You need to start by getting a baseline diagnosis...

Both compression and leakdown tests. It is pointless going further until you know the most important components are not worn out and are OK. You never know if you have a cylinder with little or no compression, in which case you will never get the car running right. A local shop can do it for your cheap if you don't have a kit.

Now look at the idle air hoses to check for vacuum leaks once you know the engine is OK. Ideally 180psi across the board with no more than 10% difference between each cylinder. As long as you are over 160psi, everything should be OK.

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2016 Monsoon Gray Audi Allroad - 21k
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2003 Gray Mercedes ML350 - 122k