Using a smoke pump to chase an intake manifold vacuum leak

My 1984 BMW 318i has a too-high idle.  Before I suspect the idle control valve, I’m eliminating vacuum leaks.

I read about a nifty technique of pumping smoke into the intake manifold and then observing where it leaks out, and that would identify the vacuum leak.

Professional mechanic’s tool companies like Snap-On make machines that generate and pump smoke but as I understand the pricing, the cheapest one starts at $750 which is about $735 more than I can afford.  So, I made my own.

I hasten to say that it doesn’t work yet, but I’m hopeful.  At least half of the functionality (generating smoke) is working VERY well.  The prerequisites (not asphyxiating anyone or causing a fire) are also being met.

I opted for the technique of dripping ATF onto a hot soldering iron. The business end of the soldering iron is held inside a plastic file folder carrying case that I got at Wal-Mart for $8.  The cool part (literally) and the cord are outside the case. I made a hole in the side of the case about 2/3 of the way up. A set of stacked rubber bands holds the soldering iron loosely in position.

My plan had been to rest the tip of the iron in a can of ATF but the iron doesn’t then get hot enough to make smoke.  So, I put an empty tuna can below the tip of the soldering iron and I made a hole in the lid right about the tip of the soldering iron. Into this hole I placed a small funnel into which the ATF goes.  I plugged the funnel with a piece of paper towel to slow the flow to a slow drip.  That worked well.  So, the pool of ATF in the funnel oozes, drop by drop, past the paper obstruction and drips onto the tip of the hot soldering iron, causing smoke.  Any missed ATF  droplets get collected in the can below.

I removed the handle of the case since it had sizable gaps around it, and I plugged the holes and the gaps around the funnel with room-temperature-vulcanizing glue (aquarium cement, basically). I missed a small hole around the funnel, and I inadvertently found it by seeing a clear smoke trail exiting from the leak.  I hadn’t intended to build a leaky smoke box, but the fact that it found its own leak was pretty cool.

In case the ATF might spill and make a mess, I put a cardboard egg carton at the bottom of the case. That way the entire contraption is elevated so if there’s a mess, it’ll be harmless, below the main components.

I tested this aspect, and the smoke-generating process works great.  Fires caused: zero.  Body count: zero.  Yay!

Also at Wal-Mart, I bought an aquarium pump for $8.  I made slots in the lid for the power cord to the pump, and for the air hose that is supposed to guide the smoke to the car.  But, even with the pump buzzing, no smoke exited the hose, though the smoke was otherwise perfectly happy oozing out of even the smallest orifice in the case.

In case it’d make a difference, I removed the intake filter from the pump, and repositioned it so as to be higher in the case, since the smoke is hot and rises to the top.  Still, no smoke exited the hose.

So, I need a better pump.  I plan to go buy an electrical bicycle pump, next.  My fear is that I might over-pressurize the intake manifold and blow out some seals and actually create the problem I’m trying to prevent.

Wow, that sounds like it has all the makings of a government program.




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