Rough idle on petrol, high idle on LPG

Thank you for your reply Jod! I did read this one as well, inspected the vacume lines, with extra attention for the elbow's. There aren't holes in them. I also tried to bent them, but still no visible cracking or aging. The problem is also a little different, as my RPM is going down on petrol, and high on LPG. I did check for DTC's, but they are 1-1-1, so no DTC's logged, and my lambda light is OFF. Oh maybe another hint, I didn't wrote in my first message. If you throttle up to say 2000 RPM on petrol and hold the throttle fixed in this position, you will switch over to LPG, the engine will increase in RPM, and will run about 2400 RPM in the same fixed throttle position. I don't known if this is normal, I didn't notice it before, but can't say if it was before or not for sure. Jesse

check your elbows again 

Jesse - did you check the elbow that connects to the LH side of the inlet manifold. Pull it off and inspect it - you won't be able to tell if its holed or collapsing in on itself by looking at it in situ Jeremy

What type and make of LPG system do you have?

Sounds to me like its bogging down on petrol, ie to rich. May be an injector sticking open and dribbling.

I do not think it is a dribbling injector. As it runs fine on start up it most likely a sensor issue. For the first few seconds the fueling is open loop - ie its not really looking at the sensors just running of base settings. Once the sensors kick in then you are having the trouble. The LPG can respond differently especialy if it is not intergrated into the petrol managment. Lambda sensor would be a good start. You could try unplugging each thing one at a time - TPS switch MAF sensor and Lambda. Do not unplug them all at onece. Once you have established its not one thing put it back and try the next.

Thank you all for your replies. My car has a Vialle AMS LPG system. I did check the rubber elbow again at the LH side of the manifold (or RH in driving direction). That does look and feel ok. Did check the sensors, and found the throttle position sensor rheostat had a bad contact just above idle. I sprayed some contact cleaner in it, then it was ok again. You can check this most easy with an analog meter instead of modern digital one. Car wouldn't run without the MAF sensor. Had the high LPG idle RPM one day only, That has been ok for the last few days. Did drive about 80 km on patrol only, and now it seems to be ok. Will order new TPS, as it will go bad again. Not sure if it is ok now due to cleaning TPS or long drive on patrol. For how long does it run without checking the sensors? Is it time or temperature based? Just wonderd about this. Thank you all for your help! Jesse

Its only a few seconds to allow the engine to settle. I'm glad you got somewhere with it.