8-9 MPG loss with A/C on - PeachParts Mercedes-Benz Forum

gwooden · #1 08-10-2007, 05:27 PM

I am seeing about 8 to 9 miles-per-gallon lost when I have the A/C on.
So, from about 29-30 down to 21 .... is this normal? This is my first summer driving the car, so I am not sure what to expect as in terms of what MPG to expect ... but 9 MPG seems kinda high?

Is 33% loss average and to be expected? Its a '77 300D converted to 134a. A/C was just serviced last week.

Thanks all,
-graham

rrgrassi · #2 08-10-2007, 05:31 PM

134a does have higher pressures, resulting in the engine working harder to turn the compressor. But, not enough to reduce MPG that much.

Has your driving habits changed? Is the tranny slipping a bit? When was the last valve adjustment? 8-9 MPG less does not sound right. I think you have other issues. A dragging brake pad or E-brake will also make the engine work harder to maintain speed.

gwooden · #3 08-10-2007, 05:39 PM

Quote:

Originally Posted by rrgrassi

134a does have higher pressures, resulting in the engine working harder to turn the compressor. But, not enough to reduce MPG that much.

Has your driving habits changed? Is the tranny slipping a bit? When was the last valve adjustment? 8-9 MPG less does not sound right. I think you have other issues. A dragging brake pad or E-brake will also make the engine work harder to maintain speed.

Well, it just got back from the mechanic from which I told him to repair/replace anything that was bad - so I would expect to hear something else about it, other than the work that was done. However, that is not to say that something was overlooked.

No, I don't think my driving habits have changed. I drive approx 140 miles a day (mostly on the interstate) and I have been monitoring this with and without the AC on. These past few days I have been sweating it to confirm my diagnosis and it's back up to 29-30MPG without the AC.

Now you got me thinking about my driving habits!

I will drive the next few days with the AC on as normal as I can, and report back. Thanks!

-graham

pwogaman · #4 08-10-2007, 08:48 PM

I am beginning to wonder if the ultra-low sulphur is impacting my mileage. I did some very smooth highway driving for 400 miles today and only got about 24.6 mpg. I did have the a/c essentially the entire time though.

#5 08-10-2007, 08:51 PM

Quote:

Originally Posted by gwooden

mechanic... I told him to repair/replace anything that was bad

Ah yes, the "hand them a blank check" statement.

BigBadJohn · #6 08-10-2007, 09:20 PM

I usually have them consult me before doing any major work w/o my knowing

LUVMBDiesels · #7 08-10-2007, 10:49 PM

Quote:

Originally Posted by pwogaman

I am beginning to wonder if the ultra-low sulphur is impacting my mileage. I did some very smooth highway driving for 400 miles today and only got about 24.6 mpg. I did have the a/c essentially the entire time though.

My mileage went down from an average of around 27.5 to around 24.5 once ULSD was the only fuel around here. I have been putting in Lucas additive to restore the lubricity and the mileage has gone back up to around 26. You only need a little additive to restore lubricity to the fuel.
Give it a try...

#8 08-10-2007, 10:49 PM

Quote:

Originally Posted by ForcedInduction

Ah yes, the "hand them a blank check" statement.

LOL, that's true. The guys I deal with usually have a pretty good idea how far to go before they need to give me a call.

GreenSD · #9 08-11-2007, 01:00 AM

I too have noticed a really extreme decrease in fuel economy when the A/C is on although I don't really have exact numbers. I have taken to using a ULSD/BioDiesel blend to help with the lubricity issue. Any thoughts on that?

#10 08-11-2007, 01:05 AM

Quote:

Originally Posted by GreenSD

I too have noticed a really extreme decrease in fuel economy when the A/C is on although I don't really have exact numbers. I have taken to using a ULSD/BioDiesel blend to help with the lubricity issue. Any thoughts on that?

The AC will reduce your mileage somewhat, but it shouldn't be 8-9 mpg, maybe 2-3 mpg. Using bio-diesel will also reduce your mileage. IMHO, using bio-diesel for "the lubricity issue" is unnecessary.

GreenSD · #11 08-11-2007, 01:12 AM

Quote:

Originally Posted by Craig

The AC will reduce your mileage somewhat, but it shouldn't be 8-9 mpg, maybe 2-3 mpg. Using bio-diesel will also reduce your mileage. IMHO, using bio-diesel for "the lubricity issue" is unnecessary.

Well truth be told I use it to feel better about the environment and keeping my money in my own economy too but I felt like the added lubrication just made it even more of good thing.

#12 08-11-2007, 01:14 AM

Even though ULSD has no lubricity issues.

chetwesley · #13 08-11-2007, 01:27 AM

I was told by a long time diesel mechanic (and head of the diesel/heavy machinery dept at the local tech college) that ULSD does provide less lubricity and that as little as 2% biodiesel returns the fuel to an acceptable level. This guy knew what he was talking about inside and out.

#14 08-11-2007, 01:30 AM

Quote:

Originally Posted by GreenSD

Well truth be told I use it to feel better about the environment and keeping my money in my own economy too but I felt like the added lubrication just made it even more of good thing.

Nothing wrong with using bio-diesel, but you certainly don't need it for lubricity. You will lose some mileage though.