Heads and Cam got swapped last week and car is running erratically (idle's rough). My mechanic applied vacuum to the Map sensor and the idle went to a rock steady 500 rpms (couldn't feel the car running at this point). We ordered the Map sensor from Turho City since it's adjustable and is supposed to work with slightly longer duration cams. What I'm wondering is what exactly does a Map sensor control and do?

 

BK,

The Manifold Absolute Pressure sensor doesn't really control anything, per se. It measures pressure on an absolute scale (31" Hg vacuum = 0 PSIA, 0" Hg vacuum = 14.7 PSIA) and reports this measurement to the ECM. The ECM decides how to adjust fuel and timing based on this input and others. Your adjustable output MAP sensor may be just the trick to "fool" the ECM into proper operation. You likely have significantly lower idle vacuum (higher absolute pressure) due to your longer duration cam profiles.

Under normal circumstances the ECM would see this as excessive intake air and tend to enrich the mixture a bit and possibly advance timing. Once the adjustment is made and the O2 sensor reports a rich mixture, the fuel would cut back, and the cycle repeats, causing the idle to "hunt" for equilibrium, never able to achieve it. Biasing the MAP output a little lower will make the ECM think the vacuum is more normal for the given idle speed and TPS position.

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Later,
Vader
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"Make Me Bad"
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