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[Plank]

My 1998 ZJ has a LPG conversion but I have a problem when the engine is cold which is getting worse.

It starts on petrol and when setting off it splutters, has no pull and will only go at all if I put my foot right down and it bucks and splutters like hell until it picks up.

It carries on spluttering and bucking until it changes over to the LPG then it picks up straight away and goes like stink.

My catalytic converter has a rattle from the heat shield and has a leak at the front joint and wants replacing at pay day, would that be the cause.

any ideas anyone

[JamesH]

Could be fuel pump, sounds like symptoms I had and running good on LPG would suggest it might be this too. Check fuel pressure at the test point on the fuel rail.

[bigjeepzj]

 
the fuel pump runs when the jeep is on LPG.
This can cause the pump to fail  as the fuel pressure  has nowhere to go.
a relay is needed to cut of the pump when on LPG
the other thing is it could be a lambda sensor as that is emulated by the lpg kit when on gas

[Delk]

Normally when the pump goes it will idle but not have any power. If you can put the peddle down and it runs I would think the pumps ok.

[Jonny Jeep]

Overpressure fuel returns to the fuel tank via the bottom of the fuel filter/regulator that sits on top of the tank. It's more likely heat build up as fuel recirculates that causes problems with the pumps.

O2 sensors aren't used by the PCM for fuel mix for at least 20-30 seconds after the engine starts, as the O2 sensors aren't accurate till they are over 300°C. If the rough running is right from the start the O2 sensor is not the cause. If the engine initially runs fine then has a short period of rough running, the heater element of the upstream O2 sensor may have failed. If this is the case the engine should run fine if switched back to petrol.

If the 02 sensor is suspected it can be unplugged. This may cause the check engine light to come on for an O2 code, but it will force the PCM to run using programmed fuel maps based on other sensor inputs. If the engine runs normally with the sensor unplugged the O2 sensor is the problem and should be replaced.

[JamesH]

Thinking about mine I think it was going for a long time, getting the odd bit of rough running and splutter and developing to rough idle but fine driving and then eventually completely non driveable.

[Plank]

Overpressure fuel returns to the fuel tank via the bottom of the fuel filter/regulator that sits on top of the tank. It's more likely heat build up as fuel recirculates that causes problems with the pumps.

O2 sensors aren't used by the PCM for fuel mix for at least 20-30 seconds after the engine starts, as the O2 sensors aren't accurate till they are over 300°C. If the rough running is right from the start the O2 sensor is not the cause. If the engine initially runs fine then has a short period of rough running, the heater element of the upstream O2 sensor may have failed. If this is the case the engine should run fine if switched back to petrol.

If the 02 sensor is suspected it can be unplugged. This may cause the check engine light to come on for an O2 code, but it will force the PCM to run using programmed fuel maps based on other sensor inputs. If the engine runs normally with the sensor unplugged the O2 sensor is the problem and should be replaced.

Set off this morning as normal and the problem was there as described previously.

Once the Beast was fully warmed up, I switched back over to Petrol. Not a splutter, blip or fart, he ran absolutely perfectly.

The great sage Jonny Jeep is obviously correct and the O2 senor wants replacing.

Thanks chaps for all the useful info, another minor issue resolved.

[Jonny Jeep]

Just to be certain i'd first disconnect it. The connector is clipped to the intake manifold near the firewall. With it disconnected it should run without fault if it is as suspected. You can even leave it disconnected till you get your new sensor. It will run slightly rich, but when I last did it I noticed no difference in mpg while waiting for an NTK sensor from Rockauto. I also just got a new discount code off them from my last purchase if you decide to shop with them.

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