Low MPG - Fuel Leak or Something Else?

[Glyn D]

Hi all, my 1996 300tdi 90 seams to be going through a lot more fuel than it used to. I might be imagining it, but it seems to have lost power too.

I recently did a 350 mile journey which was almost all on the motorway sitting at about 70mph in 5th gear. In total I went through £120 of diesel. This seemed a bit excessive to me for what is effectively from London to Sheffield and back.

I've estimated this to be 16.97 MPG:
I was probably paying about £1.28 per litre at the petrol stations.
£120 / £1.28 = 93.75 litres.
In gallons this is (93.75 x 0.22) 20.63.
350 miles / 20.63 gallons = 16.97mpg

Does this seem unusual? Looking online stated figures for mpg are about 25-27.

Appreciate it's an old car but it's recently had a brand new cylinder head with new valves, rockers etc, new glow plugs, new injectors.

It is carrying a bit of extra weight: winch bumper, winch, rock sliders, steering guard. Roof rack and LED light bar create extra drag too. And it's probably worth mentioning that it's on 285/75R16 tyres (32.8" diameter) running about 38psi.

If it was leaking, presumably I'd be able to smell it?

It has had any kind of tuning after fitting new head, plugs and injectors. Could this be the problem?

 

[tottot]

Extra weight,roof rack,knobbely or wide tires,and sitting at 70 mph all together make for low mph.

 

[windy81]

that's a lot of fuel.
Might be leaking when under load but worth checking all the usual things like leaking intake hoses etc.

 

[landowner]

If it's leaking diesel you should see it where it's blown along the chassis at speed. I would think low twenties would be about right for running at that speed.

 

[doriz]

Can't imagine it's leaking diesel. I had a diesel leak which used to pool on the chassis and tank guard, this then made an obvious puddle under the car. 70mph in what is the shape of a house brick doesn't make for good gas miles IMO.

 

[lynall]

If you were paying at motorway services it will be quite a bit more than 1.28 a litre.

I would say speed is the biggest single factor and this is why I always smile when I read people want to increase their gearing to go faster and always assume as the revs will be lower they will be saving fuel

 

[Glyn D]

If it's leaking diesel you should see it where it's blown along the chassis at speed.

Which streak of oily substance should I look at in particular? Hoping to take it for a steam bath soon and then I might be able to tell where the leaks are coming from.

I had a diesel leak which used to pool on the chassis and tank guard, this then made an obvious puddle under the car.

Rain water pools up under the car on my drive, and it has that rainbow coloured oily sheen. But this could be anything. Have replaced lots of seals which has resulted in a lot of oil on the ground and I've tried using petrol to clean some of this oil away.

Generally speaking though, you don't think 16mpg is excessive?

 

[windy81]

16mph is excessive IMO, thats V8 territory !

 

[tottot]

Looking at your landy I do not.The wind drag at 70 on your truck will be about double what it is 50mph.Roof racks are very drag inducing

 

[Glyn D]

Seems like people's opinions are split on this. I'll need to have a look around and see if I can find a leak.

Another question then, after replacing the head, valves, injectors, glow plugs etc, should I need to do any kind of tuning? Oh, new intercooler too.

For reference, here's a recent photo:

 

[tottot]

It is always worth checking for leaks,cost's nothing.

 

[landowner]

Which streak of oily substance should I look at in particular? Hoping to take it for a steam bath soon and then I might be able to tell where the leaks are coming from.

Rain water pools up under the car on my drive, and it has that rainbow coloured oily sheen. But this could be anything. Have replaced lots of seals which has resulted in a lot of oil on the ground and I've tried using petrol to clean some of this oil away.

Generally speaking though, you don't think 16mpg is excessive?

Should be looking for diesel sprayed along the chassis, could be leaking but only when running for instance

 

[doriz]

Lovely looking truck which appears at home in that wallow, no sign of any Hippos tho'

 

[Thor 1950]

Extra weight,roof rack,knobbely or wide tires,and sitting at 70 mph all together make for low mph.

easily lose 4mpg with all those extras

 

[Blizzard44]

Also noticed a problem with my mpg recently. I do less than 10 miles day, 4 days a week in a 90 with a 300tdi conversion.
Done about 80 miles since I filled up last monday and have used near enough half a tank(under drivers seat which I believe is 68 litres).
If my sums are correct then that works out to be about 11mpg.
Have since removed my roofrack, lights, snorkel, changed intercooler pipes and leak back pipes. Thinking about trying some injector cleaner too.

 

[lynall]

Which streak of oily substance should I look at in particular? Hoping to take it for a steam bath soon and then I might be able to tell where the leaks are coming from.

Rain water pools up under the car on my drive, and it has that rainbow coloured oily sheen. But this could be anything. Have replaced lots of seals which has resulted in a lot of oil on the ground and I've tried using petrol to clean some of this oil away.

Generally speaking though, you don't think 16mpg is excessive?

Patio or acid brick cleaner works quite well on oil stains, cheap as well.

 

[lynall]

Also noticed a problem with my mpg recently. I do less than 10 miles day, 4 days a week in a 90 with a 300tdi conversion.
Done about 80 miles since I filled up last monday and have used near enough half a tank(under drivers seat which I believe is 68 litres).
If my sums are correct then that works out to be about 11mpg.
Have since removed my roofrack, lights, snorkel, changed intercooler pipes and leak back pipes. Thinking about trying some injector cleaner too.

Ignore the gauge and measure the mpg tank to tank full, any other way is just guessing.

What sort of journeys are you doing?

 

[Blizzard44]

Ignore the gauge and measure the mpg tank to tank full, any other way is just guessing.

What sort of journeys are you doing?

Short drives about 5 miles each way on minor A and B roads, only one straight bit when I get up to 50mph then it’s all winding around bends at about 40mph.

 

[lynall]

Short drives about 5 miles each way on minor A and B roads, only one straight bit when I get up to 50mph then it’s all winding around bends at about 40mph.

Thats very poor then.

Ive had many land rovers and as a rough rule of thumb town work will get you at least 20mpg, rural low 20s and a decent run medium/high 20s to 30mpg and thats from 200tdi up to my current D3, obviously manuals slightly better, the later the car the lower the mpg only by a knatts cock but they are lower.

My daily commute (loads of 30/40/50 limits car only just gets to tenperature as I get to the end ff the journey) is exactly 5 miles dead drive to car park and my D3 trip shows 22/23mpg and that over reads by 10 percent so thats a genuines 20mpg from a 2.7 ton auto thats always got some sort of elcetrical loads going heated screen/seats/air con etc.

I think you need to start from the tank and check everything up to the engine, then check pump timing, is the oil lever rising? any brakes binding? really get anal about the whole car as something is obviously wrong.
Locking cap fitted? tank drain bung leaking or showing signs of being removed? sadly very easy to get to!
Is the trip registering the correct mileage? ie have you changed tyre size?

 

[landowner]

Thats very poor then.
have you changed tyre size?

Big wheels will show less miles on speedo so part of the problem would be under reading mileage