Last night I found that the compression fitting from the manifold to the vacuum gauge was so loose I could turn it with my fingers. I tightened it up, and while it was no surprise the vacuum gauge now reads slightly higher, and it seems to idle better, I could also swear the AF ratio is reading higher at all ranges except idle maybe. Is this just my imagination, or did that leak throw off the map sensor maybe, or the mixture itself? It does not seem as if that small leak could make that much difference. The idle is absolutely smoother, and slightly lower in RPM. The tap is in the plenum just below the TB.

Sam

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MAP measure manifold absolute pressure, so a vacuum leak will (A) affect the fuel mixture with unmetered it and (B) affect the MAP readings. So, the computer is being doubly fooled- incorrect manifold pressure reading and unmetered air introduced into the air stream. A small leak makes a big difference in fuel injected vehicles.

Thanks Reed. The ecu is running open loop now, so O2 sensor is not correcting micture on the fly.

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If running MAF, it will definitely affect mixture.

Speed density will treat it more like a little extra throttle opening. If the whole VE table is tuned right, it can hold the mixture the same, but if it's pushed into a cell with a slightly off tune, the mixture may change.

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"Mad Scientist" Matt Cramer
'66 Dart - turbocharged 225
My blog - Mad Scientist Matt's Lair

If you have a speed density but don't have idle control on your system it will affect idle speed & MAP readings (engine vacuum) some- that can change the pcm's idle load calculation a bit, but the pcm should be able to compensate (assuming that you are running O2 sensor closed loop at idle).

If the leak is big enough & fairly close to the intake port & injector then it can disrupt the spray pattern & cause a misfire.

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63 Valiant Wagon
225 - 4 bbl