spray carb cleaner over the vac. fittings and pipes with the engine running.if you have a vac leak the carb cleaner will be drawn in and as a result make the A/F mix rich and slow the idle or stall the motor.
work methodically dont just spray the entire vac system in one go or you'll be none the wiser.

 

If you can't find it by spraying combustible fluids, you can make a boost leak tester simular to this ONE.

 

smoke machine is the easiest. Ive never really had luck with carb cleaner or propane. Most cars adapt so easily to small leaks that adding a fuel to the intake dosent cause the reaction it gets from older carburated engines. Basically, you'd have to have a scanner hooked upa nd watch the STFT to see if it changes dramatically.

If you have a leak, you are probablly only losing about 10% of your metered air to it. If you spray propane into the intake (90% of your airflow) you have to add ALOT to really see anything. so imagine how much fuel you'd have to add in thru the leak to get a reaction. Ive tried these methods so many times in the shop and Ive come to the conclusion that with modern engines, smoke is the only way to go.

 

A great way to check for a vac leak would be to use the boost leak tester mentioned above and use some blue bubble spay soap(found at an A/C supply
store). Pressure test the system and spray the bubble soap at every fitting you wish.If you get a big bubble,wahla,leak found.I'd say that is the best way to weed
out any confusion about what's leaking or not.

 

I can already tell that my buddy and I are not going to have fun with this...

 

Any luck?

 

yup... replacing the diverter valve solved all my problems