Thank you guys for the willingness to help and your insight. My bus is my daily driver, but I haven't had time as of late to work on it. So, I sent it to a local mechanic friend of mine to run some tests. Once he got a chance to look at it, he found that removing the auxiliary air valve alleviated the high idle issue. I really have no idea what this thing does. Would any one mind explaining it to me?
Anyway, the aux air valve looks as though it is supposed to rotate and be set; it does rotate, but does not set. So, it's just connected, letting air go through it or not - I have no idea what kind of control is supposed to be placed on it.
Thanks again for the help from you guys!
Much appreciated, Scott
Generally if the cylinder head sensor is bad or disconnected it will be so rich that it may not run or the black smoke will be so thick you can't see the bus! Same goes for if the coldstart injector is not shutting off (at idle it will be waaayyy too rich.) If the fuel pressure regulator has no vacuum or low vacuum, it enrichens the mixture for passing and hill climbing. This is most likely the culprit as it will bump up the idle speed around 1000 rpms higher. If the vacuum hose is disconnected it does cause a vacuum leak, but is not significant enough in this case to kill the engine (opposite of carb engines.)
If it's a problem with the throttle cable, something may have caused the black housing to be pulled out of its seat in the steel tube that runs the length of the bus, but unlikely. This connection is directly above the transaxle and you can reach your hand up around the transaxle while lying on your back, find the throttle cable near the firewall, just right of center, follow it forward until you feel the connection. You can pull it toward the back of the bus and feel how it connects and seats. Another throttle cable problem (and more likely) can be at the gas pedal end of things. If you crawl under the front end of the bus, there is a protective shield that has one 10mm headed bolt holding it in the center at the front and three at the back edge. Remove that shield and you have access to the pedals from underneath. Check the throttle cable there. Since I'm going from least likely to most likely, how about the spring for the throttle return at the throttle body? There should be a spring around six to eight inches long that snaps the throttle closed. If totally stock, I'm not sure where it's supposed to attach, but on all of my engines it attaches to the top of the black air plenum chamber center top of engine. Be sure it is attached to the proper point on the throttle body so it has a direct pull against the cable (two little arms there.)
If your EGR is still attached, it could be the culprit. I forgot about it cuz I haven't had one for years. You could detach the line that runs down to the exhaust pipe up at the valve, plug the opening with your finger and see if the idle speed goes down. If so, you'll need to determine if you need to repair it, remove it, or leave it in place so it appears to be there and working but is actually disabled. (Let your conscience be your guide!) Easiest way to disable is to remove the valve from the plenum chamber it is attached to, make a tracing of it, use scissors and a paper punch to make a gasket (thick gasket material available at any auto parts store) - don't make a hole for the air passage!
Just re-read your message. The '77 engines had a throttle position switch on the throttle body and the '78 engines did not. If it still has all the '77 FI equipment and the '77 ECU, the TPS could be your problem. Bentley manual has testing and adjustment procedure.
I have a 77 bus (78 engine) that started idling really fast yesterday - like 2000 RPMs or more. I initially checked the timing and found that I hadn't tightened the distributor all the way last time I adjusted it (about 3 months or so ago). Well, checking the timing with my timing gun, I found it was off; but even setting it correctly didn't bring the idle speed down. Adjusting the idle-adjusting screw all the way closed did not help either. Being it a fuel injected engine, I checked all the vacuum lines and the air mixture adjustment - nothing seemed to be set incorrectly.
Does anyone have any ideas what could be causing my engine to idle so high and what I can do to correct it?
TIA, Scott