charging system and mileage - Fuel Economy, Hypermiling, EcoModding News and Forum

doclees · 10-29-2013, 03:02 AM

My gas mileage seems to be getting erratic lately. Recently replaced plug wires and went from 21.5 to 19. Didn't think much of it. Put hard top on my jeep and a long trip got me to that 24mpg mark for once. The trip back only got me 20 mpg. No change in driving style. Hess gas on the way back.
I've had a weak battery for a while. On the trip up an odd electrical event happened with my headlights going out for a moment. After returning back I checked the battery and it has a dead cell. Any relationship between charging system and mileage or would it be more plausible to assume seasonal mix and different gas companies?

California98Civic · 10-29-2013, 03:46 AM

If the battery were very seriously under-charged and the ECU can detect that and has ways to adapt via the alternator, I can see how you could lose a couple MPG. If the alternator ran full time, full tilt, then you could lose 10%. At least that's what some tests of alternator deletes here on EM might suggest, a 10% change. But I also wonder if you had a tail wind going one direction and a headwind coming back.

user removed · 10-29-2013, 08:59 AM

A dead cell can drive the charging system crazy as it is trying to charge to a certain voltage then just maintaining that voltage. Get a good battery in it before it leaves you stranded. I would give that a good chance of helping your mileage as well, but with winter coming in the northern hemisphere you will see mileage dropping.
I just noticed the difference in the winter blend fuel in my Fiesta, it's about 5% on a Sunday drive we take to a flea market out in the country. I can usually get over 50 indicated relatively easy but this time it was a battle to get over 49. Probably about a 3 MPG average difference with everything else being about the same.

regards
Mech

doclees · 10-29-2013, 10:42 AM

Thanks guys. Well its a jeep so it always has a head and tail wind at the same time. But I didn't feel or find any significant difference during rest stops either way. Amp meter hasn't been doing anything outside the slight dip right and left of center. Will be heading to GA this week with a new battery. Lets see how that averages. It was CT gas and I think thats not allowed to have any hydrocarbons and it is taxed another $.30 a gallon.