Guys, I do not know if there will actually be a detectable suction pulse at the tailpipe during cranking, but it is for certain possible. Try to picture what happens during cranking with no firing.

The engine is cranking at perhaps 300 RPM. Well less than half idle speed. Let's follow one cylinder, starting at TDC on the intake stroke. The intake valve is already open a little as the piston starts down. Atmospheric pressure pushes air into the cylinder to fill the volume of the descending piston. There is a pressure drop through the air cleaner, intake manifold, and intake valve, so the cylinder pressure never actually reaches atmospheric pressure during the intake stroke. The intake valve is still open at BDC, and it is still open for some degrees as the piston starts up on the compression stroke. During normal operation, this improves volumetric efficiency. At cranking speed, however, the air velocity is so low that soon after the piston starts upward it pushes some of the air in the cylinder back out into the intake manifold. When the intake valve closes, there is less than the theoretical maximum amount of air in the cylinder, and it is at lower than atmospheric pressure. The piston now compresses the air until it reaches TDC. The air gets hot as it compresses, and some of the heat of compression is lost to the cold cylinder head, piston, and cylinder walls. This lowers the compression pressure from the theoretical maximum adiabatic compression pressure. Also, since the piston is moving relatively slowly, there is more time for some compression leakage past the piston rings that are not yet at operating temperature. Now the piston starts down on the power stroke, only there is no power because the engine isn't firing. Some heat loss from the gas continues during the expansion stroke, further reducing the pressure from the theoretical. As the piston approaches BDC, the exhaust valve opens. Because of all the losses thus far -- the inlet restrictions, the loss of trapped mass because of the delayed intake port closure, the heat loss, and the cylinder leakage -- the cylinder pressure is LOWER than atmospheric pressure and a negative pressure pulse is generated in the exhaust system. As the piston continues downward, the pressure becomes even lower, until finally the piston is at BDC and reverses, starting upwards. Some degrees after BDC, the upward movement of the piston finally raises the cylinder pressure above atmospheric, but because of the low cranking speed and low temperature of the "exhaust" gas, the volumetric flow rate is far, far lower than during normal operation. Since the exhaust system is sized to handle much more flow, the restriction of the exhaust under these conditions is small, and very little pressure is generated in the cylinder to expel the air. Now the piston reaches TDC on the exhaust stroke and the pressure in the cylinder approaches very closely that of the exhaust manifold, which is only very slightly above atmospheric pressure. Then the piston starts downwards on the intake stroke, but because of the overlap built into the cam, the exhaust valve remains open for some degrees while the piston is descending, and the atmospheric pressure at the tailpipe acts on the whole column of air in the exhaust to push air back into the cylinder. As the intake valve opens the pressure balances between the intake manifold and exhaust manifold and both contribute to filling the cylinder. Finally, the intake valve opens more than the exhaust, and ultimately the exhaust valve closes and the cycle repeats.

So you can see that the exhaust pressure during cranking fluctuates around atmospheric -- sometimes above, sometimes below. The magnitude of the fluctuations is probably very small, and the influence of the number of cylinders and firing order and exhaust system design must be considered. There are also acoustic phenomena at play, where the pressure fluctuations can have resonances that amplify the effect, although this is probably a very small effect at cranking speed.

Don't say "it can't happen." It can, and it might.

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1987 W201 190D