So I have been having a problem starting my car and I can't seem to figure it out. Heres what I own: 2009 G37 sedan journey.

For the past week, every time I go to start the car, it wont start. Whether it has been 1 hour since I last drove it or 4-5 days since I last drove it. It will unlock with the power button and all the lights will turn on and so will the navi screen, but then it will crank a couple times and never fully start.

So I have one of those battery pack portable jump starters and I attached it to the battery the first day and turned it on and the car started right up. The odd thing was I looked at the portable battery which lets me know the percentage of charge that is left and it still said 100%. So there was no charge actually taken from the portable battery to "jump start" my car, but it did start.

Every single time for the past week that I go to start it, it doesnt start. the lights and navi turn on but the car doesnt crank. I connect the portable battery and it does start as soon as i connect it every time. The battery remains at 100% charge every time so it never takes any power from the portable battery to start.

I had my battery tested and my alternator tested and they both tested good.

I made sure my leads to both terminals were tight and cleaned off all corrosion.

I experimented some more with the portable jump starter that I have and decided to just attach the cables to my battery, but not the battery: car doesnt start
I attached the cables to the battery and plug in the portable battery, but leave the portable battery OFF: car doesnt start
I attached the cables to the battery and plug in the portable battery, now leave the portable battery ON: car does start (BUT the charge on the portable battery remains 100% so the car did not pull any power from the portable battery to start....)

I'm completely confused as to what is going on.

I assumed if the battery was slowly being drained by something, that the portable battery would have to send some power to the car to start it, but like i said it remains at 100% charge on the portable battery. I would also assume that if there is something pulling enough power to not let my car start within 1 hour of turning it off, that the same thing would drain by battery so much within 4-5 days that i wouldnt be able to power unlock my doors - but 4-5 days later the car still power unlocks door and all the lights turn on inside and so does the navi screen - it just wont crank to start all the way.

Anyone have any ideas?

Thanks!

 

Sounds like your battery cranking amps is too low and the battery pack helps give it enough to start. My portable battery doesn’t show any loss after a quick jump start. Once my car would flat out not start and turned out the ground wires were loose in the engine bay. Tightened them up! VROOOOOM!

 

So I checked the cables and grounds and they appear to be tight. I learned last week that if I just kept pressing the start button after the first try that the car did not turn on, then it would turn on the second or 3rd try without needing to add any jumper cables. Given it would crank VERY VERY slowly before starting, but it started so thats what ive been doing the past week since ive been working every single night and havent had time to get a new battery just yet.

BUT today a new problem arose after it cranked very slow to start. The car jerks forward as it shifts in to 2nd and into 3rd gear. Then it refuses to go into 4th gear unless I switch it into manual mode. at that point it lunges forward into the remainder of the gears i manually up-shift it to and as i slow down it very smoothly downshifts back to 1st gear. Any ideas? not sure if the 2 problems are somehow related or just bad luck for me that they both started occurring within a couple weeks of each other. Hoping my transmission isnt toast...

 

So plugged in an OBD and the car threw 3 codes: P0340, P0335, P0345. Thats the crankshaft position sensor AND BOTH camshaft position sensors....this seems highly unlikely to me that all 3 would go out at the same time. Any thoughts? i've ordered all 3 just in-case

 

So have you replaced the battery? I would buy a battery, try it then return if that is not the problem. My last Benz had the CPS fail & car wouldn't start at all.