Virtually all Vanagons suffer from idle control issues at some point. The following write-up is an article Lucas Valdes (President & Resident Mechanical Engineer of GoWesty) wrote to shed some light on this temperamental system.
Vanagons from 1986-1991 (rectangular headlights, 2.1 liter engine) have a Digifant fuel management system. These systems are pre-"OBD," which means there is no way to plug in a "scanner" and download "fault codes." On these vehicles you need to have good, old-fashioned common sense and experience to figure out what is wrong.
These vehicles have a separate, practically stand-alone idle control system consisting of:
1. Idle/full throttle switch on the throttle body, commonly referred to as a "TPS" for "throttle position switch"—located under throttle body. We offer a replacement TPS here.
2. Throttle body: This part is almost always worn-out. We offer a re-engineered version here.
3. Idle air control (IAC) valve, also referred to as an "auxiliary air valve" or "idle valve," located right on top of the intake air plenum, dead center top of engine.
4. Idle control unit (ICU), also commonly referred to as an "idle computer," "idle relay," or "idle control module." This part is located behind the right-side tail light assembly, and we offer a replacement part here.
The most common problem we experience with the idle control system is that the TPS is not working, either because it has failed and/or the throttle body is worn out. In either case, the TPS will not close/open when it should, so the idle control system doesn't get the information it needs. The first thing to check before you replace anything is to see if the TPS is audibly "clicking." Check with the engine off so you can hear it. It should click when you rotate the throttle just off idle, and then again at full throttle. If it is not clicking, refer to your Bentley manual for instructions on how to adjust it.
If you cannot get the switch to work, or you cannot get it to work consistently, this could be a sign that your throttle body is too badly worn. When this occurs, the shaft that passes through the housing wobbles around too much, and no amount of TPS adjustment can get the switch (your original or your GoWesty replacement) to work well. Just about every Vanagon on the road today has well over 200K miles, and the throttle body is almost always worn out on all of them. If yours is worn out, it is time for a GoWesty rebuilt throttle body, which comes already set up with a brand-new TPS.
If the switch and throttle body are okay, check to see if the idle control valve vibrates with the ignition key turned on, engine off. If it does not, it is more likely to be a bad idle control unit (ICU) than a bad IAC valve. We usually plug in a known-good IAC valve to see if the valve vibrates. If it still doesn't vibrate with a different IAC, then it is probably an issue with the ICU. If you don't have another IAC to plug in, your best bet is to try a new ICU first, since it is much more common for those to fail anyway.
Sad, but true...