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Discussion Starter #1
I had my alternator replaced at a decent repair shop. 2004 MDX with 140k miles. I believe it‘s a reconditioned one they installed. Not sure if this is coincidental or not, but my MDX is now reporting 15.5 mpg instead of the normal 17.5 to 18 mpg. No other changes to the vehicle. It starts fine. I’ve owned this car for many yrs and never saw such low gas mileage. In the first few days after getting new alternator, the vehicle seemed a bit sluggish, like the brakes were engaged (not brakes, I checked) or engine was being restrained. Anyone out there have a similar experience? Maybe a bad alternator?
 

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It's very unlikely that anything could be wrong with the alternator to lower your mileage that much that wouldn't also show up in other ways like belt squeal, bearing noise, or fast battery discharging.

If it is something mechanical, it may have just happened at the same time. I'd have someone check the brakes, including the emergency brake, again if the car feels sluggish.

You could also check to be sure it has no stored codes that didn't trigger the check engine light.

My MDX actually does vary that much in MPG tank to tank. But if yours hasn't before, have there been any other changes to the car or how and where you drive it?

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Also did you do the valve lash adjustment that you are due for just after 100k miles? That can affect mileage.

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Discussion Starter #4
It's very unlikely that anything could be wrong with the alternator to lower your mileage that much that wouldn't also show up in other ways like belt squeal, bearing noise, or fast battery discharging.

If it is something mechanical, it may have just happened at the same time. I'd have someone check the brakes, including the emergency brake, again if the car feels sluggish.

You could also check to be sure it has no stored codes that didn't trigger the check engine light.

My MDX actually does vary that much in MPG tank to tank. But if yours hasn't before, have there been any other changes to the car or how and where you drive it?

Sent from my SM-N950U using Tapatalk
Colin, thanks for the reply. I watch the mileage closely and reset occasionally. It’s never dropped below 17 mpg. I didn’t think a new alternator would have an effect but this seems too coincidental. I’ll hv the shop check it out.
 

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Discussion Starter #5
Also did you do the valve lash adjustment that you are due for just after 100k miles? That can affect mileage.

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valves were adjusted at the time of the timing belt replacement.
 

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My guess is that your higher MPG average included a lot of highway miles (maybe on long road trips) and that the shop disconnected the battery to replace the alternator and reset your statistics, and that you've driven primarily around-town miles since the repair (which will of course return a much lower MPG number).
 

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Discussion Starter #7
My guess is that your higher MPG average included a lot of highway miles (maybe on long road trips) and that the shop disconnected the battery to replace the alternator and reset your statistics, and that you've driven primarily around-town miles since the repair (which will of course return a much lower MPG number).
Thanks for the suggestion, good thought but I typically reset the mpg readout every week or so. It accounts for that. It’s weird bc I noticed a big drop in mpg after the repair and hadn’t noticed the readout showing a number less than 16 or even 17. Maybe I’m just imagining things. It was also strange that I noticed the engine dragging a bit, I thought it was related to low mpg.

I reset it a couple times since. I’ll keep that in mind, that once it is reset, mpg number will dramatically drop if going up a hill.
 

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It's funny how MPGs tend to ebb and flow. Sometimes it's because of the mandated changes to gas/ethanol blends (more ethanol means less MPG - it's one of the biggest losers in the history of "green legislation"). Sometimes it's because it's colder, so it takes your car longer to come out of it's "closed loop" operation (where it's injecting fuel based on a conservative - rich - mapping protocol until it comes up to temperature). Or, if you're letting the car warm up for a minute or two, that can make a big difference. Or it's windier, or there's more traffic, or you're just driving faster or catching more red lights. ;-)
 
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