I have heard of numerous DIY methods to detect those pesky vacuum leaks but it usually involves the use of a propane connected to a hose or carburetor spray cleaner aimed at suspected leak sites while the engine is running. Both of these methods can present a nasty fire hazzard when used around a hot engine. Smoke machines are a better method but the cost of these machines is way beyond the budget of your typical home mechanic.
Here’s an el-cheapo method I have used successfully. It’s based on the smoke generator concept.
1. Acquire a smoke generator - one cigar
2. Remove the PCV valve from the valve cover and plug the hole in the valve cover.
3. Leave the PCV valve connected to the hose that leads to the intake manifold.
4. Attach a long rubber hose to the open end of the PCV valve (the end that is usually screwed into the valve cover).
5. Lite up the cigar and start blowing smoke into the intake manifold via your hose connected to the PCV valve.
Keep blowing smoke through the hose connected to the PCV valve. The spring-loaded anti blowback valve inside the PCV valve will keep your increasing air pressure and smoke inside the manifold. If there are no leaks you should feel a significant amount of pressure build-up and NO smoke. If there is a vacuum leak the air pressure and smoke will escape through it and smoke will be visible.
6.Why did I suggest using a long hose attached to the PCV valve? Because not all vacuum leaks can be detected from the top side of the engine. You and your cigar may have to crawl under the car to observe any smoke that might be leaking out of the lower part of the manifold or its runners.
7. Give it a try - it worked for me. It’s safer than propane and carburetor spray and a cheap cigar costs only about $1.50.
Good idea.
I would however suggest a good cigar, then you can celebrate when you find the leak.
I’ve done this once a long time ago with a cigarette and personally I’m not keen on the method. This is likely because I’m a non-smoker and don’t like the lingering taste of smoke in my mouth for several days afterwards.
Questions. I can see blowing smoke into the intake. How do you pressure the intake up because this would mean that all intake valves would have to be closed, valve overlap is zero or less, etc.?
What about vacuum leaks inside the car (depending) or in lines running back inside the fenders or to the rear of the car, etc.?
Ever considered buying a fog machine like DJs and bands use on stage? A little adapting to fit a vacuum line and you can generate enough smoke to obscure the entire 'hood.
These can be had cheap and the chemical (called fog juice) is inexpensive also
I bought one for 20 bucks and this is just food for thought.
Guitar and spandex pants would be optional. 
Added to bag of tricks thanks. Awesome. Do you guys have any other good ideas for other things?
PurdueGuy
Well, if one needs to pull a vacuum on something and in a pinch one could do this.
I was out in the sticks one time and changing an A/C compressor outdoors on farm equipment.
No compressed air or electricity available to run a vacuum pump.
So what I did was to take a long length of vacuum hose and attach a Schrader valve fitting from the old compressor. This allowed me to connect the center hose from the manifold gauge set to the Schrader valve/vacuum hose. The vacuum hose was then attached to the intake manifold on another vehicle which was then started and allowed to idle with both valves open on the manifold gauge set.
About 20 minutes of this and it pulled a vacuum on the other vehicle’s A/C system. The other vehicle was turned off after both gauge valves were then closed and it was allowed to sit for about 10 minutes to make sure there were no leaks.
There were none so the A/C system was charged up sans use of a real vacuum pump and the A/C worked great.
Off the wall I suppose but considering the circumstances that was the best option and the bottom line is the A/C was ice cold.
I have an image of a car from 2010 having so many less potential vacuum leak points than let’s say a CVCC Honda (a really bad/or good example). There just are not so many places for vacuum leaks to occur on cars these days.
“How do you pressure the intake up because this would mean that all intake valves would have to be closed, valve overlap is zero or less, etc.?”
If the rings are sealing then all it takes is for the cam to be in a position where no valves are overlapping.
So the cylinder(s) on the intake stroke get pressurized too.
On a 4 cylinder with the crank near 90 deg. from TDC would satisfy this.
With more cylinders that window shrinks an disappears.
I think the 80’s Honda’s with feedback carburetors were even worse.
The CVCC system itself didn’t need extra vacuum lines.
Just a carburetor taken off the lawn mower from hell.