Loud pop then smoke, hard start
 

I've been driving a 94 E20 wagon for about 6 months, which is my first gas Mercedes after 6 years of diesel w123's.

This morning when I tried to start the car there was an audible 'pop' and then a visible cloud of smoke coming from engine bay (and the car didn't start). The sound and smoke appeared to be coming the driver side of the engine bay, with smoke most visible closest to the firewall. The smoke had a smell that I associate with burnt electrical equipment.

After this it took me few tries to start it and I had to give it gas while cranking to get it to start. I was able to drive it to work after this.

Any ideas what I might have blown?

Some additional info:

• The idle has been running high (1400 in neutral, ~900 in drive) for the last few weeks and throwing 6 flashes on the DM tool which relates to idle control. The idle has always been a little high but it has gotten worse recently and consistently trips the CE light. It also has recently been difficult to start when warm, i.e. I have to give it gas while cranking to get it to start.
• The only work I've done on this car so far is to replace the oxygen sensor when I first bought it, change the oil and air filter. A few weeks ago I cleaned the MAF with the CRC MAF cleaner which didn't seem to have any effect on the idle issue.
• Last night I built the pin diagnostic tool as outlined in another thread. I was unable to get a reading on either pin14 or pin8 for currently unknown reasons (I did get a reading on pin6 and pin7 so I think the tool is working). This did mean that I turned the ignition to 'ON' multiple times last night while I was trying to get a reading. Not sure if this might have contributed to the 'pop' this morning.

The starter is in that area and I think the main power from battery to starter goes through the floor there also.

In any event, running the car isn't a good idea. If the pop was from electrical and no fuses cleared, the unfused main power feed may have contacted ground and burned clear.