New A/C Clutch The Cause Of Excessively Bad MPG?

[Brandon Gatz]

Two weeks ago my A/C clutch was replaced, and although the A/C now works all the time, my MPG went deep south overnight. Yesterday I performed 2 26mi tests on identical stretches of roadway covering both highway and city and came to these numbers:

A/C off = 27mpg over 26 miles

A/C on = 16mpg over 26 miles - After A/C clutch repair

A/C on = 20/23 cty/hwy - Prior to clutch repair.

i've even dipped into the 12's on some days. I understand that the compressor puts a load on the system and a decrease in efficiency expected, but not this much. Prior to the A/C clutch being repairing i'd get 25% better than what I'm seeing(when it worked). I'm curious if it's possible that the clutch was tightened too much to the belt system and is pulling excess load. Being that I'm a real n00b to A/C systems I have very little insight into this. It's just too much of a coincidence for it to not be related to the repair. Any thoughts are greatly appreciated.

 

[Eddie]

I really question your numbers. Measuring mileage over 26 miles is well, not a good measurment. Most ACs will cost you ~2-4% at steady highway speeds and up to ~15% in stop and go. I can't see how your issue could be happing unless your measurements are incorrect or something else is causing this (like a disconnected vacume line).

 

[Brandon Gatz]

the measurements are not incorrect

 

[JustinH]

you need to measure full tanks, not short trips like that.

 

[Max_Wander]

did they overpressurize the system?

 

[Brandon Gatz]

I am acheiving 300-325 miles per tank as opposed to 400+

 

[Kestas]

No, you have to measure fillup-to fillup, with the tank down to at least 1/4 before each fillup. Each time write down the miles driven and gallons filled. Only this will give you the numbers you need to crunch to figure the gas mileage. In addition, this should be averaged over three consecutive fillups to get an accurate number.

On the other hand, if the a/c clutch was really eating up the gas mileage, this energy would have to go somewhere. Remember learning about energy balances and conversions in high school?... This energy would have to present itself as heat (i.e., red-hot a/c clutch) or noise.

I think your problem is elsewhere or you have a non-problem.

 

[mechtech2]

AC used energy. But those numbers are off. You need larger samples.
A bad compressor, or an overcharged one, will make an extra load, but not that much. Since the system is working, you can guess that it is pretty close to correct.

 

[mechtech2]

Someone may have knocked something loose or broken something that pertains to engine controls while doing the AC repair. It may not be directly related.

 

[oilyriser]

Did you say the a/c works ALL the time? It's supposed to cycle on and off.

 

[Junior]

Quote:

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I am acheiving 300-325 miles per tank as opposed to 400+

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Your numbers are incorrect.

Your first set of number are 40% less (27 vs 16)and these (400mi vs. 320mi) are 20% less. For me, even one tank is meaningless. Average it out over 3 or more tanks.

-I agree with other posters suggestions.

 

[Brandon Gatz]

i've gone through 5 tanks since the repair and MPG is still far below my prior average. I don't need hard numbers with an accuracy of -1/+1%, my wallet at this point is doing all the talking.

 

[Kestas]

Followup info:

http://volvospeed.com/vs_forum/index.php?s=a52f089d0be43a2db6ebc11ee0f7ebdb&showtopic=79163

It appears to be a non-problem.

 

[mechtech2]

A transmission flush solved th problem?
That's great.
But this type of problem and customer description makes it tough to diagnose things in the shop.