I have 1990 190E 2.6 liter that has always worked flawlessly until about a year ago it started to act up.....
To start-up it'd take longer than usual.
I'd put the key into the ignition, turn it for a good 3-5 seconds, and it wouldn't start up...
But if I did back-to-back once or twice it'd fire-up then almost immediately.
If I were to go anywhere in the car (after having it turned-on and running)and as long as the car's engine stayed warm, I could turn-off the car, and when coming-out of the store (or wherever it is I just happen to be at) I'd put the key in the ignition and the car would usually fire-up rightg away and start.
Normally I wouldn't have a problem with it on warm days.
But I did start noticing that when the climate got colder, it'd take long for the car to start, if not start at all!
I doubt the heater core has something to do with it...
I'll eventually replace the heater core immediately if I could get the car running again.
Do you think it might be the fuel pump or something to that extent???
ANY suggestions anyyone??? Please help me!
Thank you.

Fab- this could be just about any number of different things. I'd start with the cap/rotor. How old are they? Remove the cap and inspect. Wires? Plugs? A weak fuel pump is always possible. Start with the spark first, then think fuel...

-GH

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I'm not a techie so this is anecdotal . . .
Had a similar problem with a 300E brought to Canada from Florida.
Difficult starting in cold weather. Dealer replaced Voltage Protection Relay (problem solved). Problem recurred a year later due to moisture in relay.
Replaced again. Starts great even at 10 below!!
Hope this helps.