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Discussion Starter · #1 ·
I have a a problem with surging idle engine speeds once the engine is fully warmed up, on my 2005 R53 (88,000 miles)

RPM will reapeatedly rise to around 1400, then it sounds like the engine almost cuts out, the speed drops to around 900 before bulding up again. The whole cycle takes around 5 seconds. Engine gets hot while doing this when sitting in trafffic and that causes the fan to come on. With the fan running, the surging stops (although RPM still seems high for idle). When this is happening, my dashboard "Scanguage" shows the system switching between open loop and closed loop.

The car runs fine during normal driving and there are no (relevant) fault codes (only compaints about steering angle).

Local garage says they can't identify the cause and I refuse to be ripped off by a MINI stealership (which willl probably try making me replace things which don't need repalcing and make no difference!).

I connected an Autoenginuity Pro-line Connector to check the O2 sensor voltages. This is NOT an area I really understand. However, before the "behaviour" kicks in, sensor voltages seem to fit with what I've read - with the pre-cat bouncing around, while the post-cat looks much more stable. However, when the surging starts the post-cat voltage changes a lot - with both sensors occasionally outputting 0 volts.

Questions:

1. I don't understand why the voltages seem OK while the engine is getting warmer, then appear to go "haywire" once it's hot. Does that indicate dodgy sensors or some other fault (what could that be?)?

2. If I can get them at a reasonable cost, I'm tempted to try new sensors. I read somewhere that different sensors are needed for pre-cat and post-cat (Bosh upstream and NTK downstream?). Those I've seen online seem quite expensive "just to test a theory", but then I came across this from Neobrothers (who I believe are reputable) which seems to be for pre AND post cat: Buy Exhaust Sensors for your Mini 3dr Hatch 2005 (R50/R53) Petrol 1.6 Cooper S Hatchback Manual | Neo Brothers
Would I be foolish to use this? Is it likely to work OK without any other adjustments to the vehicle (mapping etc)?

3. Has anyone else had a similar issue and managed to solve it?

Many thanks for any info/suggestions you can share.

Cheers
 
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