Volkswagen TDI Loss of Power Issues and Resolution
Volkswagen diesel cars such as the TDI can experience a loss of power/ This could be temporary or more permanent depending on the cause. Two items to check is limp mode and the MAF sensor, just a small distance from the air cleaner.
Limp mode feels like you have the E-brake on or something is holding you back, it is an odd feeling. On a flat road, it will be subtle and much more obvious is trying to climb or getting onto a highway. Your engine light may or may not be on. Most cars in this mode will seldom go about 2000 RPM but limp mode will let you slowly accelerate to 40-50 mph. It will take much longer than normal. This mode is a safety function when the car computer detects a malfunction of some sort serious enough. There is NO signal to the driver, just this odd feeling. The only way to find out why is to use a VCDS computer to hook up to the car and see what codes. Vacuum leaks are known to cause it because the car may stumble or buck. The problem might be a boost issue, EGR, exhaust leak, clogged fuel pump, bad MAF sensor, sticking vanes in the turbo from soot. So, you really do need the code to make a determination.
A bad or going bad MAF can cause the loss of power. These are usually good to 70K miles, the cost is anywhere from $140 to $200. The MAF base is held onto by two screws. Once removed, disconnect the top connector to the MAF, install a new one. Two remove the top connector you will need a special tool to remove the two screws. A bad MAF sensor might produce a sudden reduction in car power but the normal failure mode is a gradual loss of power over weeks. Your engine light may come on if the sensor only as intermittent failure. If power is restored by shutting the car off and turning it back on, you are in limp mode. The cause of a bad MAF can because you have used an after market air cleaner that fails to keep oil residue away from it. Even a little residue will cause the sensor to fail. They are sensitive.
If you are not sure about if it is bad, simply rev the engine to 5100 rpm while in park or neutral. If the MAF is bad, you will not be able to do it. If the MAF has not failed totally, you should be able to do it. You can also unplug the plug connector. The ECU will then replace the faulty signal with a default value. If the car has more power or is unchanged, the MAF is definitely bad. The time to change a MAF is not longer than 30 min. If the engine check light is on, it may also reset when the connector is disconnected and reconnected.