O2 Sensor – Because of strict emissions these 1.8T’s are very sensitive to readings from the O2 sensors. MY 2001+ have a wideband front O2 sensor with high accuracy and a linear response. The car will adjust the fueling based on the readings from the sensor. If the sensor is over heated, exposed to lead (race gas), flooded with soot (too rich) or is just old, it can provide the car with false readings and the car will not provide the proper fueling. When the O2 sensors go bad and the ECU detects this the car will run on reserve fueling maps and will not be able to adjust for boost leaks, fuel pressure, MAF readings or any other parameter that affects fueling. To check for a bad sensor the first thing to do is run block 032 on the VAG com. If the sensors are bad, the readings will both be 0%, replace the Front O2 sensor. If the sensor is bad, it will also respond slowly, or reach a peak. Logging block 031 will show the lambda reading from the O2 sensor as well as the requested lambda from the ECU. To get air fuel ratio multiply lambda by 14.7. If the lgo shows the lambda jumping wildly when running through a gear, or perhaps it flat lines at an unreasonable level, then the O2 sensor should be replaced. It is a 50$ part. If the o2 sensors get shorted out or the wires get pulled, they will damage the ECU, be careful with the wiring on these sensors. 2000 and older cars do not have a wideband sensor, however they will still perform some adjustment of fueling based on the sensor.
Oil Cooler The Oild cooler on these cars uses a plate and plate heat exchanger. Oil on one side, coolant on the other side. Often the brazing on teh cooler can fail and oil and coolant mix together. This is often misdiagnosed as a bad head gasket. To test this remove the coolant hoses and apply air pressure to the oil cooler and watch for leaks. Oil cooler is located above the oil filter.
Spark plugs – With the weak coil packs that these cars have, and the high boost pressures that they run, the spark plugs are very important. Spark plugs on turbo cars need a nice tight gap 0.028” is recommended. Spark plugs may come “pre-gapped” however you should always check the gap, as the variation can be severe and will cause problems. Plugs are cheap, so the best way to troubleshoot is to replace them or pull them and check the gap. A good cheap replacement plug that is a bit cooler is an NGK-BKR7E copper plug. They need changing more frequently, however they are cheap.
Thermostat- Thermostats can be a source of overheating. Most common is the waterpump. Thermostats are relatively inexpensive, and can be change din a half hour. If your car overheats, or has a tendency to spike up in temp and then drop down to normal temp, it may be a thermostat.
Timing Belt If your driving along and your 60+K mile car runs great and all of a sudden dies, there is a good chance that your timing belt has broken, or stripped some teeth. This is a very costly repair and could have been prevented had the factory recommended a better service interval for the belt. Damage estimates are anywhere from 600 - 1800$ to repair this kind of failure. 60K miles is a good time to change the timing belt, some belts have lasted 90K, but it's not worth the risk. To test for this failure, pull off the timing belt cover and crank the engine by hand. If the cam gear doesn't turn you have a bad timing belt. Repair is best left to a good mechanic. - If your going full throttle and the car all of a sudden dies, check for a boost hose blown off -
TB – The throttle on these cars is drive by wire, it is an electronic throttle with a wire attached. Most common TB problem just requires adaptation, or cleaning out with carb cleaner. This procedure shows how to do a TBA. TBA can improve idle, and part throttle operation.
http://www.ross-tech.com/vag-com/cars/throttlebody.html. To clean the TB remove it, and spray inside with carb cleaner. Wipe out the residue that gets built up in there. NEVER port a TB on a 1.8T it won’t idle properly.
Race Fuel – 1.8T engines love high octane fuel, however they can have cold startup problems, and if you run leaded race fuel you will ruin your Cat and your O2 sensors. Always run unleaded fuel in a 1.8T unless you are cat less, and you have extra o2 sensors.
VAC Leak – A VAC leak will cause un-metered air to enter the engine and it will run lean. If the air is not measured by the MAF then the fuel will not be injected. The O2 sensor will compensate for much of this, however it has limits. To check for a VAC leak, log block 032 and check the idle fuel trims. If it’s more than +2% you probably have a VAC leak somewhere. Check hoses and connections for loose clamps of cut hoses here are come common areas for VAC leaks.
Turbo inlet pipe not secured, DV line leaking, Crank case breather Y pipe split, Line on Fuel pressure regulator gets worn and leaks, intake manifold gasket can leak, and PCV line under intake manifold leaks. To find leaks some people spray ether, or starter spray around in the engine bay and listen for changes in idle speed. If you get a change from spraying in a certain area look for leaks there.
Waterpump - Nearly all overheating problems I have seen on these engines has been from the water pump. VW uses a plastic impellar that is splined/molded onto the shaft. These splines strip, or the pump cracks and the impellar slips on the shaft at high speeds. If your car is overheating best bet is to change the waterpump. You can do the T-stat first to see if your lucky but every time I have seen people try this it's the water pump anyways. This happens as early as 35K miles. While your in there doing the water pump, change the timing belt. Timing belts on these can go as early as 60K miles. There are several companies that make kits to do the timing belt and water pump.
www.ecstuning.com is a good one.
Water wetter - Water wetter is an additive used to remove surface tension from water. It improves waters cooling ability in a cooling system as it prevents beading of the water, and raises the boiling point. Water wetter should not be used with G12 coolant. It is best for race cars running only water. If you add this to the coolant reservoir you get a nasty oily sludg in the reservoir. Flush coolant system and remove this.