Things that would make it hard to crank and make the header glow at idle
To little fuel or to much fuel ( this can be checked with a fuel psi gauge and a scan tool)
To little or to much ignition timing ( this can only be checked with a scan tool)
Cam timing off ( this can be checked by setting the cams and #1 Cly like your going to install a new timing belt )
Vac leak but this would also cause a very high idle , like above 2500rpm (this can be confirmed with a scan tool or checking every vac like for issues , this I do not feel is your issue)
I have never seen a CAT cause a hard cranking or a header to glow until you put a load on the engine
I think he is either running very lean , overly rich and burning the fuel in the header(rare) or has a valve open when it shouldn't be from cam timing issues , these are the most common then it gets complicated
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