Yesterday we were on a 10 hour trip and the engine was acting oddly; when braking, the engine sounded just like the engine brake, but the MPG indicated that it was consuming fuel (engine brake doesnt consume fuel). When braking, the engine would sound, and the MPG indicator indicates fuel consumption, it would go down to 50, with the heavier braking, like when stopping... then at a certain point it would suddenly stop... kinda hard to explain It seemed to happen most times but not always. This issue didn't happen before.
The battery wasn't full each time I noticed it happening; battery was like 4/5 or something like that. Or do you mean the red battery warning light? No red lights or warnings of any kind showed. It was hot at the beginning of the trip (100˚+) but later it was cooler (probably 70-80) and I think it happened that time too...
Hook the leads of the voltmeter up like you would to jump start the car and you will get a voltage. The owners manual tells you the locations under the hood.
I have had this happen to me several times too. No pattern of conditions where I can duplicate the issue. My 12v battery is fairly new so I doubt that is the issue.
Update Jan/Feb 2019 for anyone finding this thread via search: I believe this issue was caused by a vacuum leak caused by the EVAP purge valve randomly binding and being stuck open, as I diagnosed in this thread. The solution is to replace the EVAP purge valve.
