I've mostly ignored the LPG system since i managed to get it working, but i'd like to try and fix some small issues it has.

When i bought the rangey, it would barely run. A new MAF and some new petrol injectors (Rover K-Series 1.4 are a good match and much more modern!) had it running a LOT better and finally got it thru an MOT, and so i set about trying to get the LPG working. It ran, but hopelessly badly. There was no filter present, and the injector rails were quite old and after posting on here it was suggested i replace them, so they got swapped with a pair of Valtek type 30's and i fitted a filter at the same time. Eventually figured out the "autocalibration" menu in the software, ran the calibration and it all started working. Great!

It drives acceptably well, perhaps a little less perky on gas than it is on petrol, but theres no hesitations or missfiring or whatever, it just works.

So the first problem. When running on LPG, if you floor it, maybe half the time or a bit less, and you keep your foot planted to say accellerate from 20 to 70mph (so quite a while), it will stutter really badly after some period of time at full throttle. It'll only usually stutter the once, and most of the time it clears and keeps accellerating with the accellerator still floored. Very occasionally the gauge/switch thing will beep or flash its lights when this happens. Gas level doesnt seem to effect it. I've had it do it with a completely full tank, though it tends to be a bit easier to cause if the gas is on the lower end. Its different symptoms to actually running out, where it switches to petrol and beeps at me. It stays on gas when this happens.

The second issue, i'm not sure if it is an issue or not, but theres a definate performance difference between LPG and petrol, however this seems most noticeable at the lower end of the tank. Recently i was driving it on LPG and i knew it was close to running out, i was heading towards a filling station thru a sequence of roundabouts when it finally cut over to petrol. I immediately noticed it was a LOT more responsive once it switched to petrol accellerating out of the next roundabout. Seemed to need less throttle pressure to make it accellerate and it just felt better. With a full tank of gas the difference doesnt seem as large, but surely the LPG system should be compensating for gas pressure to ensure the systems always injecting the same amount of fuel?!

I'm sure that Simon will be along soon to give a definitive answer but I'd say that:

1. Is a gas flow/reducer issue. It might be set to too low a pressure or the reducer might not be able to flow enough gas due to age or being the wrong spec to start with.
2. Sounds like bad mapping. In theory there should be no difference in performance between a properly mapped LPG system and running on petrol. In practice it takes skill and experience to get it bob-on.

It sounds like you can't be sure this system has ever run perfectly so you might have to prove the design/components before getting too deep into the adjustments and tuning.

I found Valtek Type 30 injectors weren't fast enough when I tried them on my P38 - it constantly felt sluggish, no matter what I did to the tuning.

The problem is also that if it's not able to inject the LPG quick enough, then it runs a bit lean, which the petrol ECU accounts for by richening it up, and so on and so forth until you get fuel trims maxed out, and running issues (usually at idle on petrol, mine showed up as after time.

The beeping when you floor is is most likely the gas ECU detecting low pressure and then switching back to petrol automatically whilst you accelerate. LPGC or GilbertD are more versed in that stuff than me though, and I'm sure they will be along soon to give more/better info

Its a Bigas SGIS system and the ECU seems to be somewhat restricted with what it can operate. I think the original Bigas injectors were extremely expensive or difficult to get, Simon/LPGC on here suggested the Valteks as they were a close match to the original Injectors used in terms of flow rate and characteristics.

I'll need to look over the fuel trims again, but from what i remember they were fairly sensible.

Personally I'd have gone for Valtek 34s as they seem to be better that the Type 30s but a bit late now. Did you fit the correct sized nozzles when you fitted them? Easy way to check is to look at the map in the LPG software. As you've done an autocal it will have set itself correctly but the numbers want to be between 120 and 150 signifying that the gas injector times are between 1.2 and 1.5x the petrol injector times. Higher than that and the nozzles are too small.

The cutting back to petrol or losing power when held at full throttle for any length of time is a symptom of the gas pressure dropping. Most likely you are using it faster than it can get to the reducer. It could be that the manual valve on the tank isn't fully open or the pipe between the tank and reducer has been clouted and squashed partly flat.

When running normally you are picking up liquid from the tank which is turned to vapour in the reducer. As there are no baffles in the LPG tank, when getting low and with lots of cornering, the pickup pipe can come out of the liquid and be picking up vapour. That will also cause the pressure to drop as you can't turn vapour into vapour when it already is.

But, bottom line is as Morat says, if running correctly there should be no dicernable difference between running on gas and running on petrol.

This thread is spookily like my lpg running issue, which has brought me to the brink of armageddon via a new vapouriser and new fuel feed line. Hopefully the solution will be simpler for you Aragorn!

Beeping and switching back to petrol when you boot it means the reducer pressure has fallen away so the system switches back to petrol thinking you've run out of gas.

Switch flashing without beeping when you boot it means the pressure remains high enough but the system has momentarily switched back to petrol because it has sensed gas injector pulse duration was exceeding available injection window (gas pulse times too long for given rpm).

If the reducer is capable of enough flow both issues could be overcome by increasing gas pressure at the reducer, but then you should recalibrate (full calibration not just the autocal).

The V30's are a good match for Bigas injectors in terms of linearity/speed so are a good stand in for Bigas injectors in that respect, outlet nozzles for V30s are available to suit 4mm pipe so make for easy replacement of Bigas injectors which have 4mm outlets. Bigas usually use 4mm pipe, V30s usually use 4mm pipe, V34s usually use 6mm pipe but it's best to specify pipe you need to connect to when you order V30 or V34 injectors... you probably don't want to have to change manifold spuds and piping when you're changing the injectors.

Linearity/speed isn't the same as flow rate, Bigas injectors fitted with nozzles of around 3mm will flow more than V30's with biggest nozzles fitted, more even than V34's with biggest nozzles fitted. All injectors have an internal restriction (ultimately the diameter of the hole under the plunger but there can be a restriction after the plunger hole too), V30's come in 2 flavours, one flavour has an internal restriction of near 3mm the other has an internal restriction of closer to 2.5mm.... there is no point fitting outlet nozzles with bigger hole size than the internal restriction because the internal restriction will then be the limiting factor in terms of flow.

If you replaced Bigas injectors that had nozzles above 2.5mm you may need to increase reducer pressure to compensate for smaller nozzles or internal restriction of Valtek injectors.

Depending on whether you're using an old version of Bigas software / firmware I might also advise you reset the LPG ECU before running autocalibration (warm the engine, increase reducer pressure and run on LPG for a bit before resetting the ECU, re-enter all basic settings, run autocalibration and manually adjust the map). The old Bigas software / firmware doesn't show reference pressure and it isn't obvious if behind the scenes they use some standard reference pressure such as 1.2 bar regardless of pressure reading during autocal or if behind the scenes they use the pressure read at time of autocal as reference pressure (which would then be stored and used until the ECU was reset).

Also depending on software/firmware, some Bigas ECUs have petrol addition (above a certain rpm and load) set as standard - if your reducer is big enough it might be a good idea to turn off the petrol addition before doing calibration.

Sometimes the transition from running without petrol addition to running with petrol addition can cause hesitation but so too can the transition from running on gas to switching back to petrol (either in case of low pressure or in case of sensing full duty cycle of injectors).

It doesnt switch totally back to petrol when flooring it. You just get a huge miss as though the fuels cut off, and then it judders back into life, still on LPG. The light might flash and it occasionally beeps, i guess i'm not always staring at the switch though when i'm flooring it down the road :P Maybe its running out of duty cycle like you suggest, strange that its random though and doesnt always happen?

When the gas has run out, it switches over pretty cleanly to petrol (maybe a very small miss) and the switch starts beeping constantly until you press the button.

The original Bigas injectors were 3.0mm. I drilled the new Valteks out to i think 2.5 as recommended by you. You did say i might need to increase the pressure, but it seemed to work without it. The ECU was pretty much completely reset as the original settings were all over the shop, then it was warmed up and the autocalibration run.

The petrol addition stuff is turned off.

Autocalibration seems easy enough, but surely if your going to start doing a full calibration your needing a proper understanding of how it all works, and probably wideband kit and maybe even a dyno to be able to hold the engine in various load sites.

I guess it could probably benefit from a proper calibration from someone who knows what they're doing, unfortunately i suspect i'm miles away from you, and while i could find someone local, i always have a bit of a fear spending money with mechanics who are unknown to me, as theres so many cowboys around.

Would need a bit of info to be able to help from here... Pressure, Pinj and Ginj readings for all of idle, moderate load and high load. Does reducer pressure fall away at high load high rpm? What reducer do you have?

See if the switch flashes when you boot it...

What are the numbers in the map?

Don't need a wideband sensor but it helps to be able to monitor fuel trims and lambda readings, so an OBD scan tool that can show live data is handy.

Thanks, i'll see what i can do at the weekend.

I thaught i had the map saved on the PC, but the software isnt showing anything when i goto "load configuration" So i'll need to sync it back off the ECU again.

The problem i envisage when tuning without a wideband is once beyond a certain throttle position (and certainly at wide open) the ECU is likely to go open loop and the AFR will drop down into the 12/13 range. At that point, fuel trims are locked out and the lambda readings will just show "rich", whereas the proper wideband will let you see exactly whats happening and tune accordingly. But maybe you can get enough data from the lower load sites to dial in the full throttle stuff.

Yes, if we made a line from the load numbers in the AEB system map table we'd see how the graph sloped down toward the high load range. If we only plotted the graph for closed loop conditions we could imagine extending the graph for open loop high load conditions and using the numbers from the imagined further plotted points for the open loop area of the map the engine should run similarly as rich on LPG under high load open loop conditions as when running on petrol. And/or we can monitor lambda and Ginj when adjusting map numbers, lean the fuelling to the point lambda voltage comes down to say .6v equivalent (for a 0>1v probe) and increase the map numbers until fuelling should be some percentage richer, and/or we can use seat of the pants. I use all the 'ands' - the petrol system goes rich to varying extents on different model vehicles and on some the mixture goes so rich on petrol that to go similarly rich on gas would provide less performance while using more fuel as such a rich mixture is more likely to spoil volumetric efficiency on gas while a rich mixture on gas (even for same airflow / VE) isn't as likely to give the same performance increase on gas as on petrol. If the exhaust stinks when you boot it on gas it's likely to be way too rich (window open test). We still want a rich mixture on gas when you boot it but sometimes seat of the pants best response is when lambda is within range of a 0>1v sensor at about .8v or not much richer than that. Always bearing in mind that a rich mixture will speed up the burn so leads to cooler running valves and means there won't be as much remaining oxygen after the burn which in conjunction with valve heat can lead to valve wear... so we err on the side of caution between seat of the pants and rich. When you're just cruising or particularly if you've been sat idling for a while gas temp will be higher than it is after a sustained high load but the temp sensor can take time to reflect the actual gas temp... If you set high load mixture when gas temp is reading hot and don't recheck high load mixture after sustained high load use the mixture could be leaner after sustained so it's a good idea to give the car some stick and check high load mixture when gas temp reading has responded and reflects the cooler temp of the gas. Gas temp will always be cooler when using more gas because there's a combination of more cooling from turning more gas from a liquid to a vapour and the gas spending less time in the reducer being heated (if the reducer acts as a heat exchanger it becomes a comparatively smaller heat exchanger the more gas that flows through it). The fastest responding temp sensors tend to be those fitted in the same unit as an inline pressure sensor (using a sensor like those fitted in air flow meters directly in the gas flow), the slowest tend to be those simply screwed into a metal injector rail where the sensor itself isn't actually in the gas flow (picks up heat of the metal rail rather than gas temp).. response ranges from a couple of seconds to a few minutes.

Well after my radiator nonsense i finally managed to get it into a state where i was happy to really floor it and had a play this evening.

Full throttle in 2nd gear, it revved up to about 5k before getting the usual jolt. It was fairly mild this time and it continued pulling just fine after the jolt so i kept my foot in. Glanced over at the LPG guage and sure enough, red light (petrol) was on solid, and the gas light was flashing. It shifted into 3rd just fine and continued in the same manner. When i lifted off another slight jolt and the gas light went solid again.

So that would suggest as you've assumed that the gas injectors are running out of duty cycle.

As discussed, the gas nozzles i drilled out to 2.5mm, the original bigas units were 3mm i believe (3.0 stamped into the rail) I dont know wether these are the 2.5 or 3mm internal versions, how can i find out which they are to see if theres any scope to go larger before upping the reducer pressure?

Not got the computer on it yet to check the map settings.

No easy way to tell whether they're 2.5 inside, You could just drill the nozzles out anyway.. If they're restricted to around 2.5 inside and you run the same map it won't run much richer than now but if they're closer to 3.0 inside you'll have to lower the numbers in the map, which could avoid your problem.

Or you might want to come back with the info I asked for a couple of posts above first, chance you could just up reducer pressure instead of drilling nozzles out.