miss fire 3.9 discovery

ive just brought a 3.9 discovery with lpg twin under slung tanks (been stood 2 years ). dont get me wrong it needs a little tidying but its not rotten i picked it up friday night and started the drive back with in the first 10 miles it felt that it had dropped 2 cylinders , i just put it down to needing a service , we put a cap arm and a set of plugs in it and it woz back on 8 , today i started it up again and its missing again , but wot i cant get my head round is when i flick it over to lpg it revs clean with no miss fire any one got any ideas please :doh:

Hi.
I think your problem is fuel supply, not electrics. Could be a problem with the injectors or as was in my case the engine was being overfuelled by a faulty ecu, so it was effectively running on choke all the time which felt like it was missing but ran fine on LPG. Thing to do is find friendly garage and swap ecu with known good one to see if it fixs the fault and if not, isolate which cylinders are misfiring and then get the injectors checked out. The fuel system is pressurised so make sure you depressurise it before disconnecting fuel lines.

Cheers, Nick.

njh2 said:

Hi.
I think your problem is fuel supply, not electrics. Could be a problem with the injectors or as was in my case the engine was being overfuelled by a faulty ecu, so it was effectively running on choke all the time which felt like it was missing but ran fine on LPG. Thing to do is find friendly garage and swap ecu with known good one to see if it fixs the fault and if not, isolate which cylinders are misfiring and then get the injectors checked out. The fuel system is pressurised so make sure you depressurise it before disconnecting fuel lines.

Cheers, Nick.

Click to expand...

yeah thats wot im thinking fuel .. the plugs were soaked when i changed them , when reving the engine the rev,s take a while to die down hmmmm air flow meter help

chewy1969 said:

yeah thats wot im thinking fuel .. the plugs were soaked when i changed them , when reving the engine the rev,s take a while to die down hmmmm air flow meter help

Click to expand...

Would respectfully suggest you do a search. Just about every known cause of V8 misfires is somewhere in LandyZone as well as where/how to investigate and fix them.

The symptoms you describe do sound very much like an injection misfire so you need to check the injection system, start with substitute ecu then I would look at temp sensors. Good luck.

can any one help with an ecu ,, for sale ????

chewy1969 said:

can any one help with an ecu ,, for sale ????

Click to expand...

What makes you say the ECU is cream crackered?

does it have CATS? or is it pre 92.

if its running ok on gas then its likely not to be a spark issue. fuel delivery to those cylinders could be a problem.


are all the plugs soaked or just a couple?

to check the AFM on a non cat car.

Blue/Green Air flow signal- should be .2-.7 volts (no air flow). Tick over on the 3.9 is about 1.7 volts

Blue/red CO trim value. This is factory set 1.8 volts for catalyst engines, although I believe this setting is ignored with Lambda correction. Non cat cars are in the range of 1-1.5 volts, although accurate setting will require the use if an exhaust gas analyzer

Most airflow meter faults will cause the engine to run excessively rich. However if the airflow meter remains connected whilst defective then the vehicle will probably not run. In most cases the output from a defective airflow meter will be in the range 2.0-2.5 Volts, which is a viable value. This represents a moderate load and will cause heavy over-fuelling without setting a fault code.