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1995-2002 Range Rover P38A
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Discussion Starter #1
hi i have just drove 200 mile on lpg ,when i ran out it changed to petrol ran on motorway ok untill first island off m/way then engine ran but ticked over lumpy but then cut out altogether and then was a bugger to start. eventually started but it has not run right since. i have disconected maf and cleaned & refitted car would not start with maf un pluged reconected and car started but still not running propely runs great on lpg any ideas would be appreciated thanks
 

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1995-2002 Range Rover P38A
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690 Posts
It would help to know what sort of engine you have and what sort of gas system.

I had a dodgy injector simulator which would occasionally not switch the injectors off. With the engine set to gas it would miss and splutter and the lambda readings would be ridiculously high - pop the lead off the gas solenoid and it would continue to run! Cleaning up and resoldering the boards in the simulators cured it.
 

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1995-2002 Range Rover P38A
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3,952 Posts
It's flooding. If you've got a multipoint LPG system the fuelling map is much too weak so the petrol ECU winds the mixture to full rich to compensate (as the LPG system piggy backs off the petrol system). Then when you go back to petrol it is far too rich so runs like a dog until it has run long enough to sort itself out. If you've got a single point system then you could have the same problem Gordon had so it was running on both fuels (even though it had run out of LPG that only means it had run out of liquid, you've still got a hell of a lot of vapour left).
 

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LEGACY VENDOR
1995-2002 Range Rover P38A
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I'm pretty sure my LPG system runs in a closed loop system along with the petrol one, as when it's hooked up to the software, I can see the O2 sensor readings in the LPG software. (I have a Zavoli Alesei multipoint system)

I had problems with spluttering and rough idle on my 2001 awhile back, and it was the MAF, as the engine ECU had run out of adaptive value to trim the fuel trims, so it started running like a dog.

New MAF and it's running fine now. I've periodically checked since putting new MAF in for fuel trims being pulled out of range (especially by the LPG - as I was expecting the LPG system to throw it out like Gilbertd says) and the fuel trims were pretty much bang on where they were after replacing MAF/resetting adaptive values.
 

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1995-2002 Range Rover P38A
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222 Posts
Not sure about the fuel trims, but I run my 4.0 on petrol and LPG. On holiday (1400 miles) I was switching to petrol after the LPG got low and the car ran absolutely fine on both with no misfires or lack of power. I have an older single point LPG which gives no trouble at all as I think my system puts the O2 sensors into open loop when LPG switches over, so the ecu ignores the readings while on LPG, so I would have thought a new multi point system would have a similar system. I would be checking things like fuel pressure at the rail, poss checking that if its Gems the fuel filter has been changed and also a thing that happens if LPG used a lot, the petrol in the tank might be going off the boil a bit and may need changing?
 

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1995-2002 Range Rover P38A
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Sounds like we need a beginners guide to LPG systems, so here goes. On a multipoint like the Zavoli, the mixture is controlled by the petrol ECU and the LPG ECU just adds a fiddle factor. Say that at a given load and revs the petrol injectors need to be open for 5mS and the LPG ones need to be open for 7mS, the LPG ECU intercepts the injector pulses from the petrol ECU, adds 2mS and fires the LPG injectors for that long. If the LPG ECU is not set up correctly and only adds 1mS, then the mixture will be weak so the lambda sensors report this to the petrol ECU so it increases the petrol injector pulses to 6mS. Runs fine on LPG but is then running rich when switched over to petrol because the fuel trims have had to be maladjusted. The LPG ECU does not do anything with the lambda sensor outputs, they are only connected so you can see what they are doing as an aid to setting the fuelling map in the first place. It would make no difference if they weren't connected at all, in fact, some installers don't bother to connect them, it makes no difference to the running but means you need the software and a live code reader both connected when setting it up.

A single point will, hopefully on a P38, still run in closed loop. A singlepoint isn't really suitable for a GEMS P38 and definitely not good enough for a Thor but as long as the ignition system is kept in good condition, it is acceptable. While very early single point systems did run open loop, all systems available since the P38 has been about use a separate closed loop controller. When on LPG the petrol injectors are replaced with an injection emulator that simulates the injectors so the Petrol ECU doesn't get offended when it detects no injectors connected. So the petrol system doesn't know it isn't doing the fuelling. The LPG system takes a signal from a lambda sensor and runs that in closed loop by adjusting the amount of LPG going in depending on what it is told by the lambda sensor. You could have a situation where the LPG system runs very weak (due to a stuck stepper valve maybe) so the petrol system increases the pulse length to richen the mixture but as it isn't doing anything, this has no effect so it increases it a bit more but it still has no effect so etc........

Note I have used the term LPG rather than gas to prevent our transatlantic cousins from thinking I'm referring to petrol.....
 

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1995-2002 Range Rover P38A
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Thanks for the clear up Gilbert, :thumb:
 

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LEGACY VENDOR
1995-2002 Range Rover P38A
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Thanks for that! I wasn't aware the system didn't use the Lambda sensors in actual running, but for setup only.

I guess that is how the 'auto calibrate' system in the LPG software works then? It then uses the lambda sensor feedback to 'tune' the LPG system, but in normal running just leaves it to the petrol ECU.

Makes sense I guess!

Marty
 

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1995-2002 Range Rover P38A
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29 Posts
Discussion Starter #9
Hi my range rover is a 4.0 ltr se it has a zavoli multipoint lpg system as i said i have cleaned my maf unit and tried to run my engine with it disconected and it would not even start i reconnect the maf and engine starts could this mean my maf is ok. The car starts first time but does not tick over evenly it is like the tickover needs to be higher, it drives fine but seems like it is holding back. It is 100% better on lpg
It does seem to get better the more i use it on petrol thanks for all your replys
 
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