Whenever you’re replacing a timing belt or chain, or doing any kind of work that requires removing and reinstalling it, you have to make sure that everything is timed correctly. If the camshafts aren’t timed properly to the crankshaft, at worst you’ll damage valves and pistons, if you’re lucky all you’ll have is a slightly rough running engine.
One of my coworkers was getting started on a V6 timing belt replacement, and I was helping him get the engine to top dead center before removing the old belt. We were really confused when the timing marks didn’t line up. The crankshaft and front camshaft were both on their marks, but the rear camshaft was a full tooth off. He checked the vehicle history and found that the piston rings had been replaced a few years ago, so the tech that did that job didn’t get it timed properly when he reassembled it. We were all surprised that the engine had no problems with one camshaft out of time. One tooth isn’t a huge amount, but it still seemed like it should’ve caused something, even just a trouble code for not being properly timed. This tech got lucky, but this is why I almost always have someone else come check my timing before I finish putting everything together.
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