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2011 Focus 1.6 Diesel P0234 code

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Hi people, my sons diesel focus has the engine light on so we plugged the OBD reader in and it says P0234 Engine Overboost. The car drives perfectly and pulls well in all gears, there’s no loss of power. Is this a common fault and is there something thats renowned to fail or common to fail causing this?

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The variable vanes on the turbo are not moving freely and do not open fully when they should (after reaching  certain RPM), that causes the speed of the exhaust gases going through the turbo to be too high which subsequently causes too high of a pressure on the intake side (at the MAP sensor) aka overboost.

This video will help you understand how variable vane turbo works and how sticky vanes cause overboost.

Vanes getting stuck is very common occurance, the EGR returning gases full of soot in the engine and the PCV valve putting oil in the engine intake and subsequent oily exhaust gases full of soot passing through the turbo plus the design of vanes itself are recepie for "sticky" vanes.

If you have a good enough OBD reader you can monitor the MAP pressure with Forscan live capture feature. The best way to test the maximum turbo (MAP) pressure is going full throttle through the rev range in high gears up a long hill. This way you will know how much overboost you are having, MAP pressure at full throttle should be about 260 kPa I would think, any more and it is overboosting.

The best way to clean the vanes is to use a turbo cleaner (similar to oven cleaner - removing oily carbon deposits), but that usally requires removing the turbo and soaking the vanes in the cleaner, this can be labour intensive and can also expose some problems if the turbo is not "fully healthy".

Since you say your car pulls normally and doesnt go into limp mode, I suspect the vanes are not clogged badly. So you can try cleaning the vanes by burning the gunk off during driving. So you need the exhaust gases to be very hot. Best way to do that would be going on a long highway ride where your are driving as fast as possible using as much throttle as possible and having the revs above 3000 rpm for a long period of time (that way the EGR stays closed and doesnt cool the exhaust gases) and also doing pulls between around 1500 rpm and 4000 rpm inbetween to have the vanes open and close. That said this will ofcourse put some stress on the turbo due to high heat but if the turbo is realtively healthy it should be just fine, also not having to old of an oil in the engine to lubricate the turbo bearings as good as possible is advisable.

You can also try moving the turbo vane actuator in the engine bay yourself and "feeling" for sticky vanes, that may also release some junk that is stuck.

Hope it helps, I have a 1.8 tdci so I dont know your engine exactly but it should be very similar.



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Thanks for your detailed reply, much appreciated. Liked the video too. I’ll start with a clean up, I’ve changed the oil & filter so I know that’s good. I’ll update if I find anything interesting in there!

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