Wednesday, February 27, 2013

Carbon clogging and misfires in MINI engines


Earlier today we took apart an engine from a 2008 MINI Cooper S with 75,000 miles on the odometer.  The engine was clean and the services were all up to date.  The complaint:  A persistent check engine light.  The only stored codes were misfires.   The car had been to two other shops, and it had received injector cleaning, new plugs, new coils, a test for vacuum leaks (none found), and a test of the direct injection system (which it passed.)

None of those things had any effect on the engine light.  It might come back in 100 miles, or it might take 1,000 miles.  There was no pattern except this: it always returned.

This is what we found:








What you are looking at is carbon – baked on oil deposits – covering the intake port, intake valve, and valve guide.  The deposits are so think the cylinder didn’t work normally anymore. 

For purpose of comparison, here is a more typical intake port.  This photo is from a different engine – a Land Rover – but it’s got even more miles on it and as you see the valves are spotless



What caused that?  This MINI has a turbo motor so we quickly looked there.  Turbos are known to leak oil out their shaft seals.  But the turbo was clean



There was only one other possibility:  the crankcase ventilation system.  That system recirculates crankcase vapors, which can be laden with oil.  Indeed, when we looked at the hose, we found the same sort of clogging.



How do you prevent that?

Use a top-quality oil.  We use Mobil 1 and Amsoil almost exclusively.
Change the oil frequently
Keep the breather system clean

This engine is one of the new Bosch gasoline direct injection engines.  While that improves performance it can aggravate situations like this.  In a traditional engine the gas flowed into the cylinders through the intake ports, so it acted as a cleaning solvent to keep the ports clear.  Now, with GDI, the gas is fired right into the cylinder and nothing but air enters through the port.  If the air is laden with oil mist, this is going to be the result.

I expect this is not the last one of these we'll see

The final question in your mind is surely what you do about this?  There’s so much carbon that the only real cure is to pull the head and do a valve job/decarbonization.  However, this engine had a lot of wear in the lower end (separate issue) that caused the blow-by, so we realized the most cost effective cure would be an exchange motor from MINI

And that’s what we are doing.

MINI – can we repair one for you?


John E Robison
Robison Service
Springfield, MA

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