Without a scan tool you are just randomly replacing parts and without data any diagnostic done would be nothing more than a guess. I have a vagcom and an obd eleven. I like the convince of the obd eleven but the vag com has features like graph logs as well as other features that make it a better tool for the more technical diagnostics. First and most obvious would be check and double check for a vacuum leak, If you have a scan tool check your long term fuel. This is a "average" of the fuel added or subracted in the ecu by modulating the injector pulses. When the codes are cleared the long term trims will be reset to zero and you may hear or feel a noticeable change when the engine is at idle. A positive fuel trim indicates un metered air, which could also be restricted fuel so rail pressure should be checked. If the pressure is low take a look at the fuel regulator and the filter. The negative long term fuel trim indicates unburned fuel which could indicate ignition or injectors. High or low trims could indicate problems with the o2 or the maf sensors so these numbers should be observer as well. Then when the engine is warm and the o2 sensor starts to cycle the ecu goes in to closed loop you can check the short term fuel. The trim should be +or- 10 or so negative or positive numbers above 20 will probably trip the mil and set a fuel trim code. The maf should be around 2or3 at idle if the maf readings look off clean the maf and recheck the numbers. The o2 sensor (bank1) should continuously cycle from 0 to 1 volt when the ecu is in closed loop, if the voltage does not change do an ohm check. Check the coolant temp and the ambient intake temp. Sensors and codes will only get you looking in the right direction then you test the components before you replace them. There are good inexpensive scan tools that can help diagnose problems efficiently and save you money that you would have needlessly spent on parts. Log your data before you clear your codes. After the problem is resolved clear the codes otherwise the car will have to be driven a few miles before the stored fuel trim will return to normal. When the codes are cleared the trim is reset. Don't forget the basics fuel, compression, ignition.
