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I have a 1990 Corvette (110K miles) that comfortably passes the environmental sniff testing but failed because it won’t hold vacuum. And, yes, the A/C will shift from the dash to the floor under load. I’ve visually checked for cracks and breaks. I’ve changed lines that look remotely aged. I’ve checked “T”s and junctions. I’ve mentioned this at garages and get the same response you might get when you ask about an intermittent electrical problem: No one wants to deal with it!
I’ve had several vehicles that ran well but would have the vacuum-powered heat/AC/defrost doors change positions to their “parked” position under load. This can be a minor annoyance, but now states have begun checking the health of vacuum systems during emissions testing.
So what is the method for finding these vacuum leaks?
I was in your shoes a few years ago, but with the dreaded P0171 engine error code on my 1995 Lincoln Mark VIII LSC. I replaced all vacuum hoses around 2007, and they still seemed OK. Ditto for every other item on the diagnostic checklist. Rather than throw money at the problem, I bought a rudimentary smoke machine and hoped it provided a very obvious solution.
That said, you can make your own smoke machine for not much cash. I’m fortunate enough to have a “time is money” problem, so it felt good to support a fellow American with a cache of empty paint cans and a knack for entrepreneurship … but I digress.
Back to the smoke. Follow the instructions on your machine (i.e. some need a source of compressed air) and connect the machine’s output line to an engine’s major vacuum line. Think big brake booster hose and not teeny-tiny fuel pressure regulator hose. I chose a vacuum tree near the Mark VIII’s PCV valve, lit up the smoker, added compressed air, and here’s what happened:
So don’t be a dufus like me; plug every hole in the vacuum system. I blame my error on my laziness to remove the intake tube and cap off the emissions hoses leading to it. (If you’ve ever dealt with the rear-facing intake on a 1993–96 Mark Series, you might appreciate my decision.) But the smoke only leaked from the intake tube, so I replaced the next most obvious problem, the O2 Sensor, and it worked. My experience with a smoke machine was both low-stress and a major time saver.
Since your Corvette clearly has a vacuum leak, do the same test but keep an eye under your dashboard; you could have a leak near the HVAC box. If you don’t see smoke under the hood, a smartphone-enabled endoscope will be a lifesaver.
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