Awesome channel Scotty - My 2015 Malibu LS has 123,000 mi stalls 1 minute after cold start. 2nd start it runs a little rough but once it warm never a problem restarting while out and about. Manifold bolts were loose then tightened, throttle body was "cleaned" by mechanic when the manifold was tightened. Both 02 sensors changed, MAF senor changed. New plugs. Had been throwing a P0171 but no more codes, just the stalling issue. Thinking the throttle body might not have been really cleaned - cleaned.
First of all, it is possible that's there's a small, unmetered air leak that has an outsized effect on startup. Scotty has videos on looking for that.
Along that train of speculation, it's also possible that there's a cylinder misfire on startup. (Any Mode Six data on that?) Fuel Trim data (from a Live Data session) might also be of interest, if the vehicle is running very lean, particularly on Short Term Fuel Trim data on startup).
Did the mechanic use a boroscope (Scotty has videos featuring boroscopes) to look at the degree of carbon buildup on the valves?
There's some chat on the Malibu boards about this very problem, with a 2014 year model.
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You can clean the throttle body, yourself, with inexpensive throttle body cleaner. Scotty has a video on this:
https://www.youtube.com/watch?v=NWCKvU2FmDc
And, so does thecarcarenut:
https://www.youtube.com/watch?v=3B-a8WJtd8w
