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One of our cars has a bit of a rough idle which disappears at about 1,000 RPM. After eliminating everything else I concluded that this is due to a minor vacuum leak. This was confirmed by the use of a gauge connected to the system, which showed the pressure rising (i.e. losing vacuum) when it should have held steady.

How can I test the vacuum system without actually dismantling its numerous parts and testing each hose, valve and fitting individually? I've considered piping smoke through the system and seeing where it comes out but some of it won't tolerate positive pressure, even low pressure.

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Take a small propane tank, hook up a hose to it and open the valve slightly. Start following the vacuum lines until you hear the idle go up; that's where your vacuum leak is.

The reason that works is that vacuum leaks are places where the vacuum system is open and drawing in air unmetered, causing the system to have a different fuel/air mixture than the computer expects. When you get the propane near an opening in the vacuum system, it sucks in the propane and it changes the mixture again.

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Thanks, I'll give that a try but I'm not not altogether hopeful. It's an elaborate vacuum system, with much of it very difficult to get to. Still, it's well worth a shot. – John Gardeniers Mar 8 '11 at 4:32
How safe is this? That is, how safe is it to have propane near a running engine? Or, alternatively, how can I make this as safe as possible? – Don Mar 25 '11 at 17:28
@Don Probably "not very safe". Keep a fire extinguisher ready. Maybe only have the propane "on" for 20 or 30 seconds, then turn it off to let it disperse, you don't want it to pool together waiting for a spark & fireball. Wear safety glasses, gloves, a face protector... – Xen2050 Apr 28 at 1:51

Smoke and propane both work well. Another option is a rubber hose stethoscope. This isolates the noise to the end of the hose, and will make a vacuum leak hiss fairly loudly.

If you have a stethoscope with the metal rod that you use for putting on the engine block, just yank the rod part out of the hose and viola! You have a new tool.

You may be able to squeeze the hose into some places without dismantling as much stuff.

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Smoke, propane, stethoscope, and carb cleaner all work well. The last option is highly flammable and much more dangerous than propane because it sits and accumulates on the engine. It doesn't evaporate all that quickly, either. It's a very effective way to check a particular spot, to see if your hunch is right, but it's essential that you have a good fire extinguisher right nearby if you want to go down that route.

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I've used soapy water with some measure of success in the past. Just put some dishwashing liquid and water in a spray bottle and spray each section of each hose. Keep your eyes open.

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