UK Jeeper with LNG duel fuel problem

I've been working w/this chap, trying to help him figure out his problem on another forum, but there aren't many European jeepers on the other forum. I'm hoping somebody here can talk him through his problem a little better, or give me something to suggest to him. He's got problems with a stumble at idle/rough acceleration, and it is somehow related to the LPG conversion.

Model year is 2000, with the 4.0L. LPG conversion was done by previous owner. Simon's latest report is available here. Initial description of problems here.

He says it 'kangaroos' when trying to accelerate. His mechanic in the UK replaced the TPS, but apparently that did not solve the problem. It seems to me it has to be an air/fuel metering problem of some kind. I already suggested the fuel pump & all the throttlebody sensors. I am not sure, yet, whether his mechanic checked the fuel pump.

He doesn't know I'm posting here, but if you folks have some ideas I'll direct him to look here. Or, if you have an account on the other forum, you could post a reply there.

Note: the last, possibly relevant post on 's forum regarding LNG was this one from October 2005, posted here in the Modified Tech forum. I am hoping some other UK/Euro jeepers might see this & help him out.

My LPG (Propane for US readers) conversion did the same when first installed. The only reliable cure is Bosch Platinum spark plugs. I use the single electrode style and have done so for the last 5-years 85,000 miles. They are difficult to locate in the UK but Eliot at EDM usually keeps them in stock now. The first time I bought them from a Bosch dealer I had to wait 3 weeks as they were special order from Germany. http://www.edm-jeep.co.uk/jeepparts.htm

If it is a multi-point injection system he needs to make sure the LPG is being injected into the manifold as close as possible to the petrol injectors. If they are too far apart the timing for one of the fuels will be off, causing rough running etc. I wonder if the diagnostics (if applicable to his LPG system) program would help.

I also fitted Bosch platinum plugs (from EDM) to my converted XJ to help with some 'pinking' and it does have the ability to cure small problems.

Re: UK Jeeper with LPG duel fuel problem

Thanks, guys. Your suggestions are excellent, I will pass them along. I'm actually not clear yet whether the problems have more to do with the petrol function or the LPG function, but the plugs would certainly affect both.

In the meantime, Simon has noticed that the kangarooing seems to happen at 1500 RPM, but is fine at other RPM. I should mention that he says cold weather seems to make it worse. He says it starts fine. I can't decide, does this sound more like a problem of lack of air, or lack of fuel, or lack of spark? In my experience, kangarooing suggests a fuel pump problem, but that wouldn't explain why cold weather affects it.

Ah the cold weather... A lot of LPG systems can be affected by cold weather, most common problem on older single point systems is ice forming in the inlet manifold. Always run on petrol until engine is completely warm in temperatures less than 2 degrees Celsius.

My advice is to temporarily ignore the LPG. Get your Jeep technician to get the engine running purely on the fuel it was designed for. The only thing I would say is worth adding to the mix at this point is the Bosch Platinum plugs. If the technician can get the engine running on gasoline then you know your TPS, CPS and Oxygen Sensor are OK. Then start looking for a fault in the LPG. At this point you can still drive your Jeep although it costs more for fuel.

My TPS sufferred a slow decay a few years back and it was a difficult fault to spot until it completely died. I couldn't get the engine to run at all on gasoline but the LPG system ran perfectly using just the oxygen sensor for mixture adjustment.

I have also had ice form inside the LPG ECU which later defrosted and shorted out the pcb inside. In my opinion the pcb should have been potted in epoxy, three shorted ECU's later I decided to hermetically seal the unit myself at the expense of invalidating the useless warranty and that one has been running for four years in all weathers and regularly gets drenched in muddy water.