2004 C5 estate 110 HDI fuel economy

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cachaciero
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Post by cachaciero »

Citroenmad wrote:True, a C5 HDi fill up is between £90-£100 at the moment, depending on how much the fuel is and how low the tank was. That is a noticeable amount! Mind, it does last a good few miles.
You don't have a 2.2 on an autobox :-)

Well im not convinced, but there you go. We seem to disagree about a few things at times :lol:
I'm not really trying to make anybody agree or disagree, just to think outside the box as it were.

Why bother getting the injectors re calibrated when the car is fine?
I was asuming the injectors are fine regarding your theory, as its trip computer has always been out since the car was new, i doubt it had injector problems then and doubt it does now. Any main injector problem usually causes a rattle at rist, its running very sweetly with no rattle, loss of performance or MPG drop. Im very happy with its 45MPG.

Besides, surely if its actually doing better MPG than the trip says, how does that suggest an injector is at fault or not calibrated. You would think it would inject more fuel, not less :?
Why would you think that? (the answer is that we always equate wear with more leakage but it aint necessarily so)

Calibration can go both ways we are after all talking about small amounts here a 1 bar error on the fuel rail pressure is in the wider scheme of things insignificant but it will results in slightly less or more fuel being injected each stroke similarly with injector calibration there is not an absolute figure for any calibration, it will always be +/- minus a percentage and it's this percentage which gives the difference between the two readings. Now in your case you get better MPG than the trip suggest you should, which actually implies that their is a slight fueling deficit i.e if that was made good you would get slightly improved performance at the cost of slight increase consumption. However overall you are happy with the balance and don't consider that it's worth bothering about, fair enough.
Anyway, all of this is really besides the point I was initially making, its a valid point to check the actual MPG and see how or if it differs to the trip reading.
Understood and I am just trying to make people think a little more about the implications of the difference not just dismiss it "as the trip is always wrong"

In summary checking by burning of a known amount of fuel against a mileage is the MPG you get, the Trip MPG is the mileage you should be getting.
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Post by dieselnutjob »

calibrating something implies to me that something can be adjusted
there is nothing adjustable on an HDI injector

one thing that can happen is that the nozzle area can leak/wear which would manifest itself as smoke or knocking.

the other thing that can wear is the solenoid part which would normally result in failure to close after preinjection causing a pinking noise that stops at 3200 rpm.

In my experience what is more likely to cause poor MPG is blocked cat or exhaust. Behond 100,000 miles the cat will start to have restricted flow and the first sign of this is poor economy. The other thing that can cause it is an EGR valve that's sooted up in such a way that it is partially stuck open.
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Post by cachaciero »

dieselnutjob wrote:calibrating something implies to me that something can be adjusted
there is nothing adjustable on an HDI injector
Take your point guess I had the old type in my mind.

one thing that can happen is that the nozzle area can leak/wear which would manifest itself as smoke or knocking


the other thing that can wear is the solenoid part which would normally result in failure to close after preinjection causing a pinking noise that stops at 3200 rpm.
Which kind of demonstrates the analysis I was trying to show, under these conditions the injector will pass a little more fuel than it is designed to do but the ECU has no way of "knowing" that so it is not represented in the trip calculation.
At the onset of either of these problems other symptoms may not be very apparent yet the difference between actual fuel used and the trip value should start to alter.
In my experience what is more likely to cause poor MPG is blocked cat or exhaust. Behond 100,000 miles the cat will start to have restricted flow and the first sign of this is poor economy. The other thing that can cause it is an EGR valve that's sooted up in such a way that it is partially stuck open.
Agreed however both these items will tend to make the driver apply more throttle to overcome the performance limitation caused by poor combustion so fuel consumption will rise but in this case because the injectors are passing the amount of fuel expected by the ECU the trip computer value of consumption will stay in step with the actual fuel used.

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Post by dieselnutjob »

there must be some manufacturing tolerances concerning how much fuel a specific injector will flow in a given time at a given rail pressure. also the flow will be dependant on absolute rail pressure and the density of the fuel which is also dependant on the fuel temperature.

The fuel rail pressure sensor has a tolerance as does the fuel temperature sensor (I think the HDI has one of those, can't remember).

So if your sensors are all within tolerance but deviated in the same direction and if your fuel is particularly thin or thick I bet the ECUs idea of MPG might be a bit out compared with actual. Also odometers can be a percent or two out as well.

On my 607 the trip computer mileage is a bit different to the speedo trip mileage, by about 1.5 percent. Also my brim to brim mpg using the speedo trip is about 0.5 mpg better than the trip computer MPG.

My current fuel consumption is good for a 2.2 HDI auto as I'm getting 37mpg in city/M25 commuting so I don't think there's too much wrong with my injectors.

Because the rail pressure is so high (1350 bar) even the slightest problem will cause something that can be heard or seen long before it starts affecting economy. When my 806 HDI started to pink badly the economy was not affected.
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Post by cachaciero »

dieselnutjob wrote:there must be some manufacturing tolerances concerning how much fuel a specific injector will flow in a given time at a given rail pressure. also the flow will be dependant on absolute rail pressure and the density of the fuel which is also dependant on the fuel temperature.

Agreed I wonder if the ECU actually calculates injection quantities as a mass i.e weight rather than volume, it would make more engineering sense to do so.

The fuel rail pressure sensor has a tolerance as does the fuel temperature sensor (I think the HDI has one of those, can't remember) .
It does and it is the measurement of temperature which makes me think about mass, temperature would need to be known to do Mass / volume conversions.
So if your sensors are all within tolerance but deviated in the same direction and if your fuel is particularly thin or thick I bet the ECUs idea of MPG might be a bit out compared with actual. Also odometers can be a percent or two out as well.
Agreed but I doubt that the overall system variation is much more than 1 to 5% and once having established a baseline any variation will be because something has changed.

On my 607 the trip computer mileage is a bit different to the speedo trip mileage, by about 1.5 percent. Also my brim to brim mpg using the speedo trip is about 0.5 mpg better than the trip computer MPG.
I don't really understand why the odometer and the trip computer mileages should be different on the 607 I assume that they both use the same data, the C5 does and on mine there is absolutely no difference between mileage as recorded on the odometer and the trip computer. However if we assume that the 607 returns 35mpg then 1.5% difference is in simple numbers 0.5 mpg difference between the trip and measured fuel use, that's a big difference to Steves quoted 4 Mpg difference

My current fuel consumption is good for a 2.2 HDI auto as I'm getting 37mpg in city/M25 commuting so I don't think there's too much wrong with my injectors.
That's good for an auto but then yours has had some surgery I believe :-)

Because the rail pressure is so high (1350 bar) even the slightest problem will cause something that can be heard or seen long before it starts affecting economy. When my 806 HDI started to pink badly the economy was not affected.
The rail pressure is high but my gut feel is that if it dropped to say 1200 bar the difference may not be noticable, you may know better. Under these conditions the trip computer would indicate no change in consumption yet the real fuel burn as measured by brimming should indicate a reduction.
Some of these changes may be so slight as to be virtually impractical to measure without very precise measurement so in a sense may be academic but I submit that the theory is valid, I would be happy with 0.5Mpg difference but at 4 I would begin to wonder what wasn't quite right.

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Post by wright »

if i reset both the trip and odometer after filling up my trip computer is about .4 of a mile behind after about 10 miles, i don't think it gets any further behind with more miles but i will watch it more carefully now, but it could be down to my tuning box?
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Post by cachaciero »

wright wrote:if i reset both the trip and odometer after filling up my trip computer is about .4 of a mile behind after about 10 miles, i don't think it gets any further behind with more miles but i will watch it more carefully now, but it could be down to my tuning box?
The odometer? and trip computer get distance or wheel revs if you like either from a sensor in the gearbox or the ABS sensors on the rear wheels.
As regards the fuel side and your tuning box, really don't know, probably depends on what it does and how it does it.
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Post by Paul-R »

wright wrote:if i reset both the trip and odometer after filling up my trip computer is about .4 of a mile behind after about 10 miles, i don't think it gets any further behind with more miles but i will watch it more carefully now, but it could be down to my tuning box?
I used to do the same thing and also discovered that the display trip mileage would be out by a small amount compared to the speedo trip mileage.

I only use the speedo trip mileage now.
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Post by dieselnutjob »

wright wrote:but it could be down to my tuning box?
absolutely
A tuning box on a HDI works by faking a lower pressure from the rail sensor than the pressure that's really in there.
Say that the ECU would like 1000bar rail pressure.
Say that the rail pressure actually is 1000bar.
The tuning box reduces the pressure read to 900bar (10% is a pure guess)
The ECU sees 900bar and adjusts the rail pressure up until it sees 1000bar but the rail pressure is actually 1100bar.
More fuel is being injected than what the ECU thinks is being injected, which is why you get more power, but it also would throw the trip computer's MPG calculation out.

Incidentally I briefly looked at chip/box tuning the 607. However I discovered that the maximum torque of the 2.2 HDI is already very near to the torque limit of the 4HP20 transmission so I don't think it's safe to tune a 2.2HDI auto.
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Re: 2004 C5 estate 110 HDI fuel economy

Post by hamster99 »

Hi Guys
Just been reading your posts as I have had my C% for just a week or so and noticed that the trip computer shows vastly better MPG than I am experiencing in actual consumption. Actual is around 45mpg but trip says 56mpg. At first I thought wow, great news, but then reality set in when i clocked mileage between two fill ups!
Wasn't sure where to go with it but your injector info was very interesting Cachaciero. I'm going to mention it to the garage when it goes in for a service later this week and see what they have to say about it.
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Re: 2004 C5 estate 110 HDI fuel economy

Post by cachaciero »

That's a big difference and worth investigating, however I assume that we are talking 2.0 Ltr 138 here in which case, 45mpg is about what I would expect, 56 even on long distance motorway work sounds a bit optimistic. Think I would investigate accuracy of mileage readings as a first pass, drive a known distance and see how many miles the odo thinks you have done, there are calibration settings to take account of different wheel / tyre sizes. Although this still leaves me a little puzzled as odo and trip computer get their distance data from the same source and you are presumably using the odo as the basis for your calculated consumption so I would have expected the calculation to give the same numbers even if the numbers were not correct, another possibility is that the ECU has the wrong numbers for the injectors plugged in.

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