High Fuel Consumption (Low Mpg At Pump High Mpg On Trip Comp)

[iulian.arg]

Hello,

 

I'm facing a problem with fuel consumption.

Putting it strait I have a much bigger fuel consumption at the pump compared to the one in the dashboard.

This happens for a few years 3-4 at least.

My car is a Mk II C5 2007 estate 2.0 hdi 100 kw 6 speed auto gearbox.

 

The last figures I had are these:

- 640 km with 59 liters of diesel; that is 9.2 l/100km (30.7 mpg) at the pump; the trip computer shows 7.8% (36.2 mpg). The drives I did tend to be closer to the trip computer figure than the pump figure.

- an older figure I remember was 850 km with 65 liters of diesel; that is 7.6% (37.1 mpg) at the pump compared to a 6.4% (44.1 mpg) on the trip computer.

 

When I bought the car I remember  that I made 1100 km on a full tank of 65 liters with aircon active 30% of the distance. That was 5.9 l/100 km (47.8 mpg) which was very close to 5.6% (50.6 mpg) reported on the trip computer and the car's brochure (10/7.2/5.6 ext/mix/urb liters/100km) (28.2/39.2/50.6 mpg).

Since then I'm getting worse figures every fuel tank.

 

The car is in good conditions and I drive it very smooth. I don't do much city driving just shopping and visits. I rarely move the car in workdays. In the weekends is where I use the car the most and mostly for medium to long trips.

Now the car has 155k km and I bought it at 97k km.

The service intervals are in 10-12k km intervals, which is about once a year. The oil I use is Total ineo ECS 5w30.

At 118k km the car had a gearbox fluid refresh with Castrol jws 3309 oil.

I had the EGR replaced and DPF cleaned more than half a year ago and didn't changed anything in the MPG figures in a good way. By contrary, the difference seems to increase between pump and trip computer.

I know that I have 1-2-3 bad glow plugs because of bad starts in the winter under -10 Celsius but I don't think this issue has such big impact on MPG.

 

Any ideas are apreciated.

Thanks.

 

 

[paul.h]

You could check if the air filter is due for replacement and if the brakes are binding. For the brakes, after a run check to see if the wheel hubs are hot or put the suspension on max height and then jack up each wheel in turn and see if they spin freely without any sign of brake binding. If the brakes are binding then they will need cleaning to remove brake dust but also the rear ones will need the corrosion removing between the suspension arm and calipers. There are pinned topics on the brakes to help.

 

I guess you are resetting the fuel trip computer by holding in the button on the end of the wiper stalk, but if not then give it a go before some long trips.

[iulian.arg]

Hi Paul. 

Thanks for the reply.

 

The air filter was replaced every oil change (10-12k km). It's cheap and not worth keeping it 2-3 service intervals.

The brakes are in good condition. The wheels are all cool in either longer and shorter trips.

I'm reseting the "trip A" every top-up. Then I do the calculus and compare pump consumption with the trip computer figures and get those huge differences.

[aspire_helen]

Hi

What is your issue? Excessive fuel consumption or the error between the mpg figures.

 

If the latter, then there is no point looking for causes of increased fuel consumption. The trip computer simply divides distance driven (from the odometer) and fuel used (from the fuel flow meter) over a fixed time period. So, there may be a slight error in either sensor.

 

For comparison, I have recorded every drop of fuel I have put in my 2001 2.2HDi Hatch since 2008 and compared the calculated  fuel top-up consumption with the trip computer each time. Sad, but true - and a very good comparator when the engine is not performing. In that time I have driven 82,252 miles using 7638.9 ltrs in 139 top-ups giving 49.1mpg.  The average difference between trip mpg and fuel pump calculation is 3.1%, with the trip computer generally being the higher mpg, but not always. I regularly get 60mpg on longer trunk road cruising at 60mpg.

 

I have more confidence in the fuel top-up consumption being the more accurate, provided that you always top up to the same point. I top up the tank on every fill until the auto-cut off has cut the flow 3 times.

PS Ignore official consumption figures.

[paul.h]

One thing not mentioned is the speed being driven. I have found going much above about 60 mph increases the fuel consumption quite a bit.

 

Is the temperature gauge showing a normal level (about half way) and the engine warming up within a few miles - if not then maybe the thermostat needs replacing. If you have a code reader that does live data you can check the coolant temperature and should find the thermostat opens about 90°C when the radiator top hose will start to get hot and the gauge will be about half way.

 

You have not mentioned if the engine management light has come on so I assume there are no sensor faults bad enough to cause it to go in to limp mode. However, a sensor could still be faulty and giving wrong readings so a diagnostic check may be worth while. Things to check would include the coolant temperature, inlet air temperature, manifold pressure and air flow meter.

 

Faulty glow plugs probably only affect fuel consumption whilst the engine is warming up. You could reset your trip computer after this and see how it compares with official values.

 

The timing belt was due for renewal at 10 years old, has this been done and if so did the fuel consumption change then ?

 

I have never had an automatic gearbox so I am not sure how it affects fuel consumption, but if it has a fault and is not changing up the gears soon enough it could increase fuel use. Changing its oil may help but I am not sure.

[iulian.arg]

Hi,

thanks for the replies 

 

Aspire_Helen the problem is indeed the high real fuel consumption. 30 MPG for light trips outside of town is a low figure. The more appropriate figure was the trip computer one of 36 mpg. I keep using this last example because is typical for my trips. 

Your 3.1% difference is exceptionally good. My differences are in the 15-20% range, more at the pump than on the trip. I wouldn't complain at all if the figures were reversed, with less at the pump  :D. Also your mpg is very good. You seem to have a very fine running example C5.

I always fill to the maximum to have a good comparison. I actually see the diesel fuel when I lock the tank.

I wouldn't totally ignore the official figures because they seem realistic enough in this case. I'd ignore newer figures which seem to defy all the reason, being almost half of mines. Even the Touaregs V6 3L diesel, 2,2 tones SUV is officially better than my C5 <_<... riiiiight.

 

Paul.h I rarely pass 100km/h and the average for the last trip was 35 km/h.

The temperature gauge works normally and I'm always double checking it with the oil temperature gauge. I've checked it also with Lexia and it was ok.

I have no engine management light up. About the sensors you mentioned, I checked them from time to time there is nothing visibly wrong or abnormal.

 

The timing belt hasn't been replaced yet. It's planned for this year. It's a year over the recommended 10 year time interval but the mileage is not met by a good margin.

 

The automatic gearbox is changing correctly and keeps the engine running in the 1400-2000 rpm interval. That's adapted to my driving style.

 

Thanks.

[paul.h]

It does not seem as if there is anything wrong with your car. I do not know if it is possible, but maybe a Lexia could check the fuel use calculation and adjust it.

 

Newer cars do use a lot less fuel than earlier ones. Our 2015 C4 1.6 hdi manual gearbox is a good example. On long motorway trips at the speed limits it gives about 70 mpg, the previous 2011 C4 with the same engine would give a bit over 60 mpg and a previous 2008 would give over 50 mpg for the same type of trips.

[aspire_helen]

Iulian

I might dispute your "very fine running" comment when the cambelt snapped at 70K miles in 2008 and destroyed the valve rockers!! :) All those 82K miles were after I rebuilt the cylinder head on my driveway (now done 170k miles). In fact, I started recording the fuel consumption to assess my work! Afterwards, I am sure the engine had less power but retained good mpg, so is fine for my relaxed driving style.  During the re-fitting, one thing I could not do was measure the correct thickness of the cylinder head gasket (I think there are a choice of 4 thicknesses), so I just fitted a new one of the same thickness. I also allowed the cylinder bores to get a little rusty, requiring some fine emery work.  Hence, the compression may have reduced.

I agree with Paul - the mpg drops markedly above 65mph. And, as with Paul I have no experience of automatics, and I suspect your problem lies there.

H

[paul.h]

I had another thought, I do not know how the fuel use is calculated but if it is by the fuel injector opening times, then if one is worn it could let in more fuel than expected. If so, the trip computer could under report fuel use.

 

I had the timing belt/pulleys/water pump replaced on our C5 when 10 years old and everything seemed in good condition, but it had only done 30k miles. In your case at 11 years old and about 100k miles you are taking a gamble that it will not fail. If you can afford to lose the car then that is ok. I could not take this gamble on our car since I need it to tow a caravan and there are not that many cars as good for this use.

[iulian.arg]

Hi

 

I come with new data. I just traveled ~1400 km and happened to refill alt exactly 1000 km.

So I've done 1000 km with 65 liters which is 6.5l%km and the trip computer was showing 5.5l%km. That's a difference of about 20% between pump and bc. 

At least I have a constant problem: ~20% difference between pump and trip coputer. 

 

paul.h the only thing adjustable with Lexia regarding consumption calculation is to include or not the fuel consumed by DPF regeneration.

I'm always skeptical when I hear about new cars being more efficient compared to previous generation/facelift. But I have no reason not to believe you about your C4s. As for C5s mk3, from the discussion on another forum, there are no big differences between all 2L diesels mounted on them and there cannot be a big improvement without a revolution in engine technology. And there are at least 3 iterations: Euro 4 RHR (same engine as on mk2), Euro 5 140 and 163 hp and E6 150 and 180 hp.

About injection system condition, I might take a complete diagnosis as a later try cause it's a 70-100 EUR procedure in a professional dedicated service. Cheap diagnosis done at the garages are always a gamble and most of them try to hunt the problem instead of detecting or identifying it.

 

aspire_helen I'm not worried about the cam belt because the architecture of the RHR engine moves 90% of the stress from the cambelt to the cam chain. This is the reason PSA gives such a long service intervals for the belt. 

I don't know if the 2.2 HDI has the same belt+chain architecture as 2.0 RHR engine. I haven't heard of many HDI cambelt failures so far. Yours was reasonably inside the change interval of 10 years or 120 k miles.

Anyway, belt change is to come soon. I'm already in 12k km on the current oil and the next service will include belt kit as well. 

 

Thanks for the replies.  ;)