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Thread: Sudden drop in fuel economy with AC on

  1. #1
    prosin is offline Cadillac Owners Member
    Automobile(s): Allante
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    Sudden drop in fuel economy with AC on

    I have racked up over 80k miles on my 93 Allante with the Northstar engine. Recently the fuel economy has dwindled to 13 mpg when the ac is on. It has never gotten such poor mpg. When the AC is off it returns to a respectable 23 mpg. In my attempt to understand what is going on I have driven around with the instant economy display on - it plainly shows dramatic increase in mpg with the ac off. I have also noticed that when accelerating with the ac on and you let off the accelerator the mpg does not bounce back at all - also while cruising the slightest amount of acceleration will cause the instant economy to plummet. I have replaced an exploded intake plenum, a radiator, and a litany of other items(I understand the car)- the ac compressor is two years old. Is there some sort of electronic control that disengages the ac clutch- I know this sounds weird and I am a reasonably accomplished Allante mechanic but I fail to understand what it going on.


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  3. #2
    jadcock's Avatar
    jadcock is offline Cadillac Owners Connoisseur
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    Re: Sudden drop in fuel economy with AC on

    It sounds like it may be a failing A/C compressor, one causing way too much drag on the engine when engaged. If you have any type of warranty on it, I would return to the shop (if you had it done) and let them check it out.

  4. #3
    Ranger's Avatar
    Ranger is offline Cadillac Owners Member
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    Re: Sudden drop in fuel economy with AC on

    I agree.

  5. #4
    prosin is offline Cadillac Owners Member
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    Re: Sudden drop in fuel economy with AC on

    It does need a new serpentine belt- BUT I would think that when the AC kicks on there would be a hesitation. Is there anything that controls the idle when the AC clutch engages- I am thinking the AC clutch is not disengaging when the AC is on - the AC does not drop out when the passing gear grabs.....

  6. #5
    jadcock's Avatar
    jadcock is offline Cadillac Owners Connoisseur
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    Re: Sudden drop in fuel economy with AC on

    You said that you can see how the fuel economy drops with the A/C use. This tells me that the A/C clutch is working properly. If you are saying that while using A/C, the compressor doesn't clutch "on and off" like most typically do, that may indicate that you're low on refrigerant charge.

    The A/C will not normally disengage "when the passing gear grabs" unless you are at WOT. And even then, there may be a delay programmed into the system, I don't know.

    My recommendation remains to have your A/C system checked out for proper operation, if you don't have the manifold gauges and equipment to do it at home.

  7. #6
    Ranger's Avatar
    Ranger is offline Cadillac Owners Member
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    Re: Sudden drop in fuel economy with AC on

    Quote Originally Posted by prosin View Post
    It does need a new serpentine belt- BUT I would think that when the AC kicks on there would be a hesitation. Is there anything that controls the idle when the AC clutch engages- I am thinking the AC clutch is not disengaging when the AC is on - the AC does not drop out when the passing gear grabs.....
    Yes, the PCM adjusts the ISC motor (or IAC valve on later years) to compensate for the additional load.


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