Turning on A/C makes vacuum drop??

Question

Today was the first time that I've ever used my A/C since I had my AutoMeter Vacuum Gauge installed. I immediately noticed that my gauge was reading 3-5" hg LESS then it was when I turned if Off!!??

Am I correct in assuming that this means I have a vacuum leak somewhere in the system? Or is this normal?

I also should let it be known that this does NOT happen when I just use the heat. When I use the heater, the vacuum pretty much stays steady. It's only with the A/C that it's dropping quite a bit.

I was thinking that it might just have something to do with the ECM commanding a higher idle when the A/C is ON, and since higher RPMs equals lower Vacuum, I thought that "might" be the case. But I would guess that the commanded RPM by the ECM is the exact same for both the Heat and the A/C...and like I already stated, the Heat makes no difference...

Anyone know what's going on here? Is there specific hoses that could be leaking that are used during A/C operation but NOT during Heater operation?

THANKS!:)

Steve B. · Accepted Answer

I don't have a definite answer for you but I think you were on the right track.

The heat works off the hot water in the cooling system so running the heat creates no additional load on the engine. When you turn on the a/c you are turning on the compressor which can put a significant additional load on the system.

Generally there is only one vacuum line running inside the car to work all the climate control stuff. You could check for leaks by turning on the a/c then pulling this line and capping it.

Steve B.

Robert Hancock · Answer

The IAC valve is opened more with the A/C running to keep the engine at thesame idle speed. This will cause the engine vacuum to decrease.

Roy G. Bragg · Answer

The compressor adds a load to the engine causing RPM and vacuum to drop.The auto speedup is not a large RPM increase.  I observed this vacuum dropon carbureted cars many years ago.  Also putting and automatic transmissionin gear has the same effect on vacuum.Roy

Marc Westerlind · Answer

For what it's worth, my '04 has the same idle speed with AC on, or AC off. This was as seen in the "Desired Idle Speed" table in the VCM.

I do recall older vehicles commanding a higher idle speed when the AC is on though. However, I don't think that it would command the higher idle with heat... The heater only opens some flaps and lets air pass over the heater core. The AC requries the compressor to run, which puts a drag on the engine, which would normally drop the idle speed a bit.

Even if the idle speed stays the same with the AC on, I still think the engine is making up for that drag at idle by cracking the TB blade a bit.

-marc

Turning on A/C makes vacuum drop??

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