What causes MPG to decline over time?

outoftheknow

Injectors and carbon buildup around that mileage first up I would say. Injectors on slightly bigger engines than that (10,000 hp and upwards ) cause  fuel consumption increases as the seats and valves wear a bit or they are sticky. If that has spring valve lifters those cause a tail off due to timing (even if they are designed to be “self adjusting”). I’m not aware if any service takes the lid off nowadays to check valve clearances.... seems to all be plug in the laptop and see what it says

Fuel additive first I reckon. go a decent one and they do clean serious deposits. In a car you shouldn’t be so worried about it cleaning the fuel tank out and blocking things up - but the decent ones do clean the fuel tank as well and make sure the carbon is all “aligned” improving combustion a significant amount. The super dooper fuels have the additives in of course but the standalone shots should reset things for a while.

Automotive diesel is just too clean and too expensive to make the additives worth the cost every tankful. At 80,000 miles assuming 50mph average that is 1,600 hours at high revs. Fuel pumps and/or injectors need a bit of TLC in that time I reckon - presumably new injectors would cost more than the van is worth knowing VAG

youngsyp

With all the normal service items checked off, the EGR valve would be my first point for investigation. They are notorious for getting caked in carbon on diesel applications. Should be simple enough to remove and clean by hand.

I don't think you'll have a DPF on that MY so, you can rule that out.

Paul

imightbewrong

We do have a DPF, and it does flash on every now and again - I suspect it's coming to needing something drastic.

youngsyp

That is almost certainly part of the problem then. Good news is that there are services that will clean your DPF, so it would be worth trying these. Terraclean claim they'll clean the EGR system and DPF, with their service.

Paul

gizlaroc

The main reason for drop in MPG is failing thermostats.

Of course, bloked air filters, changing tyres or low pressures in tyres will also make a huge difference, but failing 'stats and the knock on effects are the largest cause.

All modern cars use the thermostat reading to control fuel, think of the old choke.

Your tdi should be getting to roughly 90ºc within around 10 miles or 15 minutes and stick there rock solid no matter what, up hill, 40ºc outside, 10mph through traffic, 90mph on the motorway, -10ºc outside.

If it isn't doing this change it.

There was a great technical document from VAG a few years ago explaining how the OBC works out the average MPG.
It explained how it uses ambient air temps, throttle position and fuel injection and how coolant temperatures dictate the amount of fuel injected.
From cold it uses roughly 40% more fuel than when it gets to 90ºc.
At 85ºc it was still using roughly 20% more.

Therefore if you have a 'stat that is failing/sticking, your car will be using more fuel thinking it is cold.

A thermostat should be a serviceable part now imho, every 4 years or 50k miles at least, they are so important for decent running, as they fail they make the car over fuel, gunking up EGRs and DPFs as well as hammering mpg.

Also, most cars won't do a DPF regen if the stat doesn't get to temp, meaning that it gets blocked up, often owners are told they need to replace or clean the DPF, which is true, but the new one goes in and 15-20k miles later that is blocked too.
A DPF will not block if the car is doing what it is meant to do, unless you never do 30 mile plus journeys, but so many dealerships, garages, miss the basics when diagnosing, and this costs the customer big time.

rubbertatti

Mechanical tappets are old skool they are mostly hydraulic, oil pressure keeps the Clarence as it’s a 2010 vw probably has a pd engine I know that has hydraulic tappets. Leaking injectors can cause a drop I’m mpg. Open the bonnet with engine running have a look around the injectors you should hear a knocking hissing type noise you might also see black plastic looking stuff around the injectors looks like oil but hard.

rubbertatti

It will also block if you only use it as a shopping trolley. the dpf stores the soot and then burns it off during regen, but for the ecu to go into regen mode the water and oil temp must meet the requirements plus time for the regen to happen. If your doing shopping trips this will never happen, the dpf will reach 100% and all that exhaust gas will find another way out eventually.

IronGiant

Bought my old shopping trolley in March, bit worried about the DPF so ran it up to North West Scotland and back.  1700 miles.  Seems to have worked, the DPF light hasn't come on yet

imightbewrong

I would guess we do 50/50 short runs to motorway runs.  We should have done quite a few 'regens' - also the VW dealer did an 'extra special regen' of some kind - whatever that means.

It's been playing up quite a bit this summer - the engine warning light comes on if you keep your foot moderately down on an incline, and it goes into limp mode.  Sometimes this is accompanied by a fan-type whirring noise from the engine.  All goes away if you restart the car.  This has been happening for a while but the dealer never found anything on the computer.  Unfortunately due to sods law it never happens on the way to the dealer.  Seemed to happen more than usual this summer but probably France is more hilly than the SE of England.

rubbertatti

Do you drop a gear when going up steep hills?
Extra special regen is code for “I ragged the poop out of your car on the way to McDonald’s for lunch, I then sat bouncing off the rev limiter while stuffing my face”

Tho they might have done a forced regen. Amusing to watch people walk by wondering why the cars revving up with no one inside