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Piston Slap: Smoking out a C4 Corvette’s vacuum leak

Frustrated writes:

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!


Sajeev answers:

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...


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Smoke is very handy finding leaks in fuel evaporative lines or fuel cap.