I've just changed a tired thermostat that wasn't closing properly and had a weak spring. The engine took significanty longer than normal to warm up.

Take the car for a shortish run and stop before the temperature gauge reaches operating temperature. Pop the bonnet open and feel the hose coming from the thermostat housing. It should be stone cold. If it is warm thenm the thermostat probably needs changing.

No, it sounds like a highly efficient diesel engine that turns most of the energy from the fuel into motion.

There is a long standing myth that diesel engines are so adiabaticly efficient that they won't warm up. In fact the proportion of fuel energy that enters the coolant is very similar to that of petrol engines-the extra efficiency comes from reduced exhaust gas energy.

Diesel engines are heavier than petrol ones and the combustion takes place in the piston bowl, not the head, so it takes longer for the coolant to warm up.

However, modern diesels use uncooled EGR during engine warm up which helps reduce engine warm up time. I'd expect the temperature gauge to hit 90 before 7 miles of travel.

Cheers guys. Basically what's happening is when I start the ngine from cold, no matter how far I drive the engine never seems to pull good MPG, and the power feels low. I am waiting until teh engine is fully warmed before measuring any MPG.

However, when I stop the car for anywhere between 5 minutes and an hour and half, if I get back in with temperature still in the engine, the fuel economy jumps about 10mpg across the board, and the power feels much better.

I also think it has a very slight shudder undle idle, but I've never owned a diesel so what I'm noticing with that could be normal.

Does anyone know why that could be?

Edited by xreyuk on 03/05/2012 at 11:29

I would assume that your 1,6CR has a DPF fitted & think that will be at the heart of your poor MPG during engine warm-up.

I recently changed from a 1.9 PD (no DPF) Passat to the 2l CR (DPF fitted) & noticed straight away that fuel consumption over the initial warm-up was about 20% higher with the CR engine.

I found some VW training info regarding this engine which states that the ECU is programmed tro get the DPF up to operating temperature as quickly as possible, & so the engine has to waste diesel heating this up from cold. I am assuming the 1.6 has a similar strategy.

Once the engine is up to temperature it is more efficient than the PD unit which is saying something, plus you have the slightly fuzzy warm feeling inside knowing there is very little particle pollution exiting the tailpipe.

Edited by dieselnut on 07/05/2012 at 00:49