[MadJoe]

Hi guys,

I know I haven't posted much lately but I still lurk and still have my trusty old 2006 E90. I wanted to post because recently I noticed a big drop in fuel mileage which I attributed to a faulty thermometer (it opens too soon not allowing the engine to fully warm up thereby not reaching peak efficiency).

Well, I still suspect the thermometer as having a negative affect on my mileage, however it seems the bigger culprit was the "Starter Ground Cable" (a simple engine to chassis ground with BMW nomenclature). If this cable oxidizes to the point where the aluminum begins to return to it's "dust" form (which is wont to happen after 13 years most of which was spent in New England), it will not cause any fault lights nor will it stop your car from starting, it will simply separate and cause rough idling, poor fuel mileage, and who knows what else. It looks a bit like this:



Removed and compared to new one:




New one installed:



Replacement is pretty straight forward, but thinking to check it might not be. Take a look under your cars once in a while if they're more than 10 years old!

Joe

ETA: fixed an image

[m_thompson]

Just replaced in my wife's E91.

[arkie6]

That ground cable is copper, not aluminum. That is why it turns green when it corrodes. The copper has a thin plating, probably tin, that gives it the silver color.

[Phil325i]

So you're saying replacing this earth strap cured poor idle and heavy fuel consumption..?

[MadJoe]

Originally Posted by Phil325i

So you're saying replacing this earth strap cured poor idle and heavy fuel consumption..?

Yes.

I'm not sure of the exact particulars, but when your engine is running a charge builds up on it from all the moving parts and needs a way to ground in order to not interfere with the electronic things connected to your engine (Specifically ignition components, but others as well) so a ground cable is connected to discharge this build up. Take that path away, and the charge starts forcing its way into other electronics, disrupting them, and can actually arc across to a ground point if it builds up enough before finding another path.

My last car to have this issue looked like the paparazzi was under the hood taking pictures when I drove at night. It's how I realized I even had a problem since that old car was already super glitchy and inefficient anyway, so I hardly noticed it was getting worse.

[MadJoe]

Originally Posted by arkie6

That ground cable is copper, not aluminum. That is why it turns green when it corrodes. The copper has a thin plating, probably tin, that gives it the silver color.

That makes a lot more sense. Thank you!