Another SC CIS Problem
 

Upon starting my 78 SC, the idle is at 2000 rpm’s for a few minutes then drops to ~950 as it should. Sometimes it will stay at 950 all day, other times it will jump to 1800. I am assuming that this is caused by a vacuum leak but would a vacuum leak be so consistent? It is always either 950 or 1800 rpm, nothing in-between. Using the A/C will also cause the rpm’s to increase to 1800. Thanks for the help.

Montroe

I'd check the deceleration valve first. You can just disconnect and plug the smaller top hose. You car might then stall when you take your foot off the accelerator, but if you're idle speed drops to 950 you've found the problem. The deceleration valve is adjustable. The a/c effect is strange, but maybe the added load increases manifold vacuum and opens the deceleration valve.

Thanks Hladun, If the deceleration valve is located on the driverside rear of the engine compartment and has one small hose coming from the top and two larger hoses connected to the bottom, I already tried disconnecting and pluging the top hose. Where should I start looking for vacuum leaks?

Do you have some kind of add-on solenoid that increases idle speed when the a/c is on? A vacuum leak is unmetered air and I'd be surprised if you could get your engine to run at 1800 rpm with an unmetered leak. It could be the auxillary air valve (not auxilary air reguator). The AAV is best removed/plugged because it causes the dreaded starting backfires. Maybe others can help you.

I have the opposite problem. When the compressor is on, the idle drops below 950 rpm. Is there a solenoid that controlls this? If so, this may be the answer to both of our problems.

A/C drops idle thread

The answer is no, but who knows what may have been added to these cars over the years.