Ok I will start out by saying that this is not on my chevy (even though it is running a little rough too) but on my wifes car. I have a question for those who understand vacuum readings better than me. So here goes to make a short story long. The car has 140k on it and runs well just idles poorly. the engines are known to have intake manifold leaks etc so I figured I would vacuum test it after not finding anything with any sprays (propane, carb cleaner etc). So the problem is that in park idles pretty fine. turn on the ac and it gets a little rougher acc belts moves a little (I have replaced tensioner, ac compressor, condensor etc already). the real problem is in D or R. the idle drops to around 500 so a little lower than I would like and gets a bit rougher. kick on the ac and the belt moves a lot more and idle gets rougher (doesnt stall and rpms stay fairly consistent but drop a little). So I vacuum tested the engine and in park no ac the needle is at 20 in/hg no fluxuating so thats good. turn on the ac and it drops to 18 and still no fluxuating (tiny vibration of needle but thats all no pressure change) rev it and let go it all checks out normal. That is how all the checks say to do it. my question now is I then put it into D and the guage drops to about 15 and with Ac about 14, but still no fluxuations just running rough (below 17 is considered low). Is this drop normal in D as that is not how the test procedures explain how to do it, and I couldnt find anything on vacuum readings in gear (probably cuz its dangerous to stand in front of the car with it in gear:lol. I was kind of hoping for fluxuations to more easily diagnose the problem but that didnt happen. I also rechecked plug gaps, it has been seafoamed, and only uses Chevron. I need to still do compression and fuel pressure readings. Just wondering if anyone had info on whether the pressure drop in gear was normal. oh yeah no codes anything like that.