I found this on another site
Before doing anything, you need to check all your intake boots, and hoses, and vacuum lines for cracks, tears, or anything that could cause a vacuum leak, since this will affect your fuel mixture reading. If your injector 0-rings are bad, replace them.
You will need a multimeter that can read milliamps, and a long 3mm allen wrench. Turn to the fuel section of the Bentley. Page 54 gives a brief explanation of the procedure, and pages 73-74 have the adjusting values. First thing you want to do is to hook up you multimeter inline with the wires going to the differential pressure regulator (DPR). The DPR is on the fuel distributor, on the side facing the engine, NOT the side facing the right fender. It has a two wire connector going to it. There are many ways to go about wiring this up. You can either get the factory test harness (which makes things a lot easier), VW special tool # VW 1315A/1 (order # TV1 315 0A1 25 ZEL), or you can put wire taps in the wires to the connectors, or you can make your own harness, using 02 sensor type connecors. If it's all hooked up, and you start the car, and it's running rough, and the meter is pinned at zero, you probably have the wires switched. Change them around and try again. If you are using an analogue meter, set it to the 10mA or 25mA scale. When you check your mixture, your car should be at normal running temperature.
That was the "hard" part. Now that everything is hooked up, start your car. After just starting the car, your meter will read a constant 8 or 9 mA. Let it sit for a minute, and the reading will start to fluctuate. THIS IS NORMAL. The reading should be bouncing back and forth consistently. The range of operation is 1-12 milliamps. The reading shouln't be below 1mA, and never above 12mA (if you want your car to run right, at least). The reading should only fluctuate about 2-3 milliamps, for example, the stock setting for a 1.8 16V is 5mA. This means the meter should be jumping between approximately 4 and 6mA, averaging 5mA.
Next thing to remember: the higher the number, the leaner your car is running. The lower the number, the richer the car is running. If your 16V is getting a reading of let's say 10-12mA, it is running too lean. If your 16V is getting a reading of 1-2mA, it's running too rich. Before actually messing with your mixture adjustment, CHECK for vacuum leaks, because they will throw the reading off. Most of this is pretty straightforward, I hope I'm not making it sound more complicated than it is.
What if your meter reads a constant number, and doesn't fluctuate? The most common cause of that is: a bad oxygen sensor.
What if your meter is hooked up correctly, but is pinned at zero? You car is set WAY, WAY too rich, and should be very sluggish to drive, and have no throttle response. Lean out the mixture slowly, until the reading starts coming up.
Turning the mixture screw clockwise will RICHEN the mixture, and of course turning it counterclockwise will lean it out. Never turn the screw more than an 1/8th of a turn at a time. You will see that it is very sensitive.
I HIGHLY recommend that all mixture checks and adjustments be done at night, so that you get consistent results. This is because the computer adjusts the mixture rich, during the day when it's hotter outside. If you check your car's mixture during the day, and it reads a lower number than what you set it to the previous night, don't worry, it's normal.
Stock settings:
1.8 16V-averaging 5mA.
1.8 8V-averaging 10mA
2.0 16V motronic-averaging 2.5mA.
Performance tuning:
Even a bone stock motor will benefit from a slightly richer setting. These are the settings I recommend for stock or mildly modified motors. The 16V settings are for maximum power between 4000 and 7000 rpms, so you might lose a little bit of low rpm power. If you would rather have the low-end, simply don't set the car as rich.
1.8 16V-averaging 3-4mA ("bouncing" off of 2.5mA)
1.8 8V-averaging 9mA ("bouncing" off of 8mA)
2.0 16V with CIS-E averaging 3mA ("bouncing" off of 2mA)
2.0 16V motronic - get a performance chip!
(what I mean by "boucing off of", is: the lowest number your reading drops to)
All cars will respond slightly differently due to mileage, local elevation, running condition, performance parts,...etc. All the settings I give you, are approximate settings.
Any questions, please e-mail me.
I explained how to do the fuel mixture adjustment, but maybe I should explain what you are actually doing.
I'm going to take the VW fuel injection, and make it sound horribly simple. It will help you in understanding the fuel delivery system of your car. This isn't exactly how the injection works, but if you think of it in this way, you'll understand what you're doing.
Your engine gets fuel in 2 ways, "mechanically" and "electronically".
The "mechanical" part, being the fuel mixture adjustment screw, which will supply x amount of fuel.
The "electronic" part being the full throttle fuel enrichment switch, which will supply a constant amount of fuel, a. a never changes. It is always the same.
a + x = total fuel to your engine.
When you adjust the mixture screw, x, you are changing the total amount of fuel going to your engine.
Once again, before the nit-pickers out there start attacking me, I know this isn't exactly how it works, but I want to put in terms that even non mechanically-inclined people will understand.
Miscellaneous stuff about changing the fuel mixture:
My friend has a CIS-E car, with a 2.0 16V. I have noticed that the fuel mixture is very sensitive. If the car is set at the stock 16V setting of 5mA, it will start up easy, idle nice, and have very good low end power, but not much high rpm power (the car will feel like an 8v motor). The setting that my friend actually runs his car at, 3mA, takes away a little bit of low end, and makes the idle a little less consistent, right after start up. But...this setting kicks ass at high rpm's.
I, once again, highly recommend that after every adjustment of the fuel mixture, you take your car out, and drive it hard. Get a good feel for your car, through the first 3 gears, and at part throttle, and full throttle. Small mixture adjustments will make the most noticeable difference at high rpms, in 3rd gear on the freeway
