LPG conversion and the petrol gauge

sorcerer · July 14, 2016

Had our 2005 RX300 converted last week and I'm very pleased with the job (couple of photos attached).

One thing that's giving me slight cause for concern though is the car's petrol gauge, which is still going down at an alarming rate even though it should hardly be using any!

Although we're new to Lexus we are not new to LPG. We covered more than 104,000 miles on gas in our old Nissan so I'm comparing it to that. As an example, if the Nissan petrol gauge showed half-full it would literally take a number of weeks for it to drop to empty and put the warning light on (indicating a truly nearly empty tank), whereas the RX300's gauge is behaving ‘normally’, ie, it dropped from half-full to 'warning light empty' over the course of a 150 mile motorway journey yesterday. I think I know what's going on and it would be nice if someone could confirm it, but first, a bit of (oversimplified) background for those who may not know how the LPG conversion works.

The car's original engine ECU still controls everything and it still sends the signals to the petrol injectors as normal. The gas ECU intercepts those signals, adjusts them as necessary and sends them on to the gas injectors rather than the petrol injectors. The car's original ECU doesn't see any difference and thinks the car is still running on petrol, which is where my problem may lie. Older fuel gauges relied on a float in the tank and measured the actual liquid level, whereas a lot (if not all) modern cars have an electronic gauge that I believe is worked by some black magic in the ECU and is a representation of what the car thinks it has left in the petrol tank.

The gas control switch tells me that all is working as it should (fire up on petrol - lights flashing. When certain parameters are reached, automatic switchover to gas and lights go steady) but the only way I could think of to check this was to fill both the gas and petrol tanks then drive around for a few days and fill both tanks again.

UPDATE – I had to go to Asda today so I took the opportunity to fill up the petrol tank again at their petrol station. I’d covered 157.8 miles since last filling it and all I could get in the tank this afternoon was just 2.37 litres, thereby proving that the car is indeed running on gas as it should and the petrol gauge is dropping in line with how much petrol the car thinks it’s using and not what it’s actually using.

The problem now, of course, is that in about 300 miles or so, that red petrol light is going to come on again indicating an almost empty petrol tank when in reality it may still be three-quarters full. Not only that, at some point the red light really will mean that the tank is nearly empty and I really will need to put some more in. Is there any way to reprogram the petrol gauge via software?

I know I’m not the first person to have an LPG-converted RX300 so are there any others on this forum and how did you deal with this little problem?

Sorry for the long post

1 - The Flashlube valve-saver fluid bottle
2 - The gas ECU
3 - The gas reducer

The 85-litre gas tank is where the spare wheel used to be and the filling point is centre of the car (so that you can fill from either side) and pokes out from below the rear bumper just as a tow bar would.

japanesebob · July 14, 2016

Hello John, what a nice looking and totally unobtrusive conversation you have. Where was it carried out? Had mine done at Leyland LPG, which isn't in Leyland, bizarrely. I too have the filler cap in the middle, makes life easier when filling up whilst towing. How much gas do you manager to fit in the tank? I've never got more than fifty litres in my eighty litre tank.

sorcerer · July 15, 2016

Hi Matthew, I was going to use Leyland LPG myself because John Ross has a great reputation, but I was having difficulty in contacting them. I later found out that John had died and I thought that the firm had gone out of business, although I now know that they are indeed still trading. Anyway, thinking that Leyland LPG had closed down, I eventually decided on a guy called Simon Andrew of Yorkshire Autogas over in South Elmsall, sort of in between Doncaster and Pontefract (http://www.lpgc.co.uk) who's done a great job and I'm very pleased with it.

I'm no expert but I'd say there's something not quite right with your shut-off valve. My Nissan had either a 69 or 72 litre tank (can't remember which now) and empty to full it always took 55 litres, which would be correct (80%) for the 69-litre or slightly out for the 72-litre. I've not yet done an 'empty-to-full' filling on this 85-litre tank but will let you know when I do.

How have you handled the petrol gauge problem? I don't particularly want the red light on for weeks, maybe even months, continuously 'crying wolf' as it were, but I will need to know when the petrol is actually getting low.

Bluesman · July 15, 2016

Mine has an 85 litre tank and I get around 70/73 liters depending on the temp. Mike.

sorcerer · July 15, 2016

55 minutes ago, piasek said:

You need emulator to sort out problems with incorrect fuel guage reading. Something like this:

http://www.lpgshop.co.uk/ac-stag-fuel-level-emulator-type-p/

That looks brilliant Adam, just what I'm looking for. Thanks very much

ChrisKaye · July 19, 2016

Thanks for taking the time to post this info. Very nice, clean install! [emoji3]

Mumoffive · July 20, 2016

Very grateful to have found this thread. We bought a 2001 Lexus300 with LPG conversion on Monday. We are on route to Birmingham eye hospital and the fuel light has come on.

Planning to fill the petrol tank to test as original poster said. Thankful that it hasn't conked out on us yet as we are heading towards Corley services.

Mumoffive · July 20, 2016

Ok we didn't stop at Corley and made it to Burmingham Eye hospital but after 128 miles the gas guage said reserve too.

When starting the car up after 2 hours the petrol guage was back at 1/4 but gas only had 1 green light. The filling seemed to be a real trickle and we got 32 litres in.

That means it is doing around 18 mpg. We were expecting nearer 25. Any advice welcome.

The invoice for the conversion says the tanks water capacity is 65ltres

sorcerer · July 20, 2016

11 minutes ago, Mumoffive said:

Ok we didn't stop at Corley and made it to Burmingham Eye hospital but after 128 miles the gas guage said reserve too.

When starting the car up after 2 hours the petrol guage was back at 1/4 but gas only had 1 green light. The filling seemed to be a real trickle and we got 32 litres in.

That means it is doing around 18 mpg. We were expecting nearer 25. Any advice welcome.

The invoice for the conversion says the tanks water capacity is 65ltres

The gas gauges are notoriously inaccurate and you just cannot rely on them. I could fill up and have four green lights but 15 minutes later it would be on one red light (that's in the old Nissan which had a different system to the Lexus). Your 65-litre tank should take a maximum of 52-litres of gas from empty to full (80% of 65) but may take slightly more or less due to temperature fluctuations.

The only way to measure your gas and/or its consumption is to let it run out, fill up, and then zero your trip meter/odometer. Let that tankful run out and then you know roughly how many miles you're getting per tankful. In the Nissan, depending on driving style, motorway or town, whether there was an 'R' in the month, what colour socks I was wearing etc., etc., I could get anywhere between 190 and 250 miles per tankful, which was around 55 to 57-litres. I've yet to find out what the RX300 will do per tankful.

The MPG figure will be less on gas than on petrol, e.g. if the car does 30 on petrol it may only do 23 or 24 on gas so you use more gas and fill up more often, but because it's cheaper you win on the cost.

Mumoffive · July 20, 2016

Yes that was what we did. I'm not convinced we are filling it completely.

Does the pump stop completely when you have button depressed?

is it normally quite slow? Would the ambient temperature make a difference?

it took ages to get to 33 litres this afternoon but was still slowly going up is it possible to overfill? I don't want any mishaps I'm a bit nervous of it to be honest.

The red reserve light was on on the little square device that has a light system to show the tank status. The first time we filled it we got 33.5 litres in but I'm convinced I let the button go rather than it stopping then at 115 miles it had gone to red again and David got 29 litres in today we got 33

It means our range is not great which is a pain.

It also means hubby is thinking he made a mistake buying it

Mumoffive · July 20, 2016

Ok Sorcerer I suspected the guage wasn't accurate but what happens if you run out of gas? Does it just switch smoothly back to petrol to allow you to get to a garage? We discovered today that it can be a chore to find lpg in a city centre.

Ps sorry for taking over your thread

sorcerer · July 20, 2016

The pump should stop completely but some of them just slow down to such a trickle that I get fed up and let the button go, then just note down how many litres I managed to get in.

Some pumps are quite quick, some slow and some very slow, just a bit of a lottery really - you'll get used to it

It shouldn't be possible to overfill as the tanks are fitted with vents. Too much pressure will cause the vent valve to open and vent the excess off to air. If you hear a hissing and smell gas, somethings wrong with the valve so get it to a gas guy ASAP but, under normal circumstances, no, it's not possible to overfill.

Honestly, forget what the lights are telling you, the gauges are about as useful as a chocolate teapot

You definitely haven't made a mistake in buying it. We did over 104,000 miles on gas in our old Nissan. Rather than get bogged down in MPG figures, let me give you a real-world example. We usually drive from our home in Preston, Lancashire, to eastern Europe for our holidays and have done for many years. Last year we drove through France, Belgium, Germany, Poland, Czech Republic and Slovakia, covering about 4,000 miles for the round trip. Doing it on petrol would have cost us over £900 for the fuel, whereas doing it on gas actually cost us £437. I honestly don't know why more people don't run LPG as I truly cannot find a downside to it. It's cleaner, better for the engine, better for the environment and, best of all, better for the wallet

Bluesman · July 20, 2016

On 20/07/2016 at 4:43 PM, Mumoffive said:

Yes that was what we did. I'm not convinced we are filling it completely.

Does the pump stop completely when you have button depressed?

is it normally quite slow? Would the ambient temperature make a difference?

it took ages to get to 33 litres this afternoon but was still slowly going up is it possible to overfill? I don't want any mishaps I'm a bit nervous of it to be honest.

The red reserve light was on on the little square device that has a light system to show the tank status. The first time we filled it we got 33.5 litres in but I'm convinced I let the button go rather than it stopping then at 115 miles it had gone to red again and David got 29 litres in today we got 33

It means our range is not great which is a pain.

It also means hubby is thinking he made a mistake buying it

Get a bigger tank fitted. If it does run to empty it will just change back to petrol automatically. I have run mine and my wife's cars on LPG for well over 17 years. We have always just kept a mileage check so when it gets near to empty we fill up. Before the tank will switch over you should get a warning beep that you are very low, I still get another 30/40 miles out of her before she switches. In all those years we have never had an accurate meter telling us what is left in the tank. Its no big deal. Mike.

sorcerer · July 20, 2016

4 minutes ago, Mumoffive said:

Ok Sorcerer I suspected the guage wasn't accurate but what happens if you run out of gas? Does it just switch smoothly back to petrol to allow you to get to a garage? We discovered today that it can be a chore to find lpg in a city centre.

Ps sorry for taking over your thread

Yes, it does. It will bleep at you to tell you that you've run out of gas but the car will seamlessly switch back to petrol with no hiccups, jerks or any other physical signs apart from the bleeping. Join the LPG forum for lots of help and information and also have a look at this website for locations of filling stations and prices.

sorcerer · July 22, 2016

On 14/07/2016 at 11:19 PM, japanesebob said:

How much gas do you manager to fit in the tank? I've never got more than fifty litres in my eighty litre tank.

Just done my first 'empty-to-full' filling and my 85-litre tank took 74.47 litres, which means I may get a little more than that in on cold winter days. It also means that it's filling beyond the 80% figure (80% of 85 is 68) so maybe my shut-off valve needs some adjustment, if they are adjustable.

sorcerer · July 22, 2016

42 minutes ago, piasek said:

Yes it is adjustable but I would leave it as it is. It will give you better range.

To be honest, about five minutes after posting that I had already made up my mind to leave it as it is. The RX300 is a big, heavy car and goes through fuel for fun, so those extra few litres will be welcome. And as we mentioned earlier in the thread, the safety valve will open to vent excess pressure if it ever came to that, which I don't think it will.