[TL;DR – rough idle was caused by a stretched timing chain and camshaft sprocket that was rotated on the shaft]

I have a 2008 Cooper S R56 that I bought about two years ago with 100k miles. After owning it for about 8 months and 6k miles, it developed a rough idle, seemingly all of a sudden. At a stop the engine would jump from 700 to 1000 rpm, then settle back down and do it again. It would noticeably shake the car. It would be smoother after an hour or so of driving, but never went away. The car also hesitated a bit when on the gas between 1500-2000 rpm. Above 2000 rpm it smoothed out and everything was fine.

I tried some basic things to fix it, like adding oil, using injector cleaner, looking for obvious leaks, and cleaning the mass airflow sensor. None of it made a difference. After about a month I got a CEL and a P1497 code: unmetered air downstream of throttle body. There were also misfire codes for all 4 cylinders that would come and go, but always all 4 at once. I’d hit the limit of what I could do without a garage at home to work in, so I took it to a mechanic that claimed to specialize in Minis.

They did a smoke test and told me that the valve cover seemed to have a crack/was leaking somewhere. Other folks on this forum said that was common, so it seemed like the cause. They replaced it, and replaced the spark plugs as well. When I got the car back it idled just as rough as when I took it in. They did another smoke test and replaced the cover again, to no avail. I bought a new vanos solenoid based on some more forum posts, which they installed, but it made no change. They gave up and handed me back my car.

I took the car to another Mini/BMW mechanic. They cleaned and checked the MAF again, along with the spark plugs and ignition coils – all of which came back normal. They boroscoped the engine and saw carbon buildup on the throttle body and intake valves, which wasn’t surprising for the mileage. They cleaned the throttle and walnut blasted the intake. That gave some benefit: the engine idled smoother than before, but it was still too rough. After that they gave up and handed me back my car.

I took the car to a third mechanic, this time one that said they were started by race car engineers, and the guy who answered the phone told me they were “intimately familiar” with Minis. They ran a bunch of diagnostics and found a lot of suspected causes that turned out to be within spec: ignition coils, spark plugs, fuel pressure, vanos solenoid movement, and engine timing. They said the last one was a mystery, as it was clear that the engine was misfiring.

They disassembled a few things and noticed that the frame for some parts that hold the camshaft and valve timing components had loose bolts, and weren’t lining up the way they should. More engine disassembly revealed the cause: the timing chain had stretched, and the corresponding sprocket on one of the camshafts had rotated away from where it should be. Apparently the sprockets are just press-fit onto the camshafts, and there isn’t a fixture like a set screw to prevent them from rotating around. The sensor that measured engine timing is only on the other camshaft, so the computer didn’t know. This all meant that the engine timing would always be off, and although the ECU tried to compensate and adjust, it was never going to be able to do so. That incorrect timing was causing the rough idle and misfiring.

They replaced the timing chain, the camshaft sprockets, and a bunch of other related internal engine components. The car idled smoothly afterwards, and didn’t hesitate at low rpms anymore. I’ve now driven the car for about 5 months, and it’s all still running fine. It definitely seems like the chain and sprockets were the root cause.

I’ll mention this because you’re probably thinking it: yes, it was a big and expensive repair for an older and higher mileage car. It may not have made sense normally, but given the prices and availability of cars at the time, it felt like the least bad option.

Hopefully this helps someone out there with a similar problem.

 

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Thank you for your Post. Your follow thru will definitely help someone in the future. Two cams and the computer only values one, who would have seen that with all the sensors on these cars. These cars have way more computer cross reading than a four cylinder motor needs to operate. One sensor shuts down 2 circuits controlling 4 un-related functions of the car. I'd keep the shop that actually stuck with you to find the real problem on speed dial. That's the shop you can can confidently recommend to friends and family.

 

Timing that is off can be identified by removing the valve cover and simply placing the timing tool on the head cams and a locking pin from below. Identifying a stretched timing chain is even easier by replacing the chain tensioner with a screw-on-tool and turning the crankshaft. Both these measures require at most or less disassembly than identifying loose bolts in the camshaft and valve timing components.

Timing chain sprockets and/or or camshaft timing units are held in position by one-time-use bolts which have to be replaced with new ones every time. If such a bolt has come loose during engine operation, it means that either it had not been replaced with new or screwed on properly the last time it had been loosened/removed.

The N14 engine has indeed only one camshaft position sensor. The N12, N16 and N18 engines have a sensor on both camshafts. All engines (including the N14) have an additional crankshaft sensor.