X5, 3.0i petrol, poor fuel consumption on LPG

[tonygale]

I have recently acquired a 2004 3.0i X5 Sport, auto, 102k miles, part service history, but reasonably looked after by previous owner. Spark plugs, oil and air filters changed recently.
The attraction in buying the car was the LPG system which should make the fuel costs somewhat less expensive than running on petrol.
The LPG system is a STAG system installed 7k miles ago , about 1 year ago.
My driving is mostly local journeys 10 - 20 miles, plus the occasional long journey. Yearly average 12k miles.
I have tried to drive with a light foot, but the best fuel consumption I can achieve on LPG is only 17mpg. Longer runs don't seem to improve the mpg.
I was hoping the fuel consumption would be about 20mpg or better.
Am I being unrealistic?
Is 17mpg the best I can hope for? The car runs faultlessly on either LPG or petrol.
I look forward to hearing from other members real world experiences for comparison.
Many thanks.

[Greydog]

I am not sure if you have owned a car with LPG before? LPG will not give more MPG it will just be at about half the cost of petrol which is why many calculations equivalent 30+ to the gallon, my V8 X5 averages about 18/19 mpg on the on board computer Petrol is around £1.31 per ltr (£5.90 per gal) LPG £0.61 per ltr (£2.75 per gal)

When I fill up with LPG it costs around £40 or so, around town I get about 130 to 180 miles mixed motoring on a run around 200 miles. My mixed motoring includes towing a couple of fat horses in a trailer.
I keep between 1/4 and 1/2 a tank of petrol and run on LPG at all times, If I were you I would find a good LPG Installer and get them to service the system and check any software is up to date but if you are using the on board computer I would think 17 is about right for a 3.0 the LPG will just make it a little cheaper to run.

[Themoog2]

Our Range Rover sport has an LPG system. If I fill it up at Morrisons it's 55p a litre. A Shell garage which is slightly nearer charge 75p so guess where I go...
So £48 fills it up and I can get 260-280 miles. That's just running about here and there so that's not bad from a 4.2 supercharged motor. I don't know what the lpg is to mpg but the computer on board says 11mpg on petrol. I think lpg has slightly worse mpg but like stated before it's a lot cheaper.

[IanP]

Lpg has less Mj/kg than petrol so you will have increased consumption on gas. I used to reckon on 10-15% less on my 4.6, but less than half the cost per litre.
If your only doing short trips as well you might only be on gas for half the time

[henrym3]

What do you get on petrol, my son used to have a 3.0 ltrs and rarely seen more than 19mpg. So the 17mpg is in the ball park. Greydogs write up just about mirrors my 4.6 consumption

[henrym3]

Should have added I switched from NGK spark plugs to LPG plugs and have not seen any improvement. They claim to put less demand on ignition system, but as I didn’t have any problems before it’s difficult to judge. I’ll probably go back to NGK when due a change.

[hennabm]

I have an 02 3.0si. It also has LPG but is currently disconnected. This is because I have a problem with the air suspension
When I first bought it the mpg was showing around 9-12mpg.

Over the year I have had mine I have changed the plugs, repaired some vacuum leaks and have seen the consumption improve to around 19 around town and around 22/23 on a 250 mile run.

The on board computer will take some time to alter if you only do longer runs occasionally. The best bet is to reset it before a known longer run and see what it gives you then.

[henrym3]

I’m struggling to understand/see the reason for disconnecting the LPG because of an air suspension problem??

[pete_g]

I am new to the forum (lurking and thinking about X5 ownership).
However, I currently drive an e46 330 on LPG (very similar engine). I had a slight MPG improvement when I changed my O2 sensors (the 2 pre-cat ones). Maybe this will help you too.

[hennabm]

Hi Henrym3

As the tank is a spare wheel donut type it had to come out to change the battery and repair the BM54 when I first bought the car.
It now has a soggy bottom and goes down overnight - not one side but both sides.

This appears to be the distribution valve which sits in the wheel well as well.

[henrym3]

:

[henrym3]

Just been over to lreland, took this pic cos l’ve never seen mine above 18.

4CDC80D0-E914-4A32-ACFA-16DD301E047D.jpeg (274.77 KiB) Viewed 3992 times

[Greydog]

Where did you get to go that far down hill in Ireland Henry?? That is an impressive average just shows what you can get from a well maintained V8 with all the wheels pointing in the same direction. What tyres are you running?
Dave

[henrym3]

ATM Goodyear rear and accelera front, if I could get the accelera for the back they’d be fitted all round. They last twice as long as rears are more than competitive on grip in rain, fuel and better than most on noise. Got 7k on the Goodyear’s and they seem to be doing far better than the Conti’s. As for running a V8 I am reluctant to admit (fear of tempting fate) but the car has been so good and reliable as to be beyond belief.
This weekend is gearbox oil change time and with 152k on the clock I’m going to look at the front prop shaft splines for wear.

[Greydog]

I did run Accelera front and rear but about halfway through their life (12k) they started wearing on the shoulders on the front just couldn't stop it. I got a deal on a set of Yoko's so that is what it is on now. Winter wheels are Wintac Extreme on 18".

Like you don't want to tempt fate but the car has run without a serious glitch since the engine rebuild full service + brake fluid due end of the month and the Gearbox will get serviced at the same time. Rear sub frame bushes will need doing at some time soon.