Gambba

Looking for anyone with a good knowledge of a 1.8T to support my theories on my problem.

Car is an '99 FWD A3 1.8T with Oettinger chip, pumping out around 210hp and what started as an occasional stutter in the rev range has now over time changed to hesitation/stutter right through the revs in every gear, unless you are extremely light on the throttle.
The cars jerks as it accelerates and occasionally in 2nd once off boost is quite happy to rev reasonably freely. The noise that goes with the problem sounds almost like the wastgate is constantly blowing/chattering but it is definately a misfire of some sort as confirmed recently. Idle is normally as smotth as a whistle and even until the cars been run for a short time and off load the engine revs very freely. Boost is good as when under acceleration the DV can be heard blowing off.

So far I have checked everything external to the engine that I can as follows;
Checked for DTC's of which there are none!
VAG-COM logging runs show MAF, Temp sensors etc to all be good and consitant with the runs and conditions.
Coilpacks swapped/replaced.
Sparkplugs replaced.
Amplifier replaced. (For ignition system)
MAF confirmed good (And was only replaced 4 months ago)
Injectors tested for frequency, flow and pressure all of which are good.
Compression test showed all cylinders good.
N75 solenoid replaced.
MAP cleaned and checked
DV swapped/replaced (Between piston and diaphragm)

The turbo was replaced around 6-7 months ago and has no leaks or problems. No smoke from exhaust and no abnormal oil consumption, and the car has only done 103,000 km's (65,000 miles)

My guess is that it may be a hydraulic lifter (Tappet) which ocassionally sticks or is worn, but any input would be grateful, as tappet replacement is costly and time consuming.

xr4tic

lots of bucking/jerking at full boost, no error codes.

Turns out it was a bad connection at the barometric pressure sensor (located next to the ECU on A4s) Only reason I found it was from looking through the VAG-COM data blocks and noticed the atmospheric pressure was way off. It should be in the 950-1050 mbar range, depending on your altitude, mine was ~2000 IIRC.

Some things to check in VAG-COM are the baro sensor readings, fuel trims, and ignition retard from the knock sensors (don't have the datablock numbers handy at the moment).

mAd1(gt3071r)

that's where I've had similar issues. Seemed that the one I got from Neuspeed was defective. Swapped it with a 5bar and no more shudder in my situation.

JAFO

fuel filter?

rabidsquirrel

I am having some similar issues as well. At first I was experiencing some hesitation in a specific RPM range when spooling up the turbo (Under boost). My A4 B6 was hesitating in the 3.5~4.5K RPM range under boost. At that time, we knew that the motor mounts were going, and that it was eating ignition coils every other couple of months (I've replaced 5 since I've owned it ~ at the 80K miles mark now).

I did some research and wound up ordering some new motormounts, transmission mount, and front dog bone mount (Apikol and "Track Density" mounts from 034 Motorsport). I also ordered their coil packs (An older S4/S6 OEM design that mounts directly to the head) and diverter valve (It seems very similar to the forge valve).

I've installed everything and the new ignition coil packs. There is a notable improvement throughout the rev range (Smoother and Stronger Revving) except for where I was previously experiencing hesitation. That hesitation has now become outright misfires with the new coils.

I've taken it to the dealership (This is the third time I've tried to get them to Acknowledge, Diagnose, and Repair this issue). Their first response was "It's the strange coils you are using!" I told them that they needed to review their notes and realize that this is the third time I've brought it in for this particular issue, and the previous two times I was running the coils that they were familiar with (I also politely told reminded them that they should be doing their jobs and not waste my time making up excuses about why they can't figure this out). I told them that these coils are better/stronger, and thus magnifying the previous issue.

They pulled the codes. I was missing randomly on all four cylinders. I said again that it isn't the brand new coils, because it was doing the same thing with the other coils. I asked them if there were any other codes, and they said there was one. It was getting some strange low pressure code on a throttle body sensor according to them. I said, "that is what you should be investigating!" In my mind, my throttle application and rev range specific misfiring is more likely to do with a faulty sensor, vacuum line, or boost pressure related causing these random misfires...

They keep talking about the coils though. I'm nearing my wits end with these twits!

I'm guessing you got this sorted out by now? If so, what was the problem?