High rev on start

Hi,
I have a slightly modified 2017 TCE 90. It's been a company car until I bought it so it's clocked now at almost 200k km (~120k miles).

Since autumn last year, it started reving up to 2000 on start and keeping it there for about half a minute before it comes down to 900. It's automatic, and if I try driving before it is done with this little tantrum, it uses the clutch to not let me go above ~10mph.
I tried cleaning the MAF, then found it has a MAP, not a MAF. Tried looking at the IACV, seems it doesn't have one (uses the throttle alone for idle). Cleaned the turbo pressure sensor. Cleaned the throttle body, but the issue is still there.

-I thought it was temperature related, though it does it even when it's warm outside.
-Usually does not do this if it was already running in the previous few hours.
-Sometimes it revs to 1600 for 10 seconds and is ready to go. Other times it goes to 2000 and holds it there for 30 seconds with little rhytmic blips every 2 seconds.

The exhaust was modded, the cat was modded, K&N filter and it's been remapped. It's significantly louder than before and when it starts revving up on start I am just waiting for my neighbours to come punch me (I know I would)... I know it's not good practice, but due to cringe, when it starts doing this, I sometimes just turn the ignition off and retry after 10 seconds. Sometimes that works.

I hope the mods are not the cause as this was not an issue when they were done but about a year after the last mod.

Anyone had a similar issue? Any ideas what could the cause be?

Thanks

Your throttle body is dirty, try to clean it.

twingoBYmercedesbents said:

Your throttle body is dirty, try to clean it.

Click to expand...

I did clean it (mentioned in original post). Took it off, cleaned from both sides with carb cleaner. It's nice and shiny. Sadly the behavior is unchanged. Neither worse nor better :/

Update: scheduled a service and diagnostics for Wednesday. I'll leave an update afterwards if they find the issue.

I hope that they can find and repair the issue tomorrow. And that it is not too expensive. Please post after you have your visit.

The service did not confirm my appointment... Drove it to a friend so we checked it together. Vacuum leak... Though I've not found it yet. I tried the Carb Cleaner method, but did not locate the leak. The Short and Long Term Fuel Trims seem to confirm it too. Other symptoms I paid no attention to also align with a vacuum leak (hissing noise (heard in video), black charr on exhaust, hickups /almost stalling from time to time, more shake when idle, smell of petrol behind, etc)
I'll try locating it again over the weekend.