Any diesel engine design experts around?

[qapmoc]

Can anybody suggest a reason why my toyota van with a diesel engine of the latest type with all electronic control and sensors everywhere is like two different engines in summer and winter.
When its summer and hot/dry it is smooth and quite (for a diesel) and the performance is almost acceptable but when its winter and cold/damp it is rougher and noisier and the performance is bl00dy awful.

Everything appears to be normal and the temperature gauge always stays at the same mid point all year round, it does not smoke or anything else and the fuel consumption is always about the same. Tried it with/ without the air filter, tried changing some sensors but it is always exactly the same.

[Clanad]

Most of the larger horse power diesel engines used on pickup trucks here in the western U.S. utilize computer controlled fuel injection. In cold weather (sometimes -20F) the computer commands injection closer to top dead center of the piston than in warm weather. This has a pronounced beneficial affect on both performance and noise. It's possible the smaller versions you are probably describing do not have this feature (for simplicity and cost reasons). We also use a different diesel fuel in cold weather than warm, but the temperature differential between seasons here can be as much as 100 degrees F., which, hopefully, you don't experience...

[stanleyman]

A deisel will take a while to warm up to give full power but with colder denser air this should give more power in winter so the difference from cold to warm is more marked. Once warmed up there should be no disernable difference to summer but bare in mind oils also tend to be thicker until thoroughly warmed up and if you are somewhere very cold this could be a constant drag as could be headwinds etc. If you only do short journeys it never gets to full temperature.

If I were you I'd get the electronics checked to see if all is ok for where you live. Personally I won't have electronics on my deisels so don't get these problems.

[qapmoc]

Yeah I thought that the colder denser air would give more oxygen and better combustion but it aint so on this one. Where I live is not really cold even in winter, 12 deg C minimum and I do mostly longish trips, but one of your point's is a good one, the van is a fairly new second hand import from Japan and I have noticed that all the others here are about the same because I can keep up (or down really) with the other same model vans on the road. I don't see why but it might make a difference that it was not designed for my location.
I have the feeling that the system is programmed to not make pollution under any conditions and it is this which makes for the odd behaviour.

[stanleyman]

12 degrees C, sounds like summer to me! I can't see that would cause any problems. As you say, this could well be emissions related where exhaust gasses are injected into the air supply to reduce oxygen and the production of emissions at lean high combustion temps.

There is a kit for Landrovers to do away with this valve that I can't think of the name at the moment and improve performance. You could blank off the pipe and valve with a blank gasket to prevent exhaust gasses reaching the inlet manifold to see if it makes a difference.