuttering, then get better, then start stuttering.
There seemed to be a spiral, until it got really bad and was just always running rich and stuttering at low RPM.
Thick blue diesel smoke clogging the air, stutter, stutter, stutter go... Once I got it over 3000 rpm, it ran better, but every time I stopped or changed gears, I'd have to stutter it up to higher revs to get it going again.
The Quick Fix
I stumbled upon a quick fix solution. The other day, I disconnected the N75 valve - essentially disabling the turbo. I was wondering if the N75 Valve might be the problem. I forgot to reconnect the wire lead and started the car. The car runs slow without the Turbo but it runs fine, it doesn't stutter or run rich.
Here's the kicker. After a spin, noticing it ran like a donkey and not a stallion, I reconnected the N75 and the car ran fine for the next couple hundred kilometers.
Today it stuttered again, I disconnected it, drove it for a few minutes, reconnected it. Problem solved again.
Are there any mechanics out there who can tell me what the problem might be? Is it the N75 valve? IS it an EGR problem? Bad gas? A wiring issue? Hoses incorrectly connected?
If you're having similar problems, try disconnecting the turbo, taking it for a spin, reconnecting it, and see if it temporarily fixes the problem.