4.0 lpg

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shealyp38

New Member
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111
Location
birmingham
just got my hands on a 99 4.0se with lpg
went to get the lpg serviced and found when running on petrol its running quite rich
the emissions were quite high
whats the best way to bring them within mot standards so i can get the lpg running right
also what mpg should i be getting on lpg

steve
 
The P38 Engine ECU uses adaptive learning for the Fuel Trims...

If it has been run excessivley on LPG the Long Term Fuel Trims will be off when running on petrol as the ECU has 'adapted' to how the LPG burns...so when on Petrol it will over or under fuel...

Also the petrol injectors could be dirty through lack of use and not atomising the fuel efficently for a clean burn...

They do say with any LPG system, you should do a few runs on petrol every now and then to a) keep the petrol injectors clean and b) so the ECU can keep its trim values within reason for petrol.

My Classic 3.9 on LPG (The 3.9 and the 4.0 are the same engine - the P38 had a few extra gubbins added externally, but the displacement is identical, was rebranded the 4.0 to differentiate the two) I would get between 13-16 MPG.

LPG is less efficent than petrol by about 10-15% but is around half the price....so real world etrol MPG would be between 16-19 or so...

My old 4.6 would rarely get above 21-22 on a decent run....
 
We've been getting between 270 - 290 miles on 80 litres of LPG on our 4.0 depending on how its driven. IMO theres little performance difference, and as I read on here, we've started putting the occasional dose of super unledded in it to help keep the injectors clean. One thing I would say, if you've got a good system , it'll switch over quite quickly, so as the others have said, make sure you give it a run on Petrol once in a while to keep things happy. Lastly - we never put much petrol in, 2 reasons, 1) you'll be lugging that little lot around, and 2) the petrol ( about 25 litres) will last for about 2 fills, so little chance of it going off. Enjoy.
 
its got a dream xxl system fitted and is quite slow switching over from petrol to lpg
i drove a mile the other day before it switched over
it switches over quicker when warmer though and as far as mpg is concerned i filled the 80 litre tank and ive done around 80 miles or so and its nearly empty
 
The system will be set to only switch to LPG once the coolant temp reaches a certain value to ensure the Vapouriser is warm enough to prevent freezing...on cold days it takes longer to warm up...and warmer days it doesn't take aslong....obviously!!

Mine switches over around 60degC after about 5 minutes or around a mile to a mile and a half in this weather....nowt wrong, just doing what it is supposed too!
 
well thats eased my mind some what
just thinking about my mpg now it terrible so far
i think by the time the lpg tank is empty i will have done about 100 miles
surely it should be better than that
or am i missing something
 
You can only fill an LPG tank to 80% capacity so my 90 litre tank will take around 72-74 litres....

my old 3.9 would get around 12-16 mpg on a good day, my current L322 does an average of 15.4 (check my fuelly account by clicking the icon on my signature - as this will show real world results) so don't bank on much more than that in all honesty...

from an 80 litre tank you should get 64-66 litres usable gas into it....about 14 gallons.

if you are only getting 100 miles from that = 7.15mpg so something is amiss, either your lead foot or it could do with a service.....I would expect closer to 175-200 if not more from 64 litres...I can usually do 200 from that amount!

Brim the tank....run it till it beeps at you or switches to petrol, note the mileage and then brim it again and do the maths....ignore the on board computer especially on LPG it will be way off in its calculations.
 
I do the same as Saint - just a mileage check between fills. If your local MOT shop normally checks your car , why not drop it down to them and get it tested for emissions on both Petrol and Gas? The gas figures should be lower than the Petrol (cleaner burn), and if they're not.... maybe time for a service or a bit of tweaking by someone in the know.
 
like i siad in my original post the emissions on petrol and then on lpg were way to high
i remember the co being something like 9.3
forthbridge said:
I do the same as Saint - just a mileage check between fills. If your local MOT shop normally checks your car , why not drop it down to them and get it tested for emissions on both Petrol and Gas? The gas figures should be lower than the Petrol (cleaner burn), and if they're not.... maybe time for a service or a bit of tweaking by someone in the know.
Click to expand...
 
Apologies for missing that. Just a thought, as you've just got it, has the usual been done new air filter and the like? Might be worth giving the MAF a clean?
 
i went and bought myself the cable for the gas system
connected it all up below is a couple of pics of the system running on gas and petrol
if someone could tell by looking at the pictures what could be wrong,if at all possible
001.jpg

002.jpg

what i have noticed is that when it switches over to gas the engine does sound a little lumpy but on petrol it sounds a lot smoother,this is on tick over

steve
 
the injection time for petrol is approx double what it should be.the gas injector time is way to small for the given petrol injector time.
plug and filter are unlikely to fix this,you need to get it on "live data" to see what the maf,engine temp sensor etc are saying
 
Last edited:
Those codes point to faulty 02 sensors. Those injector times seem high. I had mine on the comp a few weeks ago and from (a dodgy) memory I think I was getting approx timings of 4 milliseconds on petrol and between 5 and 6 on gas at idle. The injector should pulse for longer on gas but this will depend on the make of gas injectors you have as they are not all the same. Mine was running lean and the tune up has really improved my mpg and performance.
 
had it on my laptop today and heres some live data pics
ive been told the lambda sensors were changed 6 months ago
and ive also noticed a little leak on the exhaust just over the rear axle where the hanger is welded to the pipe
IMG_0017.jpg

IMG_0016.jpg

IMG_0015.jpg
 
well i fitted new lambda sensors today and there still a problem with the short term fuel trims
on tickover they are at 25% but when i rev it they drop to 0%
also had fault codes p0135 and p0155 (both codes pending)reappear
why would the codes show again with brand new sensors?
 
Enjoyed reading all your lpg experiences as I'm only getting about 9mpg from my '96 120k miler 4.0 but as it's mainly short runs round town, this seems about right.
I wouldn't mind seeing if I can improve the fuel consumption though. I've recently seen several places selling IRIDIUM Spark Plugs for LPG. Most of them cost about £120 for 8 - are they needed and are they worth it? And what about changing the plug leads?
I'm getting the oil changed in both diffs as I don't think that's ever been done.
Finally, how often should the LPG system be serviced? Mine was last done in 2008, 50,000 miles ago.
 
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