I guess this is both a question and an answer.

Yesterday my car pretty much stopped idling. When I'd hit the clutch to stop at a light or in traffic I had about a 50/50 shot at the car trying to stall. It would drop down to the normal idle speed and sometimes stay there. Other times it would wind it's way down to about 400 RPM and then just stall. It was fun holding the brake and tapping the throttle at the same time... ;)

When I got home I took a look and a large vacuum cap had popped off the line to the PCV breather. Put a new cap on and it's back to normal.

Now the question comes...

Normal still isn't quite right. My idle is really variable. Sometimes it idles at 1200, sometimes at 750, sometimes it starts at 1200 and slowly works its way down to 750. Could this be caused by another vacuum leak?

I mention this because I'm pretty sure I've got another one. My headlights stay up when I turn off the car if I forget to put them down before shutting the car off. They'll stay up forever. It doesn't matter what position that little switch under the dash is in. This tells me I've got a leak.

I wonder how many others I have.

Could this cause my idling problems?

Now the tough one. How do you chase these down? I've got a vacuum guage, but I'm assuming I also need a vacuum pump to test with?

Any and all clues appreciated.

 

Testing vacuum leaks isn't too hard -- you will need a pump to test because you need to pressurize the line to see if it leaks. Your problem very much could be vacuum related. As you found out, just taking the PCV line off will cause the car to stall.

It's probably an hour job to go through the system. It doesn't take much to cause the system to get wierd. Just be methodical and you'll get it done in no time :)

 

When you want to find out if the idle problem is related to a vacuum leak just disconnect the vacuum hose at the intake and block it with something that can not be sucked in. None of the vacuum operated things will work then but can be sure to have no leak. If the idle is fine then, start chasing the leak. It's not too bad to do this, just takes some time to check all vacuum components are without leak.

 

chris:
You don't have to have a "real" leak to get the symptom you mentioned with the headlights. There is a check valve (alum housing) that shall keep the vacuum in the storage tank after motor is stopped. These valves are prone to leaking.
You should test as others have described. In addition you might also have leaks at manifold or carb, especially at the throttle bushings.

The headlights relays are also prone to leaking. I have some info on this at my site, tech section.

Lars has a document on a potentially gasket alignment problem that might give poor idle. (corvettefaq.com)

I had success with spaying water on suspected areas, a leak will give a "slurping" sound.
My car hasn't ever idled smooth. It was very bad when I got it, after fixing various leaks it got better, but not as good as I would like. It sometimes dies at a stop light, idle is set at ca 800rpm.

I have replaced the clock with a vacuum gauge. Then you know what's normal for you car an can see if leaks develops.