Bangerla

My car did the same thing. Would start misfiring violently at 50mph. I changed the knock sensors and cured the problem. Make sure you torque exactly to what is specified in Bentley, or you will still have misfire problems.

Chris Franks

didn't read the whole thread, but if you have misfire issues, the "ignition amplifiers" commonly fail and did in my 98. They are 2 thin, square, black plastic pieces in the area of the right front shock tower, mounted back to back on a metal plate. When researching I saw that some said there's was further back toward the firewall by the BCM or ECM, but mine were not. I got one of ebay for under $50 new. You can check them very easily with a multimeter, there are 5 leads going in, and 4 coming out, test for continuity one side to the other,as they are just resistors inline. The 5th input is just 12V.

adobian

So what are the conclusions ?

Sorry for waking up and old thread.

It looks like the cause of this problem was:

Misfiring
Torque Converter
Cats
Ignition Amplifiers

What else ?

mocanlagunas

In my case, it was an empty gas tank... I just got my car two weeks ago and I didn't know the fuel gauge was defective, it didn't go below half tank, so I thought I still had gas.... Limped to a gas station with a shop, and none of us thought about the gas.... Had it towed to my mechanic $$$...

jmiddleton17

So many good thoughts and also much confusion. I own a 97 a8 4.2l. I have run into misfire situations twice now. First time I used a scope to check the ignition modules (thin black things on the left behind washer fluid, sorry I don't speak audinese). Thought I had a bad one so I replaced it to no avail. Changed spark plug on misfiring cylinder and presto chango back in business. Fast forward two years. Another misfire. First misfire was on #8. This time it is on #7 primarily but this go round it is throwing a random, multiple code (p0300). I am also getting a code on #3. This time around I didn't bother doing any diagnosis work just threw plugs at it since I was lazy last time and only did the bad one. This time changed them all and still having a misfire issue. Anyway to my real point. I did run into a problem with stalling after changing plugs and later found my air intake tube was disconnected. Car still ran and drover but would stall at stops and especially when cold. Now I can say for certain that when my audi misfires you can feel a slight lag occasionally at idle in Park. More noticeably under hard acceleration nothing really noticeable but, at slight acceleration you most definitely get a violent shake. Most of the time the shake will smooth out if you either raise rpm above 2000 or get the car up to 70, which also happens to coincide with 2000 rpm. Now that being said don't over think it. You can literally lose your mind trying to consider all of the outrageous possibilities people post on these forums. It's be said a million times before but keep it simple stupid. The car is telling you that it is misfiring. Before checking suspension, brakes, and transmission/torque converter try to figure out the problem the car is telling you it has. Back to K.I.S.S.
A car needs four things to run:
1. Air
2. Fuel
3. Spark
4. Compression

So before going any further you must check those four things first. I usually check compression last because it is a little more involved but not hard. Air is pretty easy. Check to make sure you air intake house and tubes are installed properly and not broken or torn. An air leak after the mass air flow sensor will car issues with the computer. If the computer doesn't think there is enough air it will try to compensate with a lower amount of fuel to match the amount of air. Any vacuum leak will add additional air and the computer will add fuel to compensate.
Next spark. Do you self a favor and go to an auto parts store and buy am adjustable gap spark tester. Spark testers with an illuminating light are a waste of time. They will only tell you if you have spark not whether or not you have good spark. You want to set the adjustment to at minimum the gap of the spark plug. I usually go a little more to make the coil full field and give maximum output. If it can do that you should be fine.

Next fuel.
Usually I cheat on this by spraying a little ether (starting fluid) or brake clean, carb clean, etc. In the intake and see if it starts. If it does then you will have to find out why you have no fuel. If it doesn't then fuel isn't your issue or at the very least your only issue.
If at this point you still haven't isolated the problem. Then get a hold a screw in compression tester. Not entirely sure but I think these can be purchased relatively cheaply from your local auto parts store and most likely you can return it for a full refund when you finished.
::IMPORTANT::
WHEN DOING A COMPRESSION TEST MAKE SURE TO DISABLE THE FUEL. Find the fuel pump relay or fuse and pull it prior to starting a compression test. Also, be sure to count how many seconds the engine cranks over on the first test and do the same amount on each cylinder to get compatible numbers. 5 to 10 seconds is usually sufficient.

Hope this helps. I know everyone comes on these things looking for a direct answer but, cars are like women. They never tell you what you want to hear.

pocketchange

Somewhere in all this the need for ignation comes up. Without it?
First thing I check is Voltage and ALL of the ground sources next.
pc

mike7401

Check your crank sensor wire. I had the same problem it was chewed up. I guest it would ground sometimes causing multiple misfire