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checking for Vacuum leaks with Propane?


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A mechanic friend of mine says that (in the old days) they used to check for vacuum leaks with propane. Just take a torch and open it up a little. Put it adjacent to the lines you want to check. If the engine revs, you have a leak. This assumes great ventilation etc.

Sounds questionable. Self Immolation for fun and profit? NOT FOR ME.

So, am I just chicken? or will this really work. Safely?

Assuming no, how does a DIY mechanic check for and find tiny leaks?

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Yes this is quite true. This method is used to find vacuum leaks. By putting the propane source next to the intake system, if there is a vacuum leak sucking in air, the car will be running lean, when it starts sucking in the propane, then it will run rich, and the idle will change. When that happens, you then can check out the hose that you had the propane source next to, and chances are you will find a leak!

BTW the propane is UNLIT!

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If this works at all with closed loop lambda mixture control, as soon as the DME spotted the rev increase, it would close the throttle slightly to counteract it - you will probably not perceive an increase at all.

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A safer way is to use a can of starter fluid or contact cleaner with a thin straw type nozzle. Spray in the area of the suspected leak and the RPMs will fluctuate either speeding up or slowing down. Also a vacuum gauge can be tapped into a suspecting hose and will show a leak.

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This does does work. I had my mechanic friend do it. We used a very long narrow metal pipe at the end of a valved hose. This was opened to little more than a trickle and placed next to and all along the vacuum lines and the intake couplings. No change in the engine.

We then created a vacuum leak ad placed the pipe next to it. The engine immediatly revved higher. This simple test strongly suggests no vacuum leaks. He indicates it is not conclusive because it is "low tech." For instance an intermittent pinhole might not show.

This was done outside on a windy day. He would strongly advise against doing it inside or without wind or a fan. Obviously the spark system must be checked carefully to confirm no electrical current to ignite the gas. However, the flow was so little even with a spark, you would not get a substantial ignition.

BTW I still will not do this myself.

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