Diesel economy dropped, causes?

[rrobson]

Before I get into the nitty gritty, I appreciate I’m moaning about something fairly trivial, of which many of you will likely not see as a problem to start with!

Anyway, my current 75 (a diesel tourer) had been absolutely exceptional on fuel over the last 20000 miles or so. But recently I have noticed it’s economy has dropped between 10-15%. On my current commute which is a round trip of 106 miles, it used to average at least 60mpg, normally around 62mpg. I haven’t done this commute for a while, but over the last two tanks with the same driving style it is struggling to do better than 55mpg. I used to do 260/270 miles at 1/4 of a tank mark, but it’s down to 230ish

There are a couple of things that have changed since, but I don’t believe them to be a huge contributor to the loss of economy. These include
-just passed 100000miles
-recently had the rear arms replaced
-not long since serviced

I suppose tracking could be out due to rear suspension refresh causing extra drag and scrubbing, but given I had a calliper binding last year and it still returned better economy I doubt it has a huge effect. It was quality parts for the service. Thermostat is opening correctly and it runs at 89 degrees. Drives as good as it did and doesnt feel any different.
It’s a little frustrating because it used to do 800 miles to a tank, now it’s more like 700. I’ve recently been doing lots of short journeys (for months now) so I understand it takes some time to “learn” but it would normally have sorted its self out by now. I also last week chucked a bottle of cleaner in and drove the 106 miles in third and fourth all the way to give it a good clear out. Feels perkier but hasn’t made much of a difference else where.
Thoughts??

[CivilSteve]

These seem to be really picky about suspension components being touched and alignment being thrown out. So perhaps you're onto something there.

Sure any of the calipers are not binding? Even just a little bit

I had a front binding start of the year, then one of the rears got jammed up a short time after

[rrobson]

Definitely not binding, it’s had two new rears and a good used front on in the last 6 months, plus it rolls easy after a journey where before I didn’t need to use the handbrake when stopped…

Suspension may be contributing but I don’t think it would be enough to see the reduction I am getting. It really does need tracking though, but given the state of it underneath I doubt any of the fasteners would come loose anyway!

Fuel is the same, from the same filling station so I think it’s unlikely to be the issue. Maybe I’ll try a different petrol station?

[SD1too]

Hello Reece,

Fuel consumption is an emotive subject so it is with some hesitation that I'm replying to your appeal for thoughts. But I do have one which is worthy of consideration.

The crucial fact that is missing from your otherwise detailed account is how you are measuring your fuel consumption.

Originally Posted by rrobson

I used to do 260/270 miles at 1/4 of a tank mark ...

This seems to suggest that you're using the markings on the fuel gauge. If that is the case then it will give you a very approximate result indeed and no good at all for judging fuel consumption usefully.

The trip computer will also give variable readings depending upon when it is reset. These devices also typically become inactive below a certain road speed which adds to the inaccuracy. Your comment ...

Originally Posted by rrobson

... it takes some time to “learn” ...

... suggests that you are using this method.

The most reliable measurement is a brim-to-brim calculation made on as close to a full tank of diesel that you can manage. If you're not doing so already I'd recommend that you change to this method.

I doubt very much that the mileage, the new rear arms or the service have contributed to a perceived increase in fuel consumption. On the other hand ...

Originally Posted by rrobson

I’ve recently been doing lots of short journeys (for months now) ...

... frequent short journeys will definitely reduce mpg because fuel consumption is significantly higher during the warm-up phase as well as during stop-start journeys.

Simon

[rrobson]

Hi Simon, I always do a brim to brim calculation for fuel economy, that wasn’t the first indicator of low economy it was the fact that the usual mileage covered on the fuel gauge wasn’t achieved. A brim to brim confirmed the average was down by a good 7 or 8 but total miles covered was obviously lower before it needed filling

I appreciate the comments about using the fuel gauge as a marker (we all know what these are like) but over the years it has been very consistent when travelling longer distances. I have always been able to gauge economy by the mileage traveled at the markets on the tank. The only thing I use the trip computer for is the miles remaining. This tends to be fairly accurate believe it or not!

It’s unusually low so something isn’t happy. Injectors on their way out? Maf out of spec? Would probably benefit from a t4 just to investigate. Again, I know it’s fairly trivial for some as many probably aren’t managing 50mpg, let alone 60. The problem is driving this distance every day it’s the difference between 6 round trips on a tank or 7 round trips. Couple of weeks of that and it soon adds up!
The short journeys are a killer I know, but it normally reverts back to high economy very quickly, just not this time!! Which is leading me to think something isn’t working as it did.

[clf]

I fear your economy could be much worse, as brim.to.brim, as the weather warms up, should improve in the warmer weather.

Has the fuel used changed? Perhaps the station itself has changed supplier, or the brand changing the formula?

I used to get around 10-20 miles less per tank on the cheaper fuel, with a noticeable.improvement in performance on a branded fuel (which would explain the economy).

One of your fuel floats could be failing (sagging slightly) perhaps?

Using heating ventilation more frequently, or window open?

A seizing pulley on the serpentine belt?

A change of oil type (although I would think the fuel economy difference would be low single digits over a fill for this).

A blocked air filter or fuel filter? Remove the cover for air filter to make sure no debris is in its cradle to block the intake.

A failing maf could also have an.effect on the economy.

A soft tyre or imbalanced pressures?

sent from my iToilet Pro 3□

[rrobson]

Some great suggestions there, thank you.
Quite the opposite has been seen, some of the best economy I’ve got in previous years was when the temperature was in the 30’s.

That crossed my mind, but 100 miles a tank difference? Regularly used the same petrol station for about 5 years (an esso that was a texaco) so you’d expect it to have reasonable quality fuel.

Last time I filled I put 12 gallon in and the light was just about to come on so that’s pretty normal and tied up with the fuel gauge, so I’d like to say that’s behaving.

Cracking the passenger side windows on a long journey always had negligible effect on fuel usage, and I don’t use the air con because it doesn’t work

The belt is something I could look at, hadn’t thought about that. Doing 700 miles a week a seized pulley would identify itself sooner rather than later!

I changed the air filter during its last service a couple of months ago but it is another thing I can check.

Tyre pressures checked regularly as one has a very slow leak and needs topping up once a month so pressures are good.

[clf]

Originally Posted by rrobson

Some great suggestions there, thank you.
Quite the opposite has been seen, some of the best economy I’ve got in previous years was when the temperature was in the 30’s.

that is what I meant. The economy should improve with better weather, but if yours is worse, it is maybe more of a problem than it is demonstrating.

Providing the driving is more or less identical between testing, you are right to be concerned. My own commute is month in month as close as identical with the odd run to the shops at the weekend. 10 miles each way, same route each time and I would get 420 miles per fill in winter, rising to 450 or 460 in summer. However, a couple of times per year, I would need to drive to Dublin, which would be about 120 miles from my house, and nearly all motorway or dual carriageway. When I have that drive, I will get between 480 and 500 miles to a fill depending on the temperature.

What is the operating temperature displaying on the OBD? My economy improved by around 10% when I finally sorted thermostat. However I also changed my fuel filter for the first time in my ownership, filling it with injector cleaner , running for half a minute, then leaving it to soak overnight.

Adding an inline thermostat would improve it each time - when I got the car it didnt go past 70 degrees , then with the first inline stat it wouldnt rise above 78, then 82, and finally with the current wahler it opens at 88/89, then settles at 86.

I know 10% seems extreme, however, if your coolant is only reaching 70 degrees, that along with perhaps another as yet undiagnosed issue could be contributing 70 miles of lost economy.

Another thought too, the clutch starting to slip slightly could have an effect. (try a higher gear, then accelerate as though in the gear below).

[EVS]

After my fuel pump failed and was replaced, I noticed a great improvement on fuel economy. I went from about 28 mpg to 34 mpg.