So, I've been troubleshooting my car's rough idle / inconsistent vacuum issues ever since finishing my head gasket job last fall. After taking it to a shop, turns out it has low compression across the board: #1- 130 #2- 135 #3- 125 #4- 145 The way I see it, there are two possibilities- The timing is off by a tooth or so and the exhaust valves are opening too early, or- I second-guessed the timing on re-assembly & ran it off by a tooth for about a minute. It started to sound like a diesel so I shut it off. Here's the full story- Please keep in mind that I did all this last September, so I don't really remember the specific reasoning as to why I did this, but here goes- I used white out to put reference marks on the chain & cam sprocket. With the timing chain tensioner out, I rotated the crank until the cam sprocket turned 360 deg. I think this was to bring it's arrow back to facing up without rotating the engine backward. Unfortunately, this caused me to lose my reference mark on my timing chain. However, I believe I did the rest of the head gasket job without moving a tooth. So then, I got the engine re-assembled and started it up. It struggled to fire & idled pretty terribly, so I second-guess the timing. Looking at the position of the cam gear & comparing it to a "before" reference photo (also referencing a mark I'd made on the crank pulley), It appeared to have skipped a tooth. So then, I moved the cam gear counter-clockwise by one tooth. I believe this would retard the timing? Fired it up, it ran fine for a few seconds, then slowly started to get more & more valve train noise, eventually wounding like a diesel & I shut it off after a minute or so. Wanting to make sure I had the timing correct, I pulled the timing cover only to discover that it was actually correct the first time, so I placed the chain's colored links on the crank & cam sprocket's corresponding arrows & have been running it that way ever since. If you've made it this far, I have two questions: Is it possible to have bent valves from running the engine one tooth off? Is it possible for the timing to "look correct" based on the links / marks but still be off?? From what I have heard, this is an interference engine but will only hit if it's "way off". I've also heard that if you're lining up the links & sprocket marks, your only two outcomes would be to have correct timing or to have it 180 deg. off, resulting in a running engine but with crank & cam sensor fault codes. I currently have no codes. Any input on this would be greatly appreciated.