Originally Posted by Jhill

Not always. You would be amazed at how far modern cars can compensate now days. A big clue would be if you reset te ECM and ten it runs like crap. If the vacuum leak comes on slow and gets larger over time then the ECM can constantly compensate and in theory should throw a code for fuel trim out of spec but not always. A good example is vw 1.8t, they always crack at the PCV elbow and develop a leak yet no code and no obvious poor running. Disconnect the battery to work on something unrelated and next thing you know it won't run (starts and dies).