Oddly, my factory Bi-fuel still burns Petrol
 

I consider myself very fortunate to have a Bifuel car. After sorting its initial gripes, it now runs great, and I keep the distributor slider well lubed (never use silicon, Lambdas don't like it) and clean the distributor often. Stepper motors on these are a weak point, so I keep it spotless.

Only one issue really. it still uses petrol. very odd. I don't ever select petrol (why would i?) OK, I realise it msut use some petrol to warm-up.. and it flicks to gas after 150-200 yds from cold, so can't be doing more than 2-3 miles a month on petrol. However, I reckon it burns 2 gallons of petrol for every 4-500 miles I drive, which is more than the warm-up mileage, so it must also be burning petrol when running gas. But why? Is this right?

Do the LPG cars have the same issue? (My CNG car is identical to the factory LPG cars at the business end,)

Can i ask, do the factory Necam LPG cars burn some petrol too, even when LPG is selected?

Slightly leaky fuel injector?

Yes, that thought had ocurred to me. Frankly I use petrol so rarely that I am prepared to bodge it by putting one of those solenoid cut-off valves into the fuel line, firing when the CNG comes in. You know ht kind of thing, used in conversions.

My concern is that maybe i'm supposed to be burning some petrol as a function of the Necam gas system. But that seems unlikely, but it is why I ask the question of the factory-fit LPG owners.

Aha! So it's not just mine that does this. I mentioned the similar in a post a while back, see: http://www.volvoforums.org.uk/showpo...58&postcount=9
I know the history of my 2002 S40 LPG (factory fitted Necam system) Bifuel from new. Like yourself I've never burned petrol other than to start. It usually changes over within about 200yards from cold unless its a very cold day. At new and maybe up until about the 70K mile mark the car burned very very little petrol. At one stage I reckon the petrol in the tank could of been 3,4,5 years old. It was a case of....do I put another £5 worth of petrol in now....or wait till the summer kind of thing. After about 80K miles it's LPG fuel economy began to worsen and initially I put this down to it burning both fuels at once as it had began to burn more petrol too. All of a sudden I was having to stick maybe £10-£20/year in it. Eventually at around the 100K? mile mile mark (110K? ...I forget) the LPG economy was sorted after a vap rebuild but the petrol use has remained higher than from new.
I'd like to find out what's going on.
Am I correct in saying that you can't simply cut off the petrol as its continually pumped around the circuit to cool the fuel injectors even though petrol isn't being used? Not that I want to, but seen as how CNGBiFuel has mentioned doing so, I thought it worth a mention.

My bi-fuel V70 does the same. Once while driving on a long journey cruising along on gas but with the petrol level dangerously low the car just cut out. She refused to restart on gas even though the engine was hot. I went and got 5 litres petrol and away we went again.
Why is it that the car needs petrol in the tank to stop her from stalling while running on gas?

Hmm,

Before I got the bi-fuel, I had a dual-fuel Vauxhall Corsa - I used to switch that to petrol about once a year so it didn't go stale.

With the Volvo I find I have to chuck £10 in the tank every few months. I don't do a particularly high mileage, and was blaming the warm up cycle of a comparatively large engine, mine switches after about 3/4 mile.

Looks like a theme is developing ...

I can't offer any more info on that from this end TomW as I've never let it run out of petrol but good question. My guess is that maybe it doesn't stop it stalling due to the fuel missing at the point of burning but because (if I have this right?) the LPG system is still hooked into the petrol ECU the lack of petrol puts an end to the show?

A definite theme here...
 

A definite theme here... seems it is not jsut me then. Mine uses mquite a lot of petrol, vastly more thatn it should. I have never let it run dry for fear of not being able to start it - I realise petrol is used to start and have rightly or wrongly assumed it would not start on gas. So feared not trying my luck.

I had thought there must be a petrol shut-off solenoid in there somewhere, but I CBA finding it when I can do a down & dirty fix far more cheaply... for the price of one fill up, and 2-3 hours fiddlin' I was going to take the feed for my new LPG solenoid form the existing tank solenoid so it fires jsut when required. You can get hem for £15-20 job done I thought.

I had not realised petrol is still needed once running and up to temp.

It seems it is more complex, my idea seemed tidy. Now I worry, what Pete_c20 says puts pay to that idea. Thanks, Pete, glad I didn't try it. Only I ask, WTF is going on? I'm a high mileage user and the petrol hurts when I should have no need (in my mind) of needing the stuff. More digging then?

For those of you unclear on thsi I confirm, the CNG & LPG models going into the distributor are identical, I think the ECM differs. It only pre distributor that the it all get very different, claerly the tanks etc, but anything you see as CNG you can consider relevant to you with LPG and vice-versa.

One thing to add to teh mix, my gas system is like new, yes the car has 110k on it, but has done next to no CNG miles, afterall where do you get CNG in the UK? So the problem I will assume is related to the petrol miles.

Anyone have good German? The erdgas forum has lots of CNG Bifuel blokes on there. Dutch guys too. Using Google tranlate I found out a fair bit, maybe I post this Q on there.

The still_needing_petrol_to_cool_injectors is something I've heard somewhere which kinda makes sense but like you have never wanted to find out by running out of it! The same idea had run through my mind a couple of years back and it was my searchings then that put a stop to it.
There was a chap called 'Kees' username 'kver' that used to offer much help over on http://answerweb1.askanowner.com/qa/...=15657&page=10 His website is http://www.verdampere.nl/ he used to answer all kinds of questions re LPG going back several years.

I'd love to know what causes it and what, if anything, it does to the LPG fuel economy. I'd join in in a practical way but work is a bit mad at the mo and there's another more pressing problem with the car that needs finding before I figure why the need for so much petrol. I don't know the physics /layout of the petrol injectors to offer another guess as to what's going on. To put a level head on ...I'd say - if the fuel system remains pressurised and it's been pumped round the system to cool it, and the petrol doesn't leak out anywhere or build up it must be being burned. Therefore the injector valve/valves leak. Next question - do petrol injector valves leak with time on Volvo S?? models? The people running petrol_only would never see this as it's masked by the normal operation. And even so i'd guess it would be compensated for by the petrol ECU as it would appear as a long term offset.
I'll have a search the other forums to see if I trawl anything up.

One thing is for sure - It was never there when new!

Google translate has been most useful for all kinds of things. :-)

Edited to add: After perusing the Haynes guide (and it's not the best for the bifuel S40) and a cup of tea, it would seem not a bad job to remove the fuel rail and the injectors. And if it were me, I'd jig them up so that small quantity of fuel could be sat on top of them and then I'd pressurise it to the quoted 44psi at which point it'd similar to the car. Then watch for, and if applicable measure, the leak. Can't do it just at the mo. Anybody out there fancy a go?

That is a simple enough way to check for an injector leak but I think the problem could be something else.

Before going to the hassle of fitting a solenoid pull the fuel pump fuse to see if it runs OK on LPG/CNG that way. You might find that the alternative fuel system is set to have a trickle of petrol going through.