I'm woking on a friends 1990 300 TE wagon , that has a cold starting problem.

It's been really cold here in Buffalo, (9 deg f) and when you try to start the car, it sputters a bit then dies. I've done this continually, and it won't start outside.

I moved it indoors overnight (50 deg garage) and it started, but in the same manner, misfiring, idiling at 500 rpm (smoothly though?!), backfiring through the airfilter, sluggish up until 2-3000 RPM. When it warms up, it's better.

I'm not too familiar with these cars, and did a search. I came up with an OVP relay (what does that do?)

how do I pull the codes>?
what else should I look for?
there is also a plug at the rear of the engine looks larger than a temp sensor- has four wires coming to it, and has a sheiled boot, like a spark plug boot, but smaller- can anyone tell me what that is/does?

thanks for all your help!

Percy.