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What can drain batteries when engine off?
Hi. We bought a 2000 ford excursion 7.3 a few months ago. On a long trip pulling our camper, a p1211 code came up and gas mileage went from 8.5 down to 6.5. Brought it to mechanic in town. They said oil looked bad, that could be the cause. So got new oil (switched to synthetic too). They, and everyone we talked too said we said we should be getting at least 10-12 mpg even when pulling our 28 foot 6000 pound trailer. Said to test drive it. Took it out for drive with camper, mileage was back up to 8.5. Noticed they didn't put oil cap on. Brought it back, they gave new oil cap. They took it for a spin with computer hooked up, couldn't find anything wrong, suggested we buy performance chip to increase mileage. Came home and left town 2 days later with different car. My mother in law was going to use it when we were out of town. She told us the day we left, it wouldn't start at all. When we get back, I jump start it, it worked, thought I must have left lights on. A few days pass, went to start it and it was dead. Took 1 yr old batteries out to O'Reillys to test them, told they are bad. I asked if vehicle wasn't ran in winter, can that freeze the batteries (MN). They said yes, warranty covered them, so got 2 new motorcraft batteries. Put them in and it started fine. A week goes by without being used, went to start it up and it won't start, no crank, like batteries are bad.
Wife worries mechanic did something. Like we took it in asking them to find us something wrong and that they gave it to us. And since they didn't put the oil cap back on, said it was dropped down in engine compartment.do you think this is a coincidence and just bring it back to them? Just seems so fishy that everything worked fine until they had it and couldn't find anything wrong.
What could make the new batteries drain when it's sitting there unused? Thanks so much.
Ps, what is a good computer/performance chip to get better than 8 mpg when towing??