Okay, so, I had been trying to track down a vacuum leak in my R53 for nearly a year after replacing my head gasket. The main symptom was a random falling idle when rolling up to stop lights, causing the engine to bog and occasionally stall. I fog tested, pressure tested, vacuum tested & sealed up EVERYTHING with the same symptoms. It was driving me NUTS. So, I just sorta gave up on looking & figured I'd drive it 'till the cause got worse & revealed itself.

Anyway, I ended up driving my car from California up to Idaho & back (about a 12 hour drive each way). By the time I got to Idaho, the issue seemed to have completely gone away & has been running fantastic ever since. WTF?!

So, I have a couple theories-

During my troubleshooting, I was constantly resetting the ECU. Is it possible the idle control is something the car has to sort of "figure out" after a certain number of logged miles?

Whatever functions as the Idle Air Control Valve on the R53 (I don't believe they have one) is defective & finally started working on the trip.

I would love to hear any other theories you guys may have or confirmations of mine. Thanks!!

 

What year is your R53? No codes in the DME or warning lights on? Is this symptom only present when you come to a stop, what about start up - how's the cold start? Does it happen when it's cold and warmed up? Does it fall down and then correct itself or does the idle fall and has it ever stalled out? Did you replace any parts while attempting to diagnose this issue? When's the last time replaced any routine maintenance items? Did you attempt to clean the throttle body to make sure it's not gummed up?

 

I've noticed my car runs different after a long trip, like down to NC for MOTD, but it shouldn't fix an issue, unless that issue is a cat starting to clog.

You may be experiencing a vacuum leak, if the problem went away during a road trip that would be normal of a small to moderate leak. Most vacuum leaks are most prominent at or just off of idle, so when you either transition to boost or increase throttle opening the problem would begin to dissipate.

Have you had any CEL lights, codes read, etc? I'd start with pulling the codes if there are any stored. That'll give a direction to begin diagnostic.

 

2006 R53, about 93k miles

No codes, all code history cleared by Mini dealer & no new codes have appeared

The cold start sounded pretty weak but would always start, though not enthusiastically

It would happen both cold and warmed up

The idle would fall and usually correct itself, though always idling fairly rough. Occasionally it would not be able to catch itself to correct & just fall & die.

I've basically been replacing parts on the car ever since getting it. The one problem area that I focused most of my attention on was the horribly designed supercharger inlet pipe & green gasket. I've sprayed the whole assembly down with soapy water & pressurized it so many times it's ridiculous. There are no significant positive pressure or vacuum leaks in the system anywhere. I've tested every component / sensor I could find that pertains to idle & we even have a second 2005 R53 (which runs perfectly) that I've borrowed suspect parts from & never saw any change.

The throttle body is squeaky clean with all new gaskets.

 

Is there any way to begin diagnosing a clogged cat other than pulling the header & just looking at it? I have read that cars that have had blown head gaskets will often have clogged cats because the combusted coolant vapor will start to melt the front of the converter. Any truth to this?

 

This is something I've seen more than a few times with BMW cars. The alternator starts to output too much AC voltage causing all sorts of idle issues. So, what I'd suggest you do is drive the car with a voltmeter on measuring ac volts and put the meter in MIN MAX mode. After the drive and several idle dips the MAX measured AC voltage should be less than 50mV, the BMW's I have seen with that issue would spike to 2-3 Volts. If you have access to a scope you can also observe the AC ripple, which should be a series of equally spaced and height humps on the display at ALL RPM's, regardless of loads on the charging system.