High Idle (late bus)

Most likely a leaky hose. Hard to spot because it may have a crack you can't see. Try spraying carb cleaner around likely places and see if the idle is affected.

Also check that the throttle is *actually* closing all the way.

Speedy Jim

Not likely a vacuum hose because it will generally slow down or die when it gets air that has not gone through the airflow meter. Sometimes the vane inside the airflow meter sticks, but unlikely; something may be causing your throttle to not close completely - be sure to check this. On the more likely side of things, how about the vacuum hose to the fuel pressure regulator? Is it connected all the way back to its source? Are there any new kinks or other disruptions to the fuel line on the right side of the engine or ahead of the firewall to the tank? Also try pulling the electrical connection off of the coldstart injector which is just to the right of the distributor and a bit ahead of it. If I remember right, this is a blue connector (could be green - I've swapped some around on mine!) It may be hard to remove, but all that holds it in place is a spring wire in a groove, so if you wiggle and pull enough it will come loose. Don't pull on the wires because they're really brittle.

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

Does the throttle cable return fully ? James

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

From the description, sounds like a part fell off:-( The Aux Air Valve is electrically actuated via a heated bimetal. That would have had a plug on it and a connection to the FI harness.

Speedy Jim

Thanks, Jim. The valve was connected to the electrical plug, but the valve would easily rotate. I can't see how the thing would work if its configuration was changing all by itself. What does this valve do? I assume it controls the amount of air going into the intake - and my problem was that there was too much of it in the mix.

Thanks, Scott

Sounds like the thing just plain broke:-( The "computer" tells it to admit more air in order to speed up the idle during warmup. The bimetal warps/twists to rotate the valve closed as the bimetal heats up.

Speedy Jim

This sounds like a solution to my other problem - engine easily dies while idling until it is warmed up. I thought that might be the case...

Thanks, Scott