I've asked for advice on this before, but the problem persists. The car gets about 19-20l/100, driven lightly, around town. It's the Impco factory gas system. Car has done 129k and runs nicely. It's been to several gas places who all say it's fine, inc. the people who did the original factory gas fitting for Mitsubishi. Someone on this forum suggested I take it to Mark Walker in Lonsdale, which I did. He found that the original mixer is non-adjustable and replaced it with a new adjustable unit, which he thought should give 15l/100. He says it is now set up perfectly.
Problem is, if anything it's even thirstier: the first tank after the new mixer was 22l/100, and today I filled it and it's 20.1l/100km.
There's no obvious problem with the car. I'm now running it on petrol for a while to see what it gets on that but it was about 12.5l/100km last time I checked. I'll take it back to Mark but he's away for a couple of weeks.

If anyone has any suggestions for why a gas system should be really thirsty, I'd be glad to hear it.
Thanks

Where do you live? Id be checking the data logs with evoscan. I can help if youre south

Where do you live? Id be checking the data logs with evoscan. I can help if youre south

Yea having a look at the fuel trims would be great. Thinking they might need resetting. Also, check to see what the o2 is saying when cruising around at 60 and 80kph

Does the 02 sensor work with LPG ?

I had the oxygen sensor checked a while ago, and was told that it's fine.
by the way, the odometer is accurate.

Does the 02 sensor work with LPG ?

yes, I'd like to know if the ECU is working with the gas.

yes, I'd like to know if the ECU is working with the gas.

Where you located? Flick me a pm

yes, I'd like to know if the ECU is working with the gas.

Both the LPG systems I've seen schematics for rely on the oxygen sensor to control the LPG mixtures and also feed the sensor signal to the ECU. This means that even though the injectors aren't being used, the fuel trims should still respond fairly similarly to using petrol and should be close to 0.

Both LPG systems mentioned rely on the ECU to manage ignition timing and the idle control system (as well as EGR and evaporative purge in cars that have these functions, which TWs definitely do).

Sadly, the factory dedicated LPG cars don't appear to have been given an LPG specific ignition advance map. Appropriate ignition advance mapping for LPG won't necessarily dramatically improve fuel economy directly. The main benefit of LPG specific mapping is that the car will actually drive much like it would on petrol, which in itself is where much of the gain would be had - otherwise people use a lot more throttle on LPG than they do on petrol to get the same response, which leads to increased consumption...

what's fuel trim?
it's not a dedicated LPG car; it came from the factory as dual fuel.
How does this relate to my problem, or more to the point, how could it help me to fix it?
The guy who replaced the mixer used an exhaust gas analyser to read the air/fuel ratio, which he said was perfect.

Although you have had the O2 sensor replaced, I would still suspect this as the cause. The system should have a stepper motor in the gas vapour line. That motor would be throttling down the gas supply at light loads. If not it could be a fauly motor or maybe the O2 sensor not getting its signal to it. Maybe this is not working properly?

I ran it on petrol for a while and filled it today. For the same mix of freeway and suburban driving as gave 21.3l/100km on gas, it got 10.9 on petrol. That's a massive 95% increase in consumption in gas over petrol. It's cheaper for me to run it on petrol than gas, not to mention the irritation of an incredibly short range on gas.

It's running nicely and economically on petrol, and is a fuel hog on gas. This makes me think that any shared components such as the O2 sensor must be OK.

So the situation is, yes, there must be something wrong with the gas, but I've taken it to 4 gas places in Adelaide, and they all say it's fine. It's not.

Any suggestions for improvements much appreciated.

I have 3 dual fuel Mitsis. All 3.5 litre.

I have a TH Magna with ring mixer - 14-15 litres /100. On a long run it is better. Rarely get less than 440 Kms per 65 litre fill. The car has done 456,000 Kms. I had a few issues with misfiring under load a few years ago but for the past 3-4 years it's been great.

My second car is a 2005 TW GTVI AWD. It has a Polish injected Versus system. Had a lot of setup issues after installation but I purchased the cable and software to tune it and have ironed out most problems. Is more economical than the TH. On a long run, I can get 650 Kms on 68 litres. On shorter journeys 450-540 Kms on a tank.
ut
The last car is a 3.5 Litre Pajero NM Exceed with an Impco injected gas. Runs beautifully but rarely does better than 15 l/100 and 20l/100 when towing.

Find another gas tuner/specialist. If you don't have one in Adelaide, I can recommend an excellent guy in Melbourne.

One more tip. Ask a taxi driver who fixes their cars. Most taxis are gas and so the guys who service them have the best experience. The guy in Melbourne does taxis and he has ironed out many problems on my gas cars, including an EB Falcon I used to own.

what's fuel trim?
fuel trims are the values the ECU keeps track of to optimise fuel delivery based on the oxygen sensor feedback compared to its programmed default values, to account for manufacturing variations mostly.


it's not a dedicated LPG car; it came from the factory as dual fuel.
How does this relate to my problem, or more to the point, how could it help me to fix it?
Sorry, I had the mistaken impression that this was a dedicated LPG car - in which case a reflash with an ignition advance map more appropriate to LPG would have helped a bit, though I doubt enough to get 20l/100km down to 15l/100km.

With a dual fuel car, a way of changing the ignition mapping depending on the fuel in use would be required - either:
1) a piggyback ECU with an input from the LPG system to trigger use of the LPG mapping when LPG is in use; or
2) a modified ECU ROM that can take advantage of an otherwise unused ECU input with some simple electronics to condition the input from the LPG changeover switch so that the ECU changes maps at the right time.

I know of 2 cars running option 2.


The guy who replaced the mixer used an exhaust gas analyser to read the air/fuel ratio, which he said was perfect.
On the basis of this and your other post about petrol use, and taking into account some of the other comments about "normal" fuel consumption, it seems to me there might be a problem resulting in LPG consumption approximately 33% higher than might be reasonably be expected.

That being the case, it either has to be leaking to the atmosphere or going through the engine. For it to be going through the engine without out-of-whack AFRs at the tailpipe could only mean that lots of throttle is being used a lot of the time or that there is an intake system vacuum leak that oxygen sensor feedback to the LPG processor is compensating for (resulting in more LPG being delivered than really should be for the engine load). Or any combination of these factors...

An intake vacuum leak might show up in the fuel trims (specifically the idle long term fuel trim being quite different to the mid and high trims) even on petrol. It might also be observable as the car idling at higher revs than expected and/or taking longer to get the idle revs down to normal values.

EDIT: it has occurred to me that a previous owner may also have installed some gadget that achieves a similar effect to an intake vacuum leak but only when LPG is being used, in order to get a better engine response (i.e. a proxy for extra throttle at low engine speeds). Seems pretty unlikely though.

How do you know it's getting 12.5 on petrol and 21.3 on gas?

Is this from the trip computer, or are you calculating the gas consumption by how much gas you fill up with?

over the last few days I filled both gas and petrol tanks, drove for about 100k on gas, filled it, it worked out to 21.3l/100, close to the 22 on the previous fill. I then drove it for over 100k on petrol, similar driving, got 10.9 on petrol. The figure of 12.5 was from a few years ago and exclusively stop-start driving. I never take any notice of the trip computer. The odometer by the way is dead accurate.

The previous owner was the SA govt., there's no way they would have modified it.

I've found another gas place, this will be the 5th, and they will look at it on Friday. I certainly need a competent gas mechanic.

Thanks for all the feedback and help -much appreciated.

I have 3 dual fuel Mitsis. All 3.5 litre.

I have a TH Magna with ring mixer - 14-15 litres /100. On a long run it is better. Rarely get less than 440 Kms per 65 litre fill. The car has done 456,000 Kms. I had a few issues with misfiring under load a few years ago but for the past 3-4 years it's been great.

My second car is a 2005 TW GTVI AWD. It has a Polish injected Versus system. Had a lot of setup issues after installation but I purchased the cable and software to tune it and have ironed out most problems. Is more economical than the TH. On a long run, I can get 650 Kms on 68 litres. On shorter journeys 450-540 Kms on a tank.
ut
The last car is a 3.5 Litre Pajero NM Exceed with an Impco injected gas. Runs beautifully but rarely does better than 15 l/100 and 20l/100 when towing.

Find another gas tuner/specialist. If you don't have one in Adelaide, I can recommend an excellent guy in Melbourne.

Hi Wayne,
Many thanks, that's good to know. I've found another gas tuner. Hopefully he will know what he's doing.

If it runs fine, one has to ask the question... Has it been leak tested?

I find it hard to believe it's using double the fuel but not coughing, spluttering or running like a pig.

Have you considered you may have a gas leak anywhere between the tank and the engine? Perhaps the leak only occurs when the car is hot due to expansions/contraction of different materials.

If that much fuel was actually going through the engine, there'd have to be other symptoms.

believe me, it runs really well on petrol and even better on gas. I also don't think the extra gas is going thru the engine. But I never smell gas. The tank itself was re-certified last year, which I think includes a leak test. I'll speak to the gas guys on friday about the possibility of leaks.

You'd smell a tank leak in the car so it wouldn't be the actual tank itself. But it could be anywhere between there and the engine.

Ask them to leak test the entire system - cold and hot.

ok, will do.

it runs really well on petrol and even better on gas.
If by better you mean smooth, ok. If by better you mean more power output and better engine response, from everything I've read, heard and experienced that seems very unusual for LPG without appropriate ignition advance mapping - unless the engine is always run above 3000rpm.

If the latter and you don't rev it out all the time, it seems to me that the "missing" LPG is going through the engine with the oxygen sensor keeping the mixtures sane - which suggests extra air getting into the intake somehow when LPG is being used.

I don't drive it hard, so it doesn't spend much time over 3000rpm. The power seems identical on petrol or gas. It runs very smoothly on either. I wish I knew where the extra gas is going, but I like the suggestion that it's a leak somewhere.

Most LPG fitters will have a device that sniffs out gas leaks, surely?

Gas systems are a curse..


Sorry, errr. When they work well and are tuned well they are ok. By all means, this thing should not be using the amounts of gas that it is using.

Most gas places simply put a sniffer up the exhaust pipe and rev the engine in the workshop. See if you can get a gas place which actually puts the car onto the road and under load. They can then check the mixtures underload and you get a more reliable setting. Also does the gas system have a self checking/adjusting function? If it does, maybe every time the system is disconnected it will reset.

took it to Eastside auto in Somerton park, they found a leak in a gas fitting on the tank, also cleaned the throttle body. The fitting was at the gas solenoid on the tank, so it would only leak with the engine running when on gas. They say that checking for leaks is supposed to be part of the normal gas service. We'll see how it goes now.

, so it would only leak with the engine running when on gas. They say that checking for leaks is supposed to be part of the normal gas service. We'll see how it goes now.

Great - you were a ticking time bomb I think - hope you're not a smoker :-)

no, I'm not a smoker, never have. Yet another reason to avoid the fags. Anyway, I couldn't afford it. Cigarettes cost a fortune.

Make sure you check consumption and report back. LPG shouldn't be more than about 25% worse than petrol.

on the open road, 100-110kph, it now uses 13.1 L/100km, previously 13.8; driven like a granny around town, it's using 18. A slight improvement. It's still drinking the gas, still using about 60% more than on petrol. O2 sensor is fine, no leaks any more. New mixer and converter. Newish iridium plugs. Still driving me mad.

on the open road, 100-110kph, it now uses 13.1 L/100km, previously 13.8; driven like a granny around town, it's using 18. A slight improvement. It's still drinking the gas, still using about 60% more than on petrol. O2 sensor is fine, no leaks any more. New mixer and converter. Newish iridium plugs. Still driving me mad.

Sell it...

yeah, that's a possibility, but it's low mileage and drives really well.

I had a bad run in the TW the other week. Ran out of gas at 525 Km and refilled to the usual 65 Litre. Usually I get at least 560 to 570 Km per 65 L.
I live out of town a bit and dont do much stop start heavy traffic stuff. Mostly it is open road type running. If I did more stop/start driving, then the consumption would be higher I guess.
So depending on your driving conditions, 13 to 14 L/100Km sounds about right.
The last time I filled with petrol I used about 48 Liter over 504 Km. On a dedicated run it is usually about 8.5L/100 Km. Doing short trips from cold can push the consumption higher.
Hope this helps.

13-14 is in perfect open road conditions. Yours works out to 11.6. I'd love to get that.

It might be obvious, but is the thermostat opening at the correct temperature?
A friend with a ford focus recently had problems with excessive petrol consumption and the thermo was open by 55 degrees C. It should have been 90 odd degrees C. Renewing the thermo fixed the problem.
Cant add much more.

gauge behaves normally............

How are you measuring the gas consumption? If it is by the readout on the dash, it is probably way out for gas compared to petrol. If you are checking by the number of litres and the trip meter, then I cannot offer any ideas to help.

have you tried to do some readings with wideband o2? That would instantly tell you if fueling is the problem. Have you changed the stepper motor?

How are you measuring the gas consumption? If it is by the readout on the dash, it is probably way out for gas compared to petrol. If you are checking by the number of litres and the trip meter, then I cannot offer any ideas to help.

obviously I'm reading consumption by actual distance travelled and actual consumption

have you tried to do some readings with wideband o2? That would instantly tell you if fueling is the problem. Have you changed the stepper motor?

wideband O2? what's that? The 02 sensor is OK. Stepper motor? what does that do? No it hasn't been changed. Does it work on petrol? It runs beautifully and economically on petrol.

Wideband o2 is an oxygen sensor which is normally used for diagnostic/tuning purposes.

A regular o2 only shows rich/lean readings, it is however somewhat slow to react thus it is only used during during idle/low load/low rpm conditions (closed loop). The purpose of the o2 sensor is to tell the ECU if the car is running rich or lean. Thus the ECU is constantly making small adjustment to fuel injection in order to keep the air to fuel ratio (AFR) at 14.7, which is a stoich ration for fuel. The stoich ratio is when there is just enough air to completely burn the fuel.

The wideband sensor on the other hand is able to show the actual air to fuel ratios across the rev range and different loads, for example 11.0 (which is very rich for N/A car). By temporary installing wideband o2 (usually done at the muffler) and taking the car for a drive will tell you if under certain conditions the car is running to rich or lean. This should help you identify the precise conditions where the LPG system is “dumping” gas when it should not. To be honest any decent LPG place should have done that before changing any of your components.

A stepper motor is used to adjust the flow of gas (only used by LPG), basically it is controlled by the LPG ECU in response to the readings by the o2 sensor, so if the o2 is showing rich, the stepper motor should reduce the flow of gas. Therefore, if it malfunctioning it might send excess amounts of gas, thus reducing your fuel economy.

Thanks, both the gas places who recently looked at the car said the air-fuel ratio is fine, so I'm guessing that the stepper motor must be OK.
I filled it today: mainly highway driving, 17.8L/100km, so it's still drinking the gas.

The fact that they are saying your AFR's are 'fine' is a worry, clearly they are not fine. I would honestly not take their word for it, have they taken it for a drive while monitoring the AFRs? If they just revved it out in neutral, that's hardly a proper diagnostic. Car needs to be under load, ask them to go for a drive with you in the car, while the wideband stuck at the back and see for yourself, anything richer then AFR of 13.0 on gas is too rich. Your car is munching gas while at the same time running properly on petrol tells me that AFR's are not fine, unless you got a leak somewhere (which i guess you would have smelled by now) your GAS tune is out of whack. I had mixer system at one stage and you should definitely be getting better numbers then that (the absolute worst i would get would be 16, city driving). My car with the supercharger does about 14.5 on gas, that's city driving, while i got injected system, mixer should not be that far away.

Yeah, it was thoroughly checked for leaks, and they found a small one which was fixed but hasn't helped the economy much. As I recall, the AFR was about 14. I'll get them to do it while driving.

It's got to be a leak doesnt it? Corrosion pin hole in a steel pipe. Pinhole in a flexible hose. Compression fitting not perfectly tight. Crack in a moulded part. Something been overtightened. Bloody obvious I know; but something's wrong.

Thanks, both the gas places who recently looked at the car said the air-fuel ratio is fine, so I'm guessing that the stepper motor must be OK.
I filled it today: mainly highway driving, 17.8L/100km, so it's still drinking the gas.

When the gas places check the air/fuel mixtures, the normally do it in the workshop with a sniffer up the exhaust. They rev the guts out of it and adjust until they get what they reckon is an OK reading. The engine is not under load. This works 99% of the time, because the gas system is probably OK and the readings they get are OK for normal driving. I reckon they should use a portable sniffer and go out on the road and check it under load or actual road conditions. Rarely do they do this. You probably have a stuffed stepper motor. This is a control valve which regulates the amount of gas used, and is controlled by the Oxygen sensor. If the stepper motor is OK, it will be the O2 sensor - you may be running in wide band setting.

Thanks, both the gas places who recently looked at the car said the air-fuel ratio is fine, so I'm guessing that the stepper motor must be OK.
I filled it today: mainly highway driving, 17.8L/100km, so it's still drinking the gas.

You've taken it to a couple of places and it's still on the high side ....... maybe it's an issue with your right foot!

Pretty sure that Matt Rushworth did a trip to Mannum and back recently and told me that his consumption was about 11~12 litres per 100 km, so, there is something still seriously out of wack somewhere. Hell, I get 14 around town and driving it hardish through the hills everyday.

You've taken it to a couple of places and it's still on the high side ....... maybe it's an issue with your right foot!

my right foot! I'm driving it like a granny. I couldn't drive it any slower.

I'll get to the bottom of this, and I really appreciate all the great advice: keep the ideas coming, folks.

Pretty sure that Matt Rushworth did a trip to Mannum and back recently and told me that his consumption was about 11~12 litres per 100 km, so, there is something still seriously out of wack somewhere. Hell, I get 14 around town and driving it hardish through the hills everyday.

Yup i went from Hallett Cove to Mannum and back on grand final day. Warm 35degree day. A/C on the whole time. Sitting on 110km/h

12.6L/100km. On LPG. Mixer system

I also pull consistantly 14-15s in my auto. Driving through town in peak hour everyday.

just filled it again, all city driving, 19.4L/100km. Driven like a granny. I'd love to get 15. Still don't know what the problem is, but I've spent quite a bit of money for virtually no improvement.

How many kilometres are you getting from a tank? And whats the tank size?

Can you send me a pic of your engine bay so I know what sorta system you have for advice

I fill it after about 220-240k because the gauge is showing empty. According to what's stamped on the tank, it's 85 L but I've never managed to fit in more than 59. But capacity isn't the issue, consumption is. It's an Impco mixer system, factory fitted from new.

filled it yesterday, 19.5L/100km, driven gently around town. it seems to be stuck at that level. On the plus side it drives nicely.

I cannot offer you a cure, but agree with you that sort of consumption is way too high, especially when you compare it with petrol consumption. There has to be a problem somewhere. My guess, if not a stepper motor is the mixer itself putting out too much gas, but I ran out of ideas ages ago for you. The mixer is simply a large diaphragm with a needle attached to the centre of the disc. This needle moves away from a fixed orifice, thereby opening up more flow area for the gas. If the needle is not centred properly, the mixer will be set OK for some operating modes, but not others. The only positive way to check this is to take it for a run under load with a sniffer up the exhaust. If all else fails, the sniffer will at least tell you something is wrong, even if it doesn't tell you where.

I did have an issue with my NL Pajero some eyars ago. It went rich as hell as soon as I put the foot down. It ran OK at low speeds (60 km/H) but anything over that - Phew. TOok it to a gas place in Sydney (we were visiting at the time) and the bloke there put it on his fixed analyser. As soon as the throttle opened, it went ultra rich. We couldn't find any problems. I even isolated the petrol pump (it was always isolated whenever it ran on gas anyway), but still rich as. Eventually we decided to replace the gas converter. New one and still rich as... Then he accidentally found that an orifice ring in one of the resonators in the air intake plumbing had come loose. Normally it sat in place, but as soon as a decent airflow came onto the engine, it lifted up, restricting the flow and making the mixer think it was under heavy load. Fixed that up and all fine. The resaon it came loose - I had Top Gun spark plug leads on and it backfired, causing the orifice ring to come loose. It was purely accidental that the guy found it. Check the resonators on your intake plumbing - they may be loose and start to lift and partially block the airflow to the engine. Long shot I know, but desperation....

Hi Coops,
I'm with erad. I am "out of ideas ages ago for you".

I always try to get a diagnosis of problems before spending money. Sometimes we use hunches, and if they work out, then good. If not, we have to be more disciplined. And, then sometimes, pure arse wins through. erad's post makes good reading.

In your case, many posts here advise to do a ROAD TEST with a sniffer up the exhaust. erad says this too, in post 57. My only contribution is to say that I think it is now imperative for progress, to identify whether the gas is leaking or whether it is going out through the exhaust. To this end, I believe a road test is now mandatory for you to get a handle on what is going on.

You comment on the tank capacity. The tank has a shutoff valve to limit the gas quantity to 80% of the tank capacity. My TW always shutsoff at 65 L. The tank is behind the back seat in the boot. You should get about 500 Km on your full tank.

Hope this helps.

just ran it bone dry on gas, 287 k, took 55.6 litres, = 19.4 L/100km. The previous tank was also 19.4. Still drinking the gas. Still running perfectly.

Like a drowning man, clutching at anything, I would go back to your gas man and ask him to check that the Oxygen sensor signals are actually actuating the stepper motor. It sounds to me as if your stepper motor is not controlling the mixture and you are probably running too rich. One way to prove this is to get someone with a portable gas analyser and shove the sniffer up the exhaust and go for a decent drive. If it is too rich, it could be the stepper motor or it could be the fact that the Oxygen sensor signal is either not working or the signal is not being processed correctly.

The fact that your car runs OK on petrol suggests to me that the Oxygen sensor is somehow not integrated with the gas system.

just ran it bone dry on gas, 287 k, took 55.6 litres, = 19.4 L/100km. The previous tank was also 19.4. Still drinking the gas. Still running perfectly.

I'd get close to 350kms for 55 litres on LPG and I don't drive economically.

440-420 km for 65 litres reguarly.............mixer system

thanks, yeah, so there's still something wrong with the gas setup. The problem is, what is it? I've taken it to lots of people who proclaim themselves gas experts without identifying the problem.

Most places simply plug their analyser into the exhaust and rev the guts out of the engine in the workshop without actually driving or loading it. You need someone to actually take it out on the road and check it under operating conditions - preferably with you in the passenger or driver's seat. If they won't do that, go somewhere else and find one who does. Check by phoning them first rather than taking time off work and travelling - explain the situation to them. Ideally, a dynotune place would do it because they have a better analyser and they can load up the engine and actually monitor what is happening under load. Sdaly they are the most expensive.

I did a test using the cars computer at 80kph cruising along a flat road, the Hume Hwy Campbellfield/Somerton Melbourne.

LPG, between 12's and 14's with an extra 2 with the A/C on, at idle it jumped to 18's.

Petrol, between 8's and 10's a/c on or not at idle the same extra 2.

just ran it bone dry on gas, 287 k, took 55.6 litres, = 19.4 L/100km. The previous tank was also 19.4. Still drinking the gas. Still running perfectly.

Do you go to the same service station every time? I find different fuel consumption figures depending on the servo I go to.
My TL Magna regularly gets 15-16L/100km, mix of peak hour and freeway traffic.

I usually go to the same servo, but when I've been to other places, the consumption is the same. I've also checked the odometer and it's accurate.

440-420 km for 65 litres reguarly.............mixer system

Thats pretty much what i get with the gas running out and i switch to petrol.

For what it is worth, I used to get around 600 km from 65 L of gas. This was with highway running @ 100 km/h in my wife's TF sedan - 3 L engine, manual transmission. I reckon I should have got 650 km, but it never consistently did that and that was all I could get. I used to get about 425 km from my NL Pajero - 3.5 L engine, manual transmission, not towing anything. Towing my 16.5 ft caravan, on flat country I used to get 275 - 300 km from a tankful of gas - about 65L, - highway running @ 95 km/h.

Do you go to the same service station every time? I find different fuel consumption figures depending on the servo I go to.
My TL Magna regularly gets 15-16L/100km, mix of peak hour and freeway traffic.

Pretty much what i get, 15-16L/100km, peak hour running to and from work. Out in the open i can get 13L/100