I just saw this image and I am wondering what is happening here:


The car was running and it seemed that the wind direction was out of the pipe?

  • Welcome to Motor Vehicle Maintenance & Repair! Nov 27, 2018 at 21:50

2 Answers 2


This is a quick-n-dirty test done to see if there might be an issue with a burnt exhaust valve. Another similar test uses a dollar bill (or whatever currency you happen to have). What is being checked for is reversion. This is an issue where if the exhaust valve is open when the piston is on the intake stroke, there will be the situation where exhaust gasses are pulled back into the cylinder along with air/fuel from the intake tract (with a non-direct injection setup ... just air with direct injection). If there is reversion, the flame from the lighter will be pulled into the exhaust pipe when lit in front of it. In the dollar bill test, the bill will rattle against the exhaust pipe.

Reversion can happen in one of three ways (that I'm aware of):

  • Burnt exhaust valve which no longer seals correctly.
  • Stuck exhaust valve which doesn't seat correctly or gets stuck partially open (which will quickly lead to a burnt exhaust valve).
  • Large amounts of overlap in the cam. The great the overlap, the more reversion (exhaust valve will stay open longer during the intake stroke). This is a common way to overcome the need for an EGR valve to meet emissions.
  • i had never heard of the overlap to meet emissions. interesting.
    – John Lord
    Nov 27, 2018 at 22:29
  • @JohnLord - It's why I asked this question. If you think how an EGR works, it reintroduces inert exhaust gasses back into the intake stream. If you have more overlap which causes reversion, then you are drawing exhaust back into the cylinder during the intake stroke, which also introduces the inert exhaust into the cylinder ... same same. I believe GM has done this before for performance engines ... would bet other manufacturers have as well. Nov 27, 2018 at 22:54
  • @JohnLord - This paper also explains about valve events and how they affect emissions. Pay particular attention to the Intake Valve Opening (IVO) and Exhaust Valve Closing (EVC) events. Nov 27, 2018 at 22:58

Most probably due to the velocity profile of the gases coming out of the pipe and any constrictions upstream of the exit - causing backflow at the specific exhaust gas speed.

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