Originally Posted by TyrantII
Source is a engine built from scratch by Valve for HL2 (HL1 wasn't it's own engine, but a modified engine based on Quake 1), but it's long been showing it's age as well. Valve's going to need a new engine sooner or later as well, which is why they confirmed they're already deep into development on Source 2 engine. Source itself was built to be modular, and allow easy updates that wouldn't break past functionality.
COD's engine is worse off and much more patch worked than Source. We're already up to ID Tech 5 among other propitiatory engines. COD is wasting a lot of rendering resources just trying to keep that old code running at 60FPS.
Source wasnt built by scratch:
Source distantly originates from the GoldSrc engine, itself a heavily modified version of John D. Carmack's Quake engine, as is explained by Valve employee Erik Johnson on the Valve Developer Community:
“ When we were getting very close to releasing Half-Life (less than a week or so), we found there were already some projects that we needed to start working on, but we couldn't risk checking in code to the shipping version of the game. At that point we forked off the code in VSS to be both /$Goldsrc and /$Src. Over the next few years, we used these terms internally as "Goldsource" and "Source". At least initially, the Goldsrc branch of code referred to the codebase that was currently released, and Src referred to the next set of more risky technology that we were working on. When it came down to show Half-Life 2 for the first time at E3, it was part of our internal communication to refer to the "Source" engine vs. the "Goldsource" engine, and the name stuck. ”
Source was developed part-by-part from this fork onwards, slowly replacing GoldSrc in Valve's internal projects and, in part, explaining the reasons behind its unusually modular nature. Valve's development of Source since has been a mixture of licensed middleware (Havok Physics, albeit heavily modified, and MP3 playback) and in-house-developed code.
John Carmack commented on his blog in 2004 that "there are still bits of early Quake code in Half-Life 2"
Its no different than the current cod engine, which probably bares as little resemblance to the q3 engine as source does to quake 1. I'm sure UE 4 still has some unreal 1 code in it too, same for BF1942 code in frostbite.