On cars made 15 years or more ago I could have told you exactly but now everything is electronically controlled its difficult to say. But briefly: a diesel has no control over the air intake, it takes in a full charge of air every stroke, the power is solely a product of the ammount of fuel being injected - unlike a petrol engine. An excessive ammount of fuel being injected will make it smoke but only normally when the engine is under load i.e.. when you are driving. Reducing the ammount of air going in has the same effect, this used to be caused by an obstruction in the air intake, a blocked air filter or collapsed hose, but on a modern engine it could be an electronic fault.

My 52 plate Focus 1.8 tdci 133,000 miles has a cold running fault. It starts easily, even at minus whatever, but then I get lots of light coloured smoke, smelling of diesel. If I drive off straight away it misfires violently making me stop again until it clears. I can get round the problem by starting and then waiting for the engine to warm up for five minutes. Once the engine is warm it runs perfectly,

I took the car into my local diesel specialist, Kent Diesel Centre in Maidstone and they kept the car overnight to do a cold start diagnostic check yesterday morning. They found one bad and another couple of poor injectors, basically it needs a new set. The cheapest repair cost would be for them to remove and overhaul my injectors (as opposed to fitting mew). This would cost about £930, so I'm afraid it's curtains for the old Focus.

I have decided to carry on sitting in the car for five minutes warming it up first and then get another car before next winter!

Incidently, the expert I spoke to in the garage advised to never use any oil or diesel additives, ever. He said they are corrosive and could accelererate injector problems. If you use an additive in a new Ford diesel it will invalidate the warranty!

Edited by ItsChris on 12/02/2012 at 13:15