Terrible LPG mpg

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KILOCHARLIE

Member
Posts
91
Location
East Grinstead, West Sussex / North Cardiff, South
I know poor mpg threads here are generally met with distain but my car definitely has a problem as I am only getting poor economy when using Lpg, petrol mileage seems absolutely fine.

Long trips I get above 20mpg showing on the dash but my actual LPG usage is around 11mpg.

City driving I am getting 15mpg on the dash but actually getting about 8mpg! My last fill up of lpg cost £25 and I got 50 miles for that.

I should be getting a lot more but I can't figure out what could be wrong. I'ce searched online a lot to no avail and when visiting my local lpg mechanic yesterday they said they couldn't see any problem and advised to take to a specialist place around 100 miles away for a diagnostic check which I was told won't be cheap.

Does anyone have any tips of what could be causing this? I've scoured my engine bay for any sign of a problem and found both PCV hoses are split at the back of the engine. Will be replacing these in the next few days but I don't think this could have a drastic reduction on mpg.

Thanks for any help
 
+1 Diesel is for the farmyard.
What LPG system do you have?
Hows the performance on LPG, any big difference from running petrol? You should be getting around 15% less efficiency than unleaded. I guess your LPG mechanic checked for any leaks, although if he cant / doesn't have the software to check the calibration i would look for another local LPG specialist.
Correct spark plugs are reccommended for LPG (smaller gap setting).
 
Kilo charlie, which local LPg garage did you use? I suggest you try Professautogas in Hengoed, CF82 7SW, just up the road from you.

Off the top of my head, options are: a leak in your tank (they can rust, and if you have water in the spare wheel well then it will be hard to see if it's on the bottom with the tank in place);

or it may be a split pipe underneath p*ssing out fuel instead of going into the cylinders. However a pressure test should rule this out. Did the garage do this?

And just to cock a snook at data and the other "farmers" :D I get around 28-30 mpg on my LPG, more than the 25 from my TD6! :lalala: :mooning: (have just found the emoticons page.... awesome fun!!)
 
I think my lpg system is Autogaz. It was done by the previous owner 3 years ago by the green fuel company but I don't have any documentation and there's no logo's etc on any of the kit. The mechanic did say the vaporiser looked quite small in comparisons to other range rover conversions.

The performance is fine on lpg with no noticable difference in my eyes. Strangely enough a guy at my lpg fuel station says the only thing he could think of was that spark plugs have to be at their best but by his own admission he was mo expert.
 
Split PCV hoses won't help your MPG values...

On my signature I have a link to my Fuelly Account which detaisl the kind of mileage I do and the returns I get on LPG.

LPG is about 15% less efficent than petrol, and the on board computer will only be a representative figure based on theoretical petrol injection pulses...so will be wildly inaccurate on LPG.

I return 15.1MPG on average with up to 17.5 on a run as an average and when Off Road in the IoW she returned 11mpg average!

I would say you need to get the fuel trims reset and the PCV hoses chagned asap!
 
Thanks both. Yeah I didn't think the pcv hoses would have much of an effect on mpg but I'm getting them changed asap. Also I plan to do some old school tuning like cleaning the maf, plugs, coils etc. If anyone knows how to test coils with a multimeter please let me know.

How would I go about resetting the fuel trims? Would a simple battery disconnect for a few hours do this?

The mpg figures you are both quoting seem to be exactly what I am after 15mpg with a 28mpg petrol cost equivelent.

Dolphinboy, is that where you take your RR? I've been doing some homework today and that place popped up a few times as well as Autogas worldwide in Newport. Both seem worth a try as my local place didn't even want to touch my car even for a pressure test. I have had a good check of all the lpg related hoses just now though and found a huge split in one of the big hoses that go to the centre of the vaporiser. I think this is related to heating it rather than gas flow though which from my limited knowledge comes in liquid at the bottom of the vaporiser, then out as a vapour thru the top after being warmed in the middle by the two big (an cracked) pipes.
 
^^ thanks. I guessed so. I did check fuel trims with my obd reader yesterday and the long term was 0.08% which I thought was pretty good.

I've just rechecked all the pcv hoses and only the top one is split but I'll replace both big ones anyway. The third smaller one at the bottom was fine so I'll leave that although I did see a small plastic hose in that area that is not connected to anything. It comes from the back of the car alongside another small pipe and the comes up near the back of the engine and is shaped like a umbrella handle U shape. Does anyone know what this is or what, if anything, it should be connected to?
 
KILOCHARLIE said:
^^ thanks. I guessed so. I did check fuel trims with my obd reader yesterday and the long term was 0.08% which I thought was pretty good.

I've just rechecked all the pcv hoses and only the top one is split but I'll replace both big ones anyway. The third smaller one at the bottom was fine so I'll leave that although I did see a small plastic hose in that area that is not connected to anything. It comes from the back of the car alongside another small pipe and the comes up near the back of the engine and is shaped like a umbrella handle U shape. Does anyone know what this is or what, if anything, it should be connected to?
Click to expand...

Sounds like a breather hose to me and should be left as is....

As for fuel trims, that would be the petrol trims....you need the LPG trims looking at!
 
Thank you mate. Sometimes the obvious smacks me in the face when I'm told! Will get this specialist lpg place to check the lpg trims as a first course of action, then pressure test, then see what they suggest.

As for the pcv hoses. It looks like the ones that are widely available online are for vin's starting 266201 onwards. My vin is before this and I can only find them from main dealer at £25 each. Not sure how these compare to the later version but I am thinking about getting my own length of 19mm oil resistant hose and doing them myself along with the lpg pipe that needs replacing.
 
Kilo, the other factor that WILL affect your MPG is the LPG filters.

If it hasn't been serviced every year the filters fill up with a white milky substance which certainly affects performance (i.e. it will cut out under acceleration and revert to petrol) and probably MPG. I over-ran my service by 4 months and this happened. After changing filter and injectors it was back to its old self again.

Ps use AUTOPROFESSGAS - the owner Andriej (Andy) uses Polish mechs who are tip top. Just let him have a look and I bet he gets the answer first time. They are so good I drive mine to them for servicing even though its 100 miles!
 
dolphinboy said:
Kilo charlie, which local LPg garage did you use? I suggest you try Professautogas in Hengoed, CF82 7SW, just up the road from you.

Off the top of my head, options are: a leak in your tank (they can rust, and if you have water in the spare wheel well then it will be hard to see if it's on the bottom with the tank in place);

or it may be a split pipe underneath p*ssing out fuel instead of going into the cylinders. However a pressure test should rule this out. Did the garage do this?

And just to cock a snook at data and the other "farmers" :D I get around 28-30 mpg on my LPG, more than the 25 from my TD6! :lalala: :mooning: (have just found the emoticons page.... awesome fun!!)
Click to expand...


i would have to disagree..a leaky tank you would NOTICE!!..lpg is heavier than air...so if you have a misty cloud under your car then i would get out and RUN!! PLUS IT STINKS!!!..leaky joints will show up with frost/ice covering..

lpg filters no..exactly the opposite.no gas no go.

you can buy/download for free all the lpg software you need, only thing you will need to buy/make is the laptop to lpg ecu lead.this is very useful,once mastered will save you lots of cash..

my guess is its just well overfueling..faulty injector etc..wideband o2 sensor stuck up exhaust will confirm/deny this..to fix pay good lpg company or see above.



re lpg is less economical than petrol..true and false..with lpg one can advance the ignition ALOT..this more than makes up for calorific value of propane being down on gasoline..i get better mpg on propane than gasoline., but i run aftermarket ecu..one can buy ignition advance moduels for standard car ecu (oe not lpg ecu) from aeb..it will advance timing when switched to propane/lpg..without advancing timing then yes..lpg worse than petrol.
 
Last edited:
+1 it won't be leaking. If you had a leak it wouldn't run right. When I had a leak it felt like it had a miss fire. I think get it serviced first then see how u go.
 
Hi, best suggestion is get professautogas to look at it. That said replacing my MAF worked wonders on mine (as advised by profess). as old one was out.
I know that when they work on my lpg system to monitor the fuel trim they get the car warm then turn off. Start it straight over to gas then tell me to drive 10 miles continuously ol lpg then they check it.
 
we used this mobile company to do an lpg service on our x5 (outside the house). they did a diagnostics on the engine to make sure it was running ok. then did a full lpg service (with report) £150. they were very good. hope this helps.

Mobile Automotives and LPG

i'm on here checking out problems on the l322, as we had a p38, then had the x5 for 3 yrs. but now are looking to get an l322 2002 - 2004.
 
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