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I have a Nissan primera diesel , which at the moment is only giving me mid to high 30s mpg .
Only had the car 6 months , but i remember it giving me low 40`s when i bought it .. question is will a good service bring back the mpg ?? .. its done 10,000 miles so far since the last service
my experience is that winter = lower mpg...I've noticed this more with a diesel in the past
Diesels need servicing regular to run well, the engine oil degrades etc
Clean out the EGR valve. Did my Vivaro before christmas. Was getting low 30 mpg and now getting over 40.
i drove at a constant 60mph on a long journey (140 miles) in my golf tdi and it still only got 49mpg.
Servicing may help a little but not by as much as the difference you're seeing - unless there's a fuel leak somewhere along the line? EGR valve good point too.
Try an injector/fuel system cleaner like Redex.
Make sure tyre pressures are correct.
Things like roofbars/bike racks have a surprising effect on MPG.
Driving style and type of journey have the biggest influence om MPG.
Short stop start journeys in town where the car doesn't warm up properly will give the worst MPG.
Steady 50mph journeys on A roads the best.
Maybe as you've got more used to the car you've started driving it differently and using more of the performance?
A good service should do the trick. I'm not a fan of these newer service times. My SAAB 9-3 diesel was doing nearly 50mpg but as it neared its first 18k service it was difficult getting it above 40mpg.
Hard to beat frequent servicing
Isn't there a trick of putting a bit of diesel in the engine oil which helps loosen baked on gunk over a period of time? There are all the additives you can try but many don't do anything the best value one is what is currently called 'desiel power ecomax' (seems to have a new name/forumla every year). EGR clean is a good idea you'd be supprised how clogged up they can get, in many cars you can just bypass it entirely. Also renewal and upgrade of air filter and replacement of oil and oil filter should help. You should be able to fix any problem yourself bar injector servicing, saving many pennies.
Other than servicing check;
fuel pump functioning,injectors all working, ecu not flooded, egr not blocked, air filter not blocked, mass air flow sensor not degraded,cam sensor not degraded.