Defender 110 300tdi sudden change in MPG?

[Lumpy Norman]

Art'noon all,

I recently changed the water pump, 'P' gasket and belt tensioner on my 300tdi 110 and, for reasons utterly unclear, my fuel economy has dropped from 30mpg to about 27mpg.

I'm still doing the same route, filling up at the same BP garage, driving the same way, haven't changed the amount of weight carried, using the same tyres etc, etc, but have seen this 3mpg drop. Anyone got any ideas as to why this might be please?

The 'P' gasket was changed because it had sprung a leak and got hotter than usual, but it took me a while to get the opportunity to fix it. Consequently, I was topping up the header tank before each journey. After the slight overheat, it never ran low again. Before the overheat incident and while I was constantly topping up with water, I was getting 30mpg. I do the right thing and fix the leak and I'm rewarded with 27mpg.

And that, as children might day, just isn't fair . . .

[dailysleaze]

Was it just on the most recent tank of fuel? In which case it may just be an anomaly. I get a big MPG dip if I do a long motorway journey into a headwind.

[cackshifter]

Seized viscous coupling?

[pat_pending]

How are you measuring MPG?

I ask because you'd need a pretty strict criteria to spot the difference between "30" and "about 27" over the 300+ mile range of a 110 tank.

[Lumpy Norman]

Hmmmm . . . good questions, thanks fellas. The answers, thusly:

DailySleaze: The reduction in mpg has been noticeable on every tank since the job was done. I've gone through about four or five tanks of diesel since then and it's been consistently at about 27mpg. Prior to the fettling, it was around 29-30 mpg.

CackShifter: Not entirely sure what it is you're referring to; is it the viscous coupling of the fan? If so, I don't think it's seized as it was spinning freely when I put it back on. Will check and confirm however.

PatPending: I'm using the same method each time, so any inaccuracy should be repeated for each measurement and thus cancel out. Every time I fill up, I reset the trip odometer to zero. Then when I fill up again I know how much fuel I've used and how far I went on it. The weekly journeys are practically identical, mostly dual carriageway and done at about 65mph.

There are two other minor issues which I'm not sure are related or relevant, but I'll mention them in case they're useful:

1. The fuel tank seems to be getting pressurised and blowing diesel out of the vented cap. I don't know the mechanism of how the pressure's getting in there, but the first couple of journeys after a fill up show that diesel's being ejected from the spout somehow. This was happening while the vehicle was doing 30mpg however, so I'm making the assumption that it's not related to the drop in economy. Obviously it's not helping, but the fault is common to the before-and-after scenarios.
2. The temperature gauge seems to read ever so slightly lower than it used to. Normally, the gauge's needle would sit about vertical, now however it seems to be canted over to read slightly cooler. Rather than being at 12 o'clock say, it's now at about 11:45. Not much, but noticeable. I first noticed this about a week before I changed the parts. After the overheat and while I was trundling round, topping up the coolant twice a day, it would sit vertical. But then about a week or so before I 'mended' things, I noticed it was reading that little bit low.

Any help?

[cackshifter]

Yes I meant the fan coupling, but it sounds like it isn't that. I thought you might have disturbed it doing your work.

[Lumpy Norman]

I did have to take it off for access, but I'm pretty sure I put it back on properly. Having said that of course, I haven't really checked. Will do that a little later.

Could a seized viscous coupling put sufficient load on the engine to result in such a marked increase in consumption?

[Jode]

It sounds like the replacement water pump is the cause of the slightly lower running temp - that's a good sign.

I believe the tank will normally pressurise because of the return fuel circuit (although perhaps the suction from the lift pump balances that? - I'm not certain). But that pressure/suction is 'released' via the vented filler cap. The vents can clog, however there are write-ups on this site (IIRC), and certainly on the landrovernet site, on how to take the cap apart and clear the vent.

I understand that unusual drops in fuel economy would typically relate to timing issues, lift pump diaphragm damage or turbo issues (such as manifold pressure compensator diaphragm damage, faulty waste gate actuator or damage to turbo air hoses).

[HPLP]

Its a change of 10%. I wouldnt be too worried.

At what point you stop when filling up can also change due to variations in the pump if you simply filled till the click.

H

[bluehaze]

Are the turbo and intercooler hoses ok? I ask as I had a similar issue which turned out to be a damaged one.

[simonr]

The viscose coupling could cause a 10% variation. It takes about 5kw / 7Hp to spin the fan at 3000 rpm. The VC varies the power supplied to the fan according to the temperature. The extra load on the engine could amount to 10% of the power being used on average throughout your journeys.

Si

[ballcock]

A knackered VC would also account for the lower running temp.

[TSD]

I looked at the OPs viscous coupling yesterday and it seems perfectly normal.

Suggested he takes it back off and throws it in the back for a week or two, but I don't think he has done so. With mostly dual carriageway driving and no towing, he's unlikely to be in dire need of it, and it only takes 2 minutes to refit anyway.

Coincidentally, about the same time the consumption of my D3 has changed from high 27s, to low 26s. Struggling to spot a connection though!

[Lumpy Norman]

I know it's been a while, but thought I'd post again anyway.

It looks like the problem was a knackered glow-plug timer not turning off and drawing a constant forty amps or so. Replaced the relay with one which makes the noticeable 'thunk' when everything's toasty-warm and now the mpg's back up between 28mpg and 30mpg.

Forty amps eh? Who'd-a-thunk it . . .

[cackshifter]

Good to know the outcome. And I'll be checking mine.

[jbs]

I know it's been a while, but thought I'd post again anyway.

It looks like the problem was a knackered glow-plug timer not turning off and drawing a constant forty amps or so. Replaced the relay with one which makes the noticeable 'thunk' when everything's toasty-warm and now the mpg's back up between 28mpg and 30mpg.

Forty amps eh? Who'd-a-thunk it . . .

Forgive my Ignorance here, But how does the glow plug relay affect your fuel consumption?

John

[dailysleaze]

If i'm not mistaken, it increases the magnetic load on the alternator, making it harder for the engine to turn it

[cackshifter]

It would create significant extra drag I'd imagine. I am wondering about the effect on the glowplugs of being permanently on too. Norman, did you notice whether it had any effect on the engine - eg quicker warmup? My old Mondeo used to keep its glowplugs on for quite a while after the engine started, I think for that reason.

[Retroanaconda]

Defenders have after-start warming too, the glow plug relay doesn't turn off until a little while after the light goes out (ie. when you're supposed to start the engine).

I doubt it makes much difference in the UK climate but it might at -40°C for example!

[Jode]

Forgive my Ignorance here, But how does the glow plug relay affect your fuel consumption?

John

The alternator on the standard 300 TDI puts out 65A at "normal" load. As current demand increases, especially that from large-draw items like headlights (or here, the glo-plugs - 40A is a very big load), the voltage being produced by the alternator drops; this causes the voltage regulator to signal the alternator to increase the amount of power that it produces. As the alternator output increases, friction within the alternator also increases. Overcoming that friction requires the engine to work harder (you can hear this if you turn on your main beam when idling), which results in greater fuel consumption.

[Scotts90]

On a similar vein, my evoque has an Eco screen that shows how much the gadgets affect fuel consumption....eg heated seats/screens, fan blower, lights etc. It all adds up, one seat heater at high setting is 0.3 gallons/hr iirc.

[Lumpy Norman]

The alternator on the standard 300 TDI puts out 65A at "normal" load. As current demand increases, especially that from large-draw items like headlights (or here, the glo-plugs - 40A is a very big load), the voltage being produced by the alternator drops; this causes the voltage regulator to signal the alternator to increase the amount of power that it produces. As the alternator output increases, friction within the alternator also increases. Overcoming that friction requires the engine to work harder (you can hear this if you turn on your main beam when idling), which results in greater fuel consumption.

One of the other symptoms (which, incidentally, was the one that resulted in the relay diagnosis) was that the battery wouldn't charge to the point where the vehicle wouldn't start one morning. I reckon the relay probably failed in June but, since I was trundling around in the summer months, I didn't have any additional loads turned on, so there was enough juice left to charge the battery.

Come the colder, darker (and rainier) days however, driving to work with lights, fan and occasionally wipers and heated rear window turned on, over about a fortnight the battery drained enough from regular starting duties to be unable to turn the engine over.

So yes, probably worth checking yours before the winter comes . . .