Could replacing the alternator on my '07 Camry improve my gas mileage?

I know little about this stuff, but did your odometer get adjusted? How accurate do you think it is?

No. If your alternator was charging at full amperage, the draw of engine power to drive the alternator would be about ¼ horsepower. Could be possible that less than adequate alt power could have effected the engine’s computer and caused the lower gas mileage, but I doubt it. Did you notice any excessive black exhaust smoke while this was going on?

No !

It was my belief that my gas and oil mileage has been for shit, and getting steadily worse.

There was a loud knock in the engine when it idled in gear, which evolved into a chirpy knock.

I took it in, and the alternator was beating itself to death – they showed me the dust coming out of it and had me listen.

They replaced the alternator, meaning the entire car, including computer, was without battery/power for “a while”.

There is now no unusual noise coming from the engine and the gas mileage seems better.

Am I imagining the better mileage, or could re-setting the computer have somehow corrected a gas usage problem?
The only overt problem was the noise. No smoke.

@ibstubro My guess it was your imagination. MPG could be slightly better since you don’t have the drag of the old alternator on the engine.

Imagination was always an option, @kritiper, although I’m usually pretty realistic about stuff.

It could be that I just wasn’t aware of how much I was driving until I set the odometer on ‘Trip’ so I could see exactly.

The alternator was so bad they thought I was in danger of being stranded at any minute. And I know these guys. They charged me cost on the alternator plus $40 install. No point in blowing smoke up my butt.

@ibstubro It was a wild guess. You are quite realistic.

Yes, but indirectly.

The ECU controls your ignition timing, injector timing, and fuel/air mixture. My old carb-and-distributor Corolla would require adjusting screws in the carb and manually adjusting the distributor with a timing light, but most modern cars replace those carb adjustment screws and the distributor with a computer.

Under normal conditions, the engine computer learns and improves. I can’t help but recall when Sport Compact Car took their “daily driver” Subaru WRX to the dyno. They strapped it down, did a few pulls, disconnected the battery for a few minutes, then did a few more pulls. It turned out that once it forgot about their ~30k miles of lead-footing, the thing lost a little MPG… and about 20-something horsepower, or about 10%. So forgetting the settings can be a bad thing. And unless the engine is brand new, odds are that the default settings that it reverts to after reconnecting the battery are pretty far off from what the engine actually needs to run correctly; the older the engine, the more off it will be.

However, when conditions are sub-optimal, it can learn bad habits as well. If your alternator is wonky, it won’t get the clean 12-volt power, and computers do “interesting” things when their power supply (which, on cars, is generally the alternator; the battery is pretty much just there for starting) is crap. For instance, if the computer decides to fire off the plugs too early, you get something like this; a kind of knocking sound. Or maybe the sound was the alternator bearings going out. In either event, firing the spark plugs too early or too late can affect MPG, as can having the computer telling the injectors the wrong things and screwing up the mixture.

Once the ECU gets a steady 12v instead of whatever it was getting before, it will function properly and tell your engine to do the right things; the ignition timing will be properly set and the mixture will be far closer to optimal. And if the timing and mixture are right, the engine will run stronger and more efficiently than if it’s all out of adjustment due to a “drunk” computer.

Hmmm, @jerv.
The knock was the bearings going out of the alternator.

I would swear that just prior to replacing the alternator the gas mileage was so bad I could almost watch the gas gauge go down. I’m doing a rough check now but the mileage seems to be 30ish.

I’m glad that the noise has stopped and glad that I’m no longer freaked out by the gas mileage.