A simple question to all boost gauge owners:<p> What's your boost gauge reading (@ idle) when the temperature is below ...say, 40 (F)?<br> <br>At idle, the gauge has been reading ~ 1-5 Hg at start-up.... once the engine warms up, it will drop down to maybe 10 Hg. <br>This is completely different than in the summer time, when it would read ~18Hg @ idle.<p>Change in ambient temperature = change in intake manifold pressure??<p>Thanks in advance!<p>Cris<br>

I assume, since you say idle & therefore you should not be in boost, that you mean you are giving vacuum readings from your boost/vacuum gauge.<p>Hg is Mercury. Most gauges show inches of mercury or "in Hg".<p>Now for some outloud thinking:<p>I think you've got a lot of variables there...it is possible the computer idles the engine higher in cold temps for a longer period. <p>It is unlikely the ambient temperature would cause this discrepancy since you are measuring engine vacuum (how hard the engine is sucking air in through the idle air bypass valve, since throttle is closed at idle).<p>However, we could assume that the computer is compensating for ambient temperature (knowing cooler air is denser & therefore the engine requires less air volume than in the summer)...but then the vacuum should be higher in the cold temps cuz the Idle air bypass valve could be closed more in the winter = higher vacuum.<p>Dunno...let's see what others come up with<br>steve

My readings are as follows:<p>Cold: ~16 in HG at idle.<p>Warm: ~20 in HG at idle with compressor off. ~1-2 in HG less with compressor on.<p>Sean<br>97 1.8Tqm<ul><li><a href="http://www.ns.net/~seang">Sean's A4 Page</a></li></ul>

<br>The difference in idle vacuum gauge readings is due to the difference in duty cycle of the Idle Air Control valve, which varies in order to maintain a constant idle RPM, under control of the ECU. When a larger average idle "opening" is required, to maintain the same RPM, such as due to the additional load of a cold engine (oil pump), or AC compressor, the intake manifold vacum will be "lower" (less vacuum) just as if you had cracked the throttle open abit to bring the idle speed back up.