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There are a few harness grounds around the cylinder head and side of the block, the main ground for the engine to body is from one of the engine mount arms.
Intermittent issues starting issues mostly when restarting from an ECO stop. I changed the battery, battery sensor, voltage dip limiter, starter relay, and the starter itself is new. I have explored all the other common electrical issues. So I was thinking about grounding points.
You changed the aux battery as well? What is your voltage dropping to when starting? Any issues when it is not an eco start?
In that case it would only be the large ground strap, you should be able to access it from the top with the vehicle on the ground easily enough on an M274, to check it for corrosion. I've seen a W212 corrode on the body side and we could not see it until we tried to remove and the stud snapped off the body rather than loosen the nut. Required a trip to the body shop to weld in a new ground stud under the gas pedal area. Easy enough to do a resistance check from the engine to the body and see, or voltage drop across the same when starting.
1. Aux battery was replaced about 2 yrs ago. No warnings or codes regarding that. My understanding is that is to power the electronics and doesn't affect starting.
2. I changed this large cable (See pic) that connects the battery to the bolt via the sensor. The bolt otherwise was not corroded.
3. Voltage doesn't drop according to the hidden battery screen.
4. Maybe it occasionally occurs when I first start car but I hit the Keyless GO button one or twice and it starts up.
There is a cable from the engine mount arm on the engine side, to the body of the car, that is your ground path for the starter. That I have seen corroded at the body attachement but only one time. I believe it's on the left side, but I could be mistaken, it has been a while. Can't miss it though, big brown wire running from right on the aluminum arm back to the body of the car.
Intermittent issues starting issues mostly when restarting from an ECO stop. I changed the battery, battery sensor, voltage dip limiter, starter relay, and the starter itself is new. I have explored all the other common electrical issues. So I was thinking about grounding points.
Have you checked the relays as well?
I checked online with regards to ECO start/stop and Circuit 15 and 87M was mentioned.
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