I own a 2005 volvo S40,and i’ve been having some inconsistent issues on start up. Usually after it’s been sitting for awhile and it’s cold outside, I start it up and it has it’s usual high idle, but after a few seconds it idles even higher and a red warning light comes on with the message “try slowing down or switching up gears” and if j leave this long enough, grey smoke comes out of the front of the car. as soon as I turn jt off and turn it back over, the problem is virtually gone. Additionally, the check engine light will come on but also turn off randomly. I plan on running the codes and finding the problem that way, but if j could get any suggestions on where to check first that’d be great.

 

cold start issues can often be caused by fuel pressure issues, particularly on older models like the Gen 1 s40s which have a fuel pressure regulator that can fail and bleed off rail pressure over time. Since you've seen codes, there's something stored. My guess it will report on fuel trim issues. Another cause is intake air leaks or a stuck idle air valve. Cold engines like rich mixtures to start (effectively what a choke does) so if your intake / throttle body is not enriching the mixture or is letting in more air through an unintended source, you will have longer cold cranking. These causes may not be in play for all models/years as the fuel management tech changed over time so best to research other posts specific to your model year/engine type.