I would greatly appreciate any advice you may have.

[mortimerPAS]

So, I own a 59-plate Mini Cooper S. It recently got a new gasket valve cover, spark plugs and a blow off valve. However, when I took it out of the garage, it was the same as when I had brought it in: no acceleration, no engine lights and a boost in power. The mechanics checked to see if all the hoses were tightened, so they tightened a few. The car seemed fine, so I drove it home, to work the next day and, after parking it, I went for a quick trip to the store's petrol station. It happened again: I parked, then forgot my keys and had to go back in. It started out fine, and after seven hours it started acting up again. I rushed it back to the garage, where the mechanic confirmed that it was still acting up. We went for a test drive, and although it didn't perform as expected, the mechanic found no fault codes. They kept it for a few more days. They told me it wasn't doing it when I called to enquire about a progress report.This time around, I'm hoping they can find the problems more precisely, so I gave them permission to let one of the mechanics use my car over the weekend.I was wondering if anybody else had this and if so, what happened? I feel like I'm doing their job for them right now because I'm researching everything that could be the cause. The fact that they insist it isn't doing anything should not make me feel like I'm arguing that it is.

[mortimerPAS]

https://www.amazon.co.uk/s/?search-alia ... ini+Cooper

[GoodBoyCar]

Before anything else, I would make sure there are no leaks by conducting a thorough smoke test with the intake, exhaust, and all cylinders closed to identify any potential issues. As the temperature rises, the issue may become apparent.

[mortimerPAS]

Thank you, but I still haven't heard back from them, and I'm afraid to call because I know they'll conduct the tests regardless of my warning.

[GoodBoyCar]

I would not necessarily recommend