We see this as a common problem as people making major changes to the engine such as porting heads, stroking, custom CAM, etc

In this case custom mods, esp the head work and CAM causes enough loss of vaccum that if effect functions such as startup and idle.

To show this better we have below 2 engines, one stock and one with the above mods.

The PCM commands as what it thinks the engine mode is thus at a set RPM range and set vaccum level seen the PCM assumes as example that engine is at idle.

At the bottom you see each fuel trim cell the engine used, the amount of vaccum and MAP reported in KPA.

Notice comparing a stock LS1 as the order of the fuel trims at the bottom and then each of those cells what the reported vaccum and MAP was.

Clearly seen with the mods has greatly reduced the amount of vaccum, as a pointer look at what the values are for Idle ( which is cell 19)

This greatly effects what the PCM is doing as to timing and commanded fuel as the PCM sees in the modified engine the cross points of vaccum and MAP PCM thinks engine is in a different mode then idle

Stock engine is bottom graph and as reported via OBD-II scanner at idle

MAP = 32 KPA Vac = 70 KPA ( MAP is inverse of vaccum) and for modified engine
MAP = 67 KPA Vac = 34 KPA ( about 50% VAC loss due to mods design)

Thus quite of bit of vaccum is lost at the low end which does not help engine to run well at low engine loads and effect the quality of startup and idle conditions

As you see idle now is all the way over where normally headed to WOT when the butterfly of throttlebody, ie little vaccum, high MAP.