What next?

[jseaman]

Following on from my earlier thread (http://www.lpgforum.co.uk/viewtopic.php?f=28&t=10959), I have an XJ6 with a Stefanelli multipoint system that trips off under load (full throttle every time, >50% throttle up hills etc). I have now changed the liquid and vapour filters and it HAS improved things but not cured the problem. I have checked for kinks in the LPG feed line and it looks OK so has anybody got any other ideas on where to look?

Could the vapouriser be a problem?

Is there any kind of pressure regulator normally? It ocassionally feels like it is surging a little (as though the gas supply pressure fluctuates) before cutting off

Any thoughts welcome

James

[Gilbertd]

The vapouriser is the pressure regulator and it could be reaching it's sell by date. It shouldn't after only 3 years but who knows.

[jseaman]

Thanks, is there much I can do to check it? Can they be serviced? Can I get any old vapouriser and bolt it on?

[Gilbertd]

In theory most multipoint vapourisers can be used. You need to choose one that is rated for at least the bhp of your engine and different systems run at different pressures so, although the pressure is adjustable, it needs to be one that is in the right range. You'll also need the software and a lead to connect to the system to set it up. The lead and software would appear to be your next step anyway, then you can see why it is dropping back to petrol. Although it appears most likely that the pressure is dropping under load, there is always the chance that it may be something else.

[rossko]

Can I get any old vapouriser and bolt it on?

Not really. There is vast difference between injection types and singlepoint types. Within injection types, there is a range of working pressures and horsepower handling capability. Many are broadly similar and so fairly interchangeable, though you have to watch out for compatible temperature sensors and so on.

It's quite possible the original fit wasn't up to scratch. I think old style Stefanelli SIS vaps are rated 140-150kW, X300 3.2 is 155kW

Problems can also arise from tank slosh, depending on tank type and position, or poor choice of tank valving.

It's not impossible that the problem is not actually related to gas pressure, but to excessive injector durations.

[jseaman]

As usual, thank you for the good information.

I guess a lead and software is on the cards next!

Regarding tank location - it is a toroid in the spare wheel well, not sure about 'valving'?

If injector durations are wrong then it is getting to the stage where I need to find someone willing to tackle it (I presume they use a wideband and rolling road to set these up normally?)

[rossko]

With a toroid tank, it depends whereabouts on the circumference the valve is, as to how prone it is to slosh uncovering the pickup pipe. If you do have a slosh problem, you'll only normally notice it when less than half-full. They ALL suffer from slosh as you get down to the last dregs.

Some tank valve types are too small for 3+ litre engines, but if it is less than ten years old that will probably be ok.

No rolling road, no wideband sensor; the car comes with perfectly capable sensors and roadwheels of its own for this task

[jseaman]

So it pretty much rules out slosh because this happens even with a full tank

This system is 3 yrs old so hopefully that ticks off the valve too

So you don't do AFR tuning and knock monitoring etc normally?

[rossko]

Nope; these are functions of the engine management system. This type of LPG system is fuel only, slaved from the original EMS, which still runs the whole show. "Tuning" is matching actual LPG delivery to the delivery the EMS expects.

[jseaman]

OK so how do you match the LPG to the fuel requirements, is it normally just by 'feel'?

[mat_fenwick]

In a nutshell, it's when the petrol injector times are the same at a particular engine load and speed, regardless of which fuel it's running on. The petrol EMS will trim the petrol injector times (and hence the gas times) as the lambda sensor detects rich or weak running, so if the gas isn't set up correctly it can compensate to a certain extent (up to 100% injector duration).

But beyond a certain point (WOT), the petrol EMS will ignore lambda feedback and just give a slightly rich petrol mixture for maximum power and engine protection. With the software, you need to make sure that it does the same on gas.

[jseaman]

OK so if I understand correctly, you trim the gas injector times with the LPG ECU/software to give the correct mixture for given load/rpm etc whilst the original ECU is attempting to monitor mix as normal based on open/closed loop and lean/rich afrs. The pulse width for the gas injectors is set up by monitoring the pulse width for the petrol injectors to see what the stock ECU is doing i.e. if it increases the on time then the LPG mix is lean - is that right?

[mat_fenwick]

Yes. So you would then adjust the multiplier between petrol and gas injector times (repeatedly switching between petrol and gas to check), until the petrol ECU isn't having to make any large adjustments. Then repeat at different engine speeds/loads. If you took it to a pro for a thorough service they should check this, although those at the cheaper end may only change the filters as you've done yourself.

If you run out of injector duration then it's a case of either increasing gas pressure, or increasing injector jet size. I would hope that jet size was sselected correctly during the initial install, but pressure may change over time as the vaporiser ages. Back to needing the software to check...

[jseaman]

Many thanks indeed - I'm not sure how much I will be able to do with the software and my lack of experience but I think we'll soon see!

Thanks for taking the time to provide the info

[Gilbertd]

Assuming you can get the lead and software for the Stephanelli system (WTV are the lead and software specialists, if they can't supply one then you may have a problem, they are on http://www.wtv-uk.co.uk), the first thing you would do is save the existing settings. That way you can play around to your hearts content and when it refuses to run on gas at all, you can revert it back to how it is now.

[rossko]

A word of caution; don't expect some magical fix by connecting it to a laptop and twiddling. You've got a fault of some kind that needs diagnosing.

[jseaman]

This is on ebay:

http://www.ebay.co.uk/itm/Stefanelli-LP ... 5644bcf5bc

But I've no idea where you plug it in!

[Gilbertd]

If you look at your LPG ECU, you should find a dangling cable with a plug on the end. You plug it in there. That cable is a serial port one, you'll need an older laptop with a 9 pin serial port. Some people have managed to get them to work using a USB to serial adapter but it's a bit hit and miss. As for operating systems, the older the better. Windows 95, 98, 98SE and 2000 all work well, XP works usually, you are into very dodgy territory trying to use Vista and Win 7. This is where that old laptop that is too slow for anything and not worth enough to put on eBay really comes in useful.

[jseaman]

Thanks, will check out the ECU

In my experience, prolific chip sets for USB/serial comms (which are in most converters) can be difficult with older serial ports. I have some high end high speed serial cards so hopefully can get one of those to work. Thanks for the tips, I'll keep this in mind.

Rossko - I completely agree, I'm just trying to figure out whether I'm getting out of my depth but so far I've managed to acquire enough knowldege off here to keep going - just got to decide if I can keep that up!