4.6 stroker from Russia

[darkcorp]

MAP - manifold absolute pressure.
So when engine is off it shows 101kPa aka 1 atmosphere.
When engine is running (idle) it should show 33-35 kpa. Mine shows 41-43 kpa so ECU thinks that engine works with load and adds some extra fuel. Then the mixture is corrected by the O2 sensor and I see it via negative long term fuel trims.
Is that point clear?
As I drive my fuel trims go to near zero. As I decelerate MAP shows 30 kpa and less as it supposed to be.
So I'm very confused by this.

[Cummins90]

Hissing sound and LTFT with positive increase at idle.

High idle?

Ever occlude that IAC bypass port?

[Cheromaniac]

MAP - manifold absolute pressure.
So when engine is off it shows 101kPa aka 1 atmosphere.
When engine is running (idle) it should show 33-35 kpa. Mine shows 41-43 kpa so ECU thinks that engine works with load and adds some extra fuel. Then the mixture is corrected by the O2 sensor and I see it via negative long term fuel trims.
Is that point clear?
As I drive my fuel trims go to near zero. As I decelerate MAP shows 30 kpa and less as it supposed to be.
So I'm very confused by this.

OK, so your reading of 43kPa was the MAP, which means the idle vacuum is 17.2"Hg. That's normal for a cam that has a longer valve opening duration than stock. Mine was 17"Hg when I originally had the Crane 753905 cam in my stroker.
When you slam the throttle shut to decelerate quickly, the vacuum reading will be momentarily higher until air begins to flow past the idle air bypass port, so your MAP reading of 30kPa (vacuum 21"Hg) under this condition is normal.
As Cummins90 suggested, try blocking the idle air vacuum port while the engine's idling and see if it stalls. If it doesn't, you have a vacuum leak.

[darkcorp]

How can I block the idle air vacuum port while the engine's idling? Remove the idle valve? Or close the inner hole with cloths?

If I have a vacuum leak my idle revs would be high, don't they?

[Cummins90]

No leave the IAC valve in place. With the engine at operating temp take intake hose off the throttle body. The throttle plate should be fully closed. Do not put anything inside the throttle body while the engine is running! Place your thumb over the bypass port inside the throttle body.

If your engine continues to run without any changes you have a vac leak that is just the right size to keep the engine running correctly. Your engine may decrease rpms slightly and you may be able to identify the vac leak you have as the hissing should increase in volume.

Your engine should choke and die quickly when covering the bypass port in a normal situation.

To test the movement of the IAC valve you can partially cover the port for 5 seconds being careful not to stall the engine. Then release the port and observe your rpms. They should increase to about 2000 to 2200 and rapidly fall back to regular idle speed as the IAC valve returns from wide open to idle position.

And don't worry I still have both my thumbs.