Slowly dropping MPG
 

I've been keeping an eye on my fuel consumption graph in my garage and I've noticed that the FE has dropped very slightly over time from June until now. I've covered 35,000 miles since April '09 and now my car has 97,000 miles on the clock - is it possible that age/wear & tear on either the engine or the car itself would increase consumption slightly over time, or could it be something else? Or is it just a blip? I haven't done anything on the car since August apart from oil changes, and I've used the same oil since buying the car.

After all, I used to have 3.7 L/100km but now I get 4 L/100km :-/

Check your fuel pressure, make sure the pressure doesn't drop off (it will slowly overnight but not to zero) to quickly. If it does, it's an indication of a possible leaky injector, which would have a bad spray pattern.
How's the engine temp. going down the road? A stuck open t-stat will cause the engine to takelonger to warm up, & might not reach full operating temp.
How's the tune up parts? Do an inspection on the plugs, wires, vacuum hoses,& filters. Also a good time to check compression & see what vacuum the engine is making. If it has a maf senser, try cleaning it.

Check fuel pressure - how do I do this?

Engine temperature - the thermostat was replaced a couple months ago as the engine takes 15 minutes to warm up, and sometimes never reach operating temperature. After discussions with other Astra DTi owners, and another ECO4 owner, seems that the engine burns so little fuel, it takes such a long time to warm up the heavy cast iron block. After all, after 4 hours of being parked up in 0 deg C temps, the engine is still hot to touch. Brand new Astra 1.7 CDTi (same engine as mine, but with common rail injection) takes 10 minutes to warm up. So it's normal fare, apparently.

Plugs? Do you mean glow plugs? Wires - it's all digital nowadays, so if a wire is faulty, I don't think the car will work. What wires do you mean exactly? All filters are replaced following the service schedule, but now I no longer have a 140 mile commute, I will be replacing the filters twice as often.

How do I check the compression, and I didn't know diesels had a MAF sensor?

Thanks for your advice though!

Sorry, I didn't know it is a diesel, so no map senser, spark plugs, ect. I think you don't have to worry about fuel pressure either. If compression is low you'd probably have trouble starting it. I'm not a diesel expert. Hopefully you figure out your loss of mileage.