Judder / Misfire on LPG - Fine on petrol

Hi all,

My 3.9 Discovery 1 has started to misfire/judder really bad below 2000 rpm and at light throttle. Seems to go away at higher rpms

It is particularly bad on a cold morning and results in a big drop in power / judder.

• The system is a Bigas Sequential (SIGN).
• I have replaced the plugs to NGK BPR6ES and gapped to 0.8
• New leads
• New distributor, rotor, cap and ignition module
• New coil

Could it be a sticking injector?

I have purchased a data cable to hook into the Bigas ECU, lambda is reading very high (over 4v)

Video of mis-fire here:

I have no idea about LPG but if it was petrol I would say check ignition timing, check vac advance is working properly. If all that checks out then possibly MAF or ECU. Be aware that sometimes on a cold morning you will have a misfire due to condensation in the distributor cap, once the engine is warm it goes away.

You say you replaced dizzy cap, rotor arm and ignition module ( I assume you mean the ignition amplifier). Are the arm and cap genuine Lucas? If not - fit genuine, more expensive I know, but oh so worth it.

After replacing the distributor, amplifier etc. I had to retime the ignition, rehooked up vac advance and tested that it works.

I have started looking at the MAF and O2/Lambda sensors as a next port of call... With the engine at idle the MAF reads ~0.45 Which seems correct? HOWEVER As soon as the ignition is turned on, both O2 sensors read over 3 and 4 volts and continue to be that high while the engine is running? Does this indicate they are broken?

Could be as simple as kev says. LPG can take 16 ish btdc but 95 petrol will start to pink around 12. Turn dizzy a degree anti clockwise first and see if it improves

Sure, I'll advance and use premium petrol as far as possible. Will update with findings

May also be an issue with using resistor plugs, though others do. Bp6es are non resistor.

Update on this:

Advancing the timing to 14 degrees made a big difference but there was still a slight miss at low RPMs.

I took the car to Leeds LPG and it turned out the lambdas had not been hooked up to the LPG ECU (which explains the duff readings) and it was running a V8 base map. He hooked up the lambdas and did a full retune. £90 quid well spent!

Running beautifully now.

To summarize: LPG loves ignition advance and LPG specialists are totally worth the cash!