Any D4-d Experts Out There ?

[Johnaa]

Mk 1 Yaris - 1.4 D4-D engine - low mileage (20,000miles). I have developed a problem with slow running when the engine is first started from cold. For about the first minute the engine runs smoothly. It then starts to run with what can only be described as a blub,blub,blub rhythm. It sounds as if the engine fires four times then misses four times then fires four times and so on. Increasing the revs stops it but it returns when when dropped back to idle speed. Once the engine warms up this strange rhythm disappears (after a couple of minutes) although as time goes on it is taking longer to disappear. Once it is warm, the engine runs absolutely perfectly at all speeds. Fuel consumption is very good ( between 55 and 65 MPG depending on conditions) as it has always been.

I have tried the following but with no improvement:

Checked for water in fuel filter - none found.

Tried premium diesel fuel - no difference.

Tried BD44 injector/fuel system cleaner additive - no improvement.

The fault appears to be engine temperature related so I have the feeling it is to do with some kind of sensor malfunction or blockage. I have wondered about air flter change but cannot see how this would relate to temperature.

Has anybody experienced anything like this? - or have any constructive ideas? I am reluctant to book the car in as I think the problem could be difficult to reproduce as it only happens for a short while when engine is cold. Thanks.