can someone tell me what this plug is, friends selling his 93 fd3s for $8500 and 75,000km, it doesn't wanna start until you've tried a dozen times and sounds like a vac leak once its running and the heat light comes on after a minute of driving and he just had to replace the coolant reservoir, i wanna know if this plug could cause some of these troubles

 

I don't know what that plug is, but I can say pretty confidently it is not causing those problems, caution against purchasing without diagnosing the problems.

 

Obviously it's a JDM car. If it were a LHD car, I'd say that plug is for the cruise control actuator, which the car was either never equipped with, or removed by one of the previous owners. It would have nothing to do with the car's issues.

Regarding those issues....I can only speculate, but some might mistake an over-flowing coolant reservoir with a leaking coolant reservoir and change it. An over-flowing reservoir, combined with hard cold starting, stumbling idle, white smoke (steam) for several seconds after it starts would point toward a coolant seal failure. With alot of steam/coolant going through the combustion chambers and out the exhaust, it conceivably could also explain the exhaust heat light coming on as well.

 

In fact looking at it again, looks like the actuator cable was left behind.