Resolving Loss-of-Power Issues in the Volkswagen TDI
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 possible causes are computer-imposed "limp mode" and the MAF sensor, which is just a small distance from the air cleaner.
Limp mode is a safety function when the car computer detects a serious malfunction of some sort serious enough. There is NO signal to the driver, just this odd feeling. It feels like you have the E-brake on or something is holding you back. On a flat road, it will be subtle; it is much more obvious when trying to climb or get 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.
The only way to find out the cause for this limping is to use a VCDS computer to hook up to the car and see what the codes are. Vacuum leaks are known to cause it because the car may stumble or buck. Or the problem might be a boost issue, EGR, exhaust leak, clogged fuel pump, bad MAF sensor, or sticking vanes in the turbo from soot. So, you really do need the code to make a determination.
The MAF (Mass Air Flow) Sensor
A bad or going-bad MAF can cause the loss of power. These are usually good to 70K miles.
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 has intermittent failure. If power is restored by shutting the car off and turning it back on, you are in limp mode.
One cause of a bad MAF is that you have used an aftermarket 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 whether 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.
If the engine check light is on, it may reset when the connector is disconnected and reconnected.
The time to change a MAF is not longer than 30 min. The cost of a MAF is anywhere from $140 to $200. The MAF base is held onto by two screws. Once removed, disconnect the top connector to the MAF and install a new one. To remove the top connector you will need a special tool to remove the two screws.