With ac on -8 on the boost gauge.. - GM Forum - Buick, Cadillac, Chev, Olds, GMC & Pontiac chat

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Old 07-03-2007, 09:30 PM   #1
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Default With ac on -8 on the boost gauge..

went to get the A/C fixed and upon turning on the A/C the boost gauge went to -8 while in drive at idle but in park it goes down just a bit, like -5 or -6. The dreaded SES came back on again.. :( :( :( :( :( . The ses probably is the battery cables I have yet to change but this high boost gauge while the A/C is on is that normal? I sprayed all over the engine vac lines with carb cleaner and replaced the canister hoses also, no change in idle.
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Old 07-03-2007, 10:15 PM   #2
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It is normal to lose some vacuum with the ac on. The ac draws enough power from the motor to do that.

I don't think battery cables are the cause of your SES light. Go to Autozone they will scan your car for free if you go in and tell them you have a light like that on.

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Old 07-03-2007, 11:02 PM   #3
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Not LOSE vacuum, but GAIN, as his symptoms show.

When the AC is on, the PCM requests a slightly higher RPM to compensate for the load, the raised RPM'* produce more vacuum when in bypass, resulting in a more negative number.

This observation is expected and correct.
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