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Discussion Starter · #1 · (Edited)
I have been having a problem for the past 6 months, where I have been getting miss fire on 1 bank 3 cylinders at idle. When I am driving, I can't tell if the miss fires are present and car feels fine otherwise.

My mechanic explained that it could be a vacuum leak.

Car is a 2004 E46 330i Sedan 200K mile 6 speed manual. I bought this car last summer to replace my turbo build e90 335 single turbo with 528whp. I wanted a cheaper car to play around with and possibly add a turbo at some point, when i get it running well. I have own an E46 before and I liked it.

The weekend after bought the car home it started with a problem of miss firing after 5k RPM. I found out it was the catalytic converter being clogged.

Initially I did a tune-up and replaced a few items that needed attention:

- Rebuild Vanos unit by DR Vanos - (Vanos solenoid was bad in new unit/fixed that)
- Upper and lower Intake boots (cracked in multiple places)
- New Valve cover Gasket
- New DISA Valve
- Oil Catch Can (CCV Delete)
- Spark plugs (Bosch)
- Ignition coils
- Oil change

After the first issue, I took to a Mechanic#1 with the clogged catalytic converter. He replaced with Headers and test pipe back to stock mufflers. It resolved the issue of miss firing over 5K. however, a new issue of surging at idling after being running for a 5 mins arose.

Took it to mechanic#2 and he said the O2 was install incorrectly and he had to place them in the correct order and he also replaced the Intake gaskets and repaired a leaking vacuum port in the back of the manifold. It sort of fixed the issues however, now it idling funny and a code for the IACV was presented. The issue seemed to improve after I replaced the IACV however, I still have an idle issue where at idle 3 cylinders on the same bank is miss-firing

Mechanic #3 a Friend suggested to using;
- Brake cleaner to spray around the engine bay for vac leaks.
- Or get a smoke tester to check for vacuum leak.
- He also mentioned the getting a new valve cover because it could be a clogged internal PCV issue.

I am pulling out my hair with the car. I love driving this car however, I have gone way over budget for a cheap beater / fun car and have not been able to driver it much.
I would like some advice to troubleshoot this issue. What would be the way forward?
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