How old is your battery and have you had it tested? If the battery is marginal it won't spin the engine fast enough and just like in the winter it just won't start. Does it seem to crank as fast as it used to or should? That may be contributing to the trouble you have starting it at high altitudes.

There is a compensator on the IP, it is on the back and looks like a can with an open port (I believe the port is right on top). Often times people assume there is supposed to be a vacuum hose attached to it and add one but it is supposed to be open to the atmosphere to sense pressure and adjust fuel accordingly. Make sure yours has the port open and there is no dirt or crud in it too.

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Marty D.

2013 C300 4Matic
1984 BMW 733i
2013 Lincoln MKz