Car not running properly on LPG when car has been idling for some time

Question

I have a Renault Clio 1.4 16v 72kw from 2003. I have this strange problem where when the car has been idling for some time (when in traffic for example) and gets more warm (temperature indicator more than half way. This means it's > 90 C right?) it wil start to misfire when it's running on LPG. If I let it Idle for too long it will eventually stall. When I start to drive at higher speeds again and the engine cools from the resulting air flow it will run perfectly again (also runs good on idle again)(temperature indicator just below half way now). The engine runs well at higher RPMs on LPG even when it's warm from idling.

Note:
-On petrol I have no problems at all.
-The radiator fan will eventually come on when I let it idle so there is no problem with that.
-I have expensive NGK Iridium spark plugs which are supposedly better for LPG. They've had about 50.000km.
-I still had the old filter when this started happening.

Now I have already been doing some research and one thing that is very likely is that the injector rail is clogged. This is very likely because I replaced the dry filter waaay to late (it did 100.000km lol) because I didn't know it needed replacement. However, I really doubt this is the problem because the car runs fine when not getting warm from idling to much. I wonder why would The injector rail start to behave differently when the temperature of the engine becomes higher? (during idle) It's not welded to the engine block although it's near it. So yeah it shouldn't get that much hotter right? It might to idk.

So my guess it's either spark plugs or the injector rail. The problem is that i'm not sure because:

1.The spark plugs work fine with any temp on gasoline and lower temps with LPG.
2.The injector rail works fine with lower temps and why would it start failing with higher temps? shouldn't it start to work better lol?

Btw I have this LPG system (dutch): https://agit-lpg.nl/portal/files/2014/09/Eurogas-Folder-Evo-360-NL.pdf

Should be the same as this (english): https://landirenzo.com/sites/default/files/upload/impianti/evo-lpg.pdf

So what I'd like to hear is: What should I try first, plugs or rail and if that doesn't work what else could it be?

Thanks for helping out.

MasterWaster · Answer

As someone who also has a motor with LPG then I suggest going to your local LPG specialist to have it sorted out - the thing is that usually these kits have special software to fine-tune them (including diagnostics) and the last thing you would want is your engine being ruined.

LPG is great but it also needs specialist maintenance. Just my 2 cents.

Criggie · Answer

This answer is based around a conversion with a carburettor, which doesn't match OP's descripion of fuel injection and rails, but hopefully leads OP to look for cold spots in the engine bay while idling.

Your gas/air mix is running lean because of ice/frost forming in the vapouriser while idling.   I had experienced this myself.   I suspect your hot water piping needs checking - the radiator/water pump etc.
LPG setups have a vapouriser to help turn the Liquid into a gas.  The process of evaporating means it gets cold - a phase change from liquid to gas will chill the surrounding area.
Mine looks vaguely like this and has one chunky pipe going straight to an injection ring immediately above the carb.  Your car's a lot newer so will probably look different.

When idling, the engine is not pumping a lot of hot water through the cooling system.  So there's not a lot of hot water getting to the vapouriser, so it gets cold.  You'll see frost forming on the outside.
  Frost on the outside of a regular LPG cylinder that is in-use.
So as this frost forms in your vapouriser and chokes up the gas flow, leaning out the engine and it eventually stalls.
You should be able to see this effect by simply idling the car on LPG, and watch for the frost forming (though a warm garage, and having the bonnet/hood up might interfere.)

Find a control tap in the water hose to the vapouriser and open it a bit more.
Rev the engine a bit more when idling (downside, you look like a spanner wanting to race)
If its an electrically-warmed unit, check the voltage at the heater block, and that the heater is functional.

Car not running properly on LPG when car has been idling for some time

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