Where is the air bypass valve located on a 2001 1.8L VW beedle?

There could be a few possible causes for the rear heater not blowing hot air in your 2003 Chevrolet ASTRO van. Here are a few things you could check:

1. Make sure that the rear heater control valve is fully open. This valve is located near the heater core and controls the flow of coolant to the rear heater.
2. Check the coolant level in the radiator. If the coolant level is low, the heater may not be able to produce hot air.
3. Check for any leaks in the coolant system. If the system is leaking, it may not have enough coolant to produce hot air.
4. Check the thermostat. If the thermostat is stuck closed, it may not be allowing enough coolant to flow through the system to produce hot air.

If these steps do not help, it is possible that there is a problem with the heater core itself, or with other components of the heating system. It may be necessary to have a mechanic diagnose and repair the issue.

the expansion valve is in the evaporator case. you can tell easily if your car has a bad expansion valve. connect the gauges and turn on air . observe the low side gauge it will pull down to a vacuum. the green numbers below zero.

here is a schematic , should help you locate the air bypass valve

also follow u-tube link for a visual of what your looking for
Charged Air Cooler Bypass Mazda Miller Cycle

The idle air control valve (IAC) is located on the throttle body.this valve allows for sir to bypass the throttle plate when closed in order get air in to the engine

Idle Air Control Valve Idle Air Control Valve Operation:
A computer-controlled air bypass valve located in the throttle body. It helps control idle speeds by allowing air to bypass the throttle plates as to increase the idle spedle Speed Actuator Idle Speed Actuator Operation:
It regulates the position of the throttle valve plate(s) to compensate for additional loads on the engine, like the air conditioner. It measures engine RPM readings, compares them to the reference RPM in the engine-control computer memory and adjusts the idle air by-pass.ed.

P0411- Means Secondary Air Injection System Incorrect Flow Detected!
Possible causes - Vacuum hoses damaged.
- Air Pump inoperative.
- Air bypass solenoid circuit open.
- Air bypass solenoid leaking/blocked or stuck open/closed.
- Air diverter valve leaking/blocked or stuck open/closed.
- Air air hoses r Possible solution - Visually inspect vacuum lines for disconnects in the AIR systems and visually inspect for proper vacuum line routing. Repair if necessary.
- Disconnect the air pump hose at the solenoid valve, start the engine and see if there is air being pumped P0445- Means Evaporative Emission Control System Purge Control Valve Circuit Shorted! Possible causes - Damage harness or connectors to the Purge Control Valve
- The valve circuit is open or shorted
- EVAP canister purge volume control solenoid valve may be faulty
P0445 Description The EVAP canister purge volume control solenoid valve uses a ON/OFF duty to control the flow rate of fuel vapor from the EVAP canister. The EVAP canister purge volume control solenoid valve is moved by ON/OFF pulses from the ECM. The longer the ON pulse, the greater the amount of fuel vapor that will flow through the valve. When is the code detected? - An excessively high voltage signal is sent to ECM through the valve

Operation

The Idle Air Control (IAC) valve adjusts the engine idle speed. The valve is located on the side of the throttle body. The valve is controlled by a duty cycle signal from the PCM and allows air to bypass the throttle plate in order to maintain the proper idle speed. Do NOT attempt to clean the IAC valve. Carburetor tune-up cleaners or any type of solvent cleaners will damage the internal components of the valve.
The Idle Air Control (IAC) valve adjusts the engine idle speed. The valve is located on the throttle body. The valve is controlled by a duty cycle signal from the PCM and allows air to bypass the throttle plate in order to maintain the proper idle speed.
Removal & Installation

1. Disconnect the negative battery cable.
2. Detach the IAC solenoid connector.
3. Remove the two retaining bolts and remove the IAC solenoid and gasket from the throttle body.
4. Installation is the reverse of the removal procedure. Use a new gasket and tighten the retaining bolts to 71-97 inch lbs. (8-11 Nm).

The Idle air control valve (IAC) is on the back of the throttle body housing on the 4.6 V8, it's half the size of a can of Red Bull. Try tapping on it with a small hammer and if the truck clears up then it's time to replace the IAC.
You can clean it with Throttle body injector but that's just a quick fix for now. Good luck and hope this helps.