Thread: MAP/MAF readings to detect a vacuum leak?

Just doing some thinking and wondering if this is possible. On my OBDII scanner I can see live engine data from the sensors, Im suspicious I have a vacuum leak from the plenum and I want to monitor how its doing for permanence after I fix it, instead of pulling the fuel rail over and over checking to make sure all the bolts are tight.

Would having a vacuum leak from the plenum gasket (i.e. plenum bolts not torqued right) be noticable on MAP or MAF readings, since a vacuum leak there would change the manifold pressure, and the way I see it, having air coming in unmetered in another location would cause the MAF reading to be different than expected as a specific RPM, no? Or is my logic totally flawed?

So provided that it is possible, what would a healthy value be for NA l26 at idle for the MAP and MAF sensors? ... hv3, plog, dp if that changes anything.

MAP in kpa/hqs or whatever, should be at -19ish, just like if you were to hook a vacuum gauge up to it.

Other way to tell is Fuel Trims.

Short term and long term, you want both to be around + or - 3-4ish.

If the long term fuel trim is pegged at 16.4 or higher than normal, you have a vacuum leak.

Monitor your IAC. Reason being is that controls your idle via a "controlled" vacuum leak. Therefore it adjusts when you have a leak to keep the idle where the pcm is commanding it to be.