Hello Everyone.

I purchased a 2014 E350 coupe about 3 weeks ago. Car runs great but it did have the "loud engine" noise associated with fatigued engine mounts.
I didn't drive the car much the first 2 weeks with the holidays but I had no issues when I did.
Last Sunday I decided to try and change the engine mounts. I successfully replaced the passenger side but ran out of time before I had a solution as to how to remove the driver side.
When I went to drive the car down off of the ramps I pushed the Start/Stop button and everything went dead. No dash lights, no interior lights, no power. I had the trunk open
the whole time so initially I thought I must have drained the battery with the light on (yeah, I couldn't believe that either) anyway, I opened the hood to look for a jump terminal,
didn't see one so I assumed the battery was in the trunk. I closed the hood. When I did, interior lights came back on, dash lights on, car started right up. So, I'm thinking OK first
car I've had with keyless-Go must be something I don't know, maybe a key, something, I don't know. The car started and ran fine every time since then.
Changing the passenger side engine mount didn't do much for the noise though. We're expecting a big snow here tomorrow and I had a little time today so I thought the trans mount might
be easy enough to do in the time I had and I'd see if that makes any difference. Which it did the car is significantly quieter. However, I finished the trans mount replacement.
Then I went to the store. When I came out of the store. Opened the door interior lights on, power good. Put the groceries in the car, get in and push Start/Stop, everything dead again.
So, I know from last week, I open the hood, nothing, close it lights come back on. Push Start/Stop, dead again. Open the hood, close it back on. I did this four times, each time it failed
when I pressed Start/Stop. The fifth time, I removed the Start/Stop button and used the key and the car started. I replaced the button and I restarted the car just fine, several times now with the button.
I mention all of this detail because I am an IT professional by trade. Part of my job is noticing patterns that machines/software make.
I this case I'm seeing two. 1.) The failure has only occurred in the 2 instances where I had the car jacked up. 2.) Each time I open the hood and then close it the power comes back on. (What the hell)
That looks like some kind of a feature, like a reset. But I have no clue as to what. A bad key, a bad Keyless-Go module. Does these symptoms sound familiar to anyone?
I'm not a mechanic but I've had motorcycles, boats, SeaDoos, a bunch of cars. I have an 83' 500SL Euro I've been working on for 22 years. If someone told me each time they closed the hood electrical power was restored
I would say at best its a coincidence, at worst, you're nuts.
It's cold as **** here now (Pittsburgh) and 12-18" of snow expected. The last thing I want is to end up stranded somewhere with this car I just bought. But, I don't even know where to begin with this one. All thoughts are welcome.

I appreciate you indulging me.

Thanks Marc

 

Great detail on noticing everything it sounds like u got a loose negative cable under the hood I would check the battery on the passenger side by the firewall theres a big cover that covers the battery. u must remove that first to locate the battery. I have a e550 cabrio and theres a battery for me and one in the trunk but the trunk one is the auxiliary one it's small compared to the front engine bay by the firewall one. chexk those out and see hopefully it solves your issue.

 

I had seen the battery cover in the spare tire area. I did some more online digging last night and saw that there was another battery under the hood.
I have not had a chance to check, but I will go back and check all connections.
Thanks, I appreciate the follow-up.

 

Can you keep inserting your ignition key instead of using Start button. The KeylessG module in the trunk is very capable of ignoring your key or not being able to chat with ignition module.

Test a few things:
Battery voltage above 12.5V
GND Strap chassis to engine
Keyfob 3V cell
OBD2 bus scan for errors

 

I don't drive the car everyday but it has not failed since. Having said that we've been frigid and under snow for almost two weeks now and I don't want to chance it stranding me.
I will check each of your suggestions as soon as it warms up a bit. I would think though if the battery were weak enough to cause a complete loss of power: A) it wouldn't come back on and eventually start the car
and B) after sitting outside for days at a time in this weather it would not turn over or at least crank slower, which it hasn't.
Grounds, I have thought of, but how does that equate with opening and closing the hood bringing back the power each time?
I did in fact receive a key fob battery low message, and replaced the key fob battery, shortly before this incident occurred. I have not received the message after replacing the battery. I have used both keys since.
I do have a bluetooth scanner here somewhere. I will look.

I appreciate the reply