Causes of Low TD Fuel Economy?

[CitroJim]

I've been seeing on here a few posts recently that suggest later TDs are not returning wonderful fuel economy. Xac and his 33mpg from his 2.1TD is a case in point.

It has got me wondering on what can affect fuel economy in an ECU equipped TD. I assume that the timing and duration of the diesel injection on these are controlled by computing engine speed, how far open the throttle is, the needle lift sensor for fine timing control and the warmth of the coolant via the Coolant Temperature sensor. Is there an air mass meter too. I've never spotted one so I guess not.

On petrol powered XUs, the first suspect if fuel economy is bad and everything else is in order is the Coolant Temperature Sensor. It is notorious for fooling the ECU into thinking the engine is cold and thus keeping the mixture rich with obvious consequences.

If a TD has a duff Coolant Temperature Sensor, does it cause the diesel injection period to be significantly lengthened and thus use more fuel? Or does it simply adjust the timing as on a fully mechanical injection pump.

What effect might a slightly leaky EGR valve or a sticky needle lift sensor have on economy?

[weety]

speedo accuracy could be an issue..... checking my speedo against GPS my speedo is always 10% slower than actual speed....i assume this means that my mileage is 10% lower than it actually is

so my 36mpg could actually be 40 mpg (ie 36 plus 10 percent)

anyone else checked xantia speedo accuracy against GPS?

[Peter.N.]

Both my XMs have mechanical fuel injection, Jan '96, the very last to, and both do similar mpg. I would have thought that for the ECU to cause poor consumption you would have noticable symptoms i.e. smoke, excessive noise (over advanced) or very quiet and getting hot (retarded), or faulty thermostat (low temperature reading). If the engine sounds and performs normally you probably need to look elsewhere, like drag due to binding brakes or of course a heavy right foot.

[CitroJim]

anyone else checked xantia speedo accuracy against GPS?

Yes, I checked my TD against GPS and it was a good 5MPH fast at 30, 40 and 50, that being the old cable driven type. Most speedos do run on the fast side usually so an indicated 30 is in reality more like a true 25MPH.

I've not seriously checked the electronic speedo in the Activa but according to our little smily faced speed indicators the Thames Valley Police like to put out around these parts, it is pretty well spot-on. If I get a smily face saying I'm doing 30 then the speedo agrees.

[TehAgent]

Jim i think your looking in the wrong place about this MPG business,

Its not under your bonnet you should be looking, but at the price of the damn fuel

When filling up, its always best to fill your car up by the liter, rather than by the cost, this way, you can always be sure how far your going for your money, also, you can check just how much we are being ripp..... i mean taxed by Mr brown.

But on a more serious note, could it be that the ECU is getting the mixture wrong and pumping too much Fuel into the chamber?

[Xaccers]

Well my 2.1TD is currently going like a rocket, but there is definitely something at around 3000rpm that it doesn't like.
It's as if there's fuel starvation or something.
Noticed some air in the clear fuel line yesterday when I gave it a couple of pumps on the bulb, so will check my leak off pipes when the weather gets better, incase they're leaking like TehAgent's.

[jeremy]

Extra fuel being pumped in will simply make the engine run faster or smoke if too much is pumped in. Diesels run on the principal of free air - there is no mixture as such - just enough fuel pumped into excess air to meet requirements - so more fuel - more power.

The timing is more important although generally if it is wrong the things will smoke or sound noisy or both.

Its only the Bosch pumps which are affected by the condition of the leakoff pipes. Roto-diesels and presumably their derivatives - Epic and Delphi just leak and smell but the running is unaffected.

[deian]

Just a thought.... have you checked the fuel filter at all? This wouldn't necessarily cause bad fuel consumption but it may provide weirdness at 3000rpm maybe.

Tyres levels can affect fuel consumption too, knock them up a psi or two and see how it is. Maybe one tyres is quite bad, then again you'd feel that in the handling.

[Xaccers]

Tyres and pressure are fine, check them every week or so.
Fuel filter and air filter were changed for Halfords ones a day or two after getting the car at the begining of March because at the time the car was gutless (it suddenly opened up one evening and has been pretty good since, just bad mpg)
I'm "treating" it to shell v-power diesel as from what I've read it's low sulpher diesel with a higher cetane number and shell's version of redex added at the refinery, and I can claim most of my fuel costs back.
Just wondering if I could be losing fuel from pipes anywhere.

Rear calipers used to rub against the discs, but I've sorted that and now the rear wheels run free.

[philhoward]

Try an italian decoke....but also consider that if you only do town driving then you might only get somewhere in the 30's mpg. High 40's can only usually be achieved when you're in 5th gear most of the time. 42mpg in my 1.9TD Xantia (43 miles each way to work - half of it motorway, rest mixed) became 48mpg after an italian decoke.

[tomthered]

Whats an Italian Decoke?

[philhoward]

http://www.frenchcarforum.co.uk/forum/v ... hp?t=21888

[Cpt_Spalding]

Hmm.

Waiting to do a full tank calculation on my new Xant 1.9TD (97) but by the way the needle is dropping on the gauge I reckon I'll be lucky to get 420 miles out of this tank.

I'd have thought that figure would be closer to 550 miles.

I don't feel too much resistance, in that the car will roll freely down a gentle slope given a nudge.

Did anyone ever resolve this? How far should I be looking to get on a tank while being lightish on the shoe leather?

[Kowalski]

I've been seeing on here a few posts recently that suggest later TDs are not returning wonderful fuel economy. Xac and his 33mpg from his 2.1TD is a case in point.

It has got me wondering on what can affect fuel economy in an ECU equipped TD. I assume that the timing and duration of the diesel injection on these are controlled by computing engine speed, how far open the throttle is, the needle lift sensor for fine timing control and the warmth of the coolant via the Coolant Temperature sensor. Is there an air mass meter too. I've never spotted one so I guess not.

On petrol powered XUs, the first suspect if fuel economy is bad and everything else is in order is the Coolant Temperature Sensor. It is notorious for fooling the ECU into thinking the engine is cold and thus keeping the mixture rich with obvious consequences.

If a TD has a duff Coolant Temperature Sensor, does it cause the diesel injection period to be significantly lengthened and thus use more fuel? Or does it simply adjust the timing as on a fully mechanical injection pump.

What effect might a slightly leaky EGR valve or a sticky needle lift sensor have on economy?

The only things that electronics on a 1.9 TD with the ECU can do to the injector pump is to adjust the timing or raise the idle speed. The injection period corresponds to the amount of fuel injected on a mechanical (ECU controlled) injector pump, so the injection length can't be altered but it can be injected at the wrong time. If your timing is off, you get bad fuel economy, can a faulty coolant temp sensor make your ECU run the engine on "cold start" timing and would that affect your economy?

The ECU can also open the EGR valve, the effect of which would be to dump boost pressure if the car is driving i.e. not idling or very low throttle. The EGR valve will only recirculate exhaust gas when the intake manifold pressure is lower than the exhaust manifold pressure, i.e. no turbo boost. The needle lift sensor is some sort of inductive sensor, i.e. no moving parts.

I've got an ECU controlled 1.9 TD ('97), my fuel economy is just fine. In the last 22k miles, I've not got less than 50mpg thats over 34 tanks of fuel, brim to brim, the last two were over 55mpg. My company has just moved its office, 2 weeks ago in fact, so my route home is different, we'll see what effect it has on the next tank. The fully mechanical pump on my other Xantia ('94) never gave so good economy.

[Village]

I find that the usual cause of low MPG is me driving the car. Have you inadvertently let me drive your car too?!