1989 Volkswagen Van engine losing power

After reading your post the first thing that comes to mind is the engine may be getting back pressure. Check the catalic converter. Do you hear any rattling at the converter when the engine is running. It might be very slight and hard to hear. With the engine and exhaust COOL. Tap the converter with your hand to hear if anything is loose in the converter. It might not be the problem but it has happened to me after having a car sit for over a year. Once the converter gets warm the lining breaks apart and starts clogging the exhaust making for back pressure and finally creating enough to shut the engine down.

We did as you suggested and it did not fix the problem. The problem seems to get worse as the van warms up. Turning over the ignition restarts the engine. There is dark/black smoke when accelerating which increases as engine warms. Opening the throttle bogs down engine more. We were wondering if it could be a sensor. If so, which one.

you had not said the vw was putting out black smoke, Go to the air intake check the sensor inside the air intake. See if there is a buildup of carbon. Sometimes the air intake sensor gets clogged with car
bon and sends a signal to the ecu for more fuel pressure. Sometimes this can be fixed by using a throttle body cleaner to clean out the throttle body. If you decide to do this make sure you follow the instructions. Also check all the wires going to the sensor making sure they are connected and making good contact.