intermittent misfire 3.9 doin my nut in!

Hi all,

I have searched this, but struggling to find a decent answer.

Well, my 3.9 fender conversion was running like a dream for about 5k miles. . .until recently when it's developed an intermittent misfire (I think), causing an occasional backfire and Its down on power. It's also recently been converted to LPG(500miles ago), and runs lumpy on both petrol and lpg. It was running better pre-LPG, but not perfect recently.

Today I've replaced the HT leads, which seemed to work for about 20mins. . .

What should I do next? Planning to check all sparks on Saturday, but they're all recent.

I'm quite new to this v8 malarky, so apologies if I'm being a total spanner.

Help!

S

i would run on petrol to see if affecting just LPG or both fuel systems.

If both when the LPG was installed did they adjust the mix or set up ?

LPG does require a good spark so work back plugs leads coils.

Hope that helps

Which version of the engine have you got? Which engine management?

Peter

Hi chaps, thanks for the replies.

The engine is a 1997 3.9v8 from a disco auto. It has a custom ecu that's been rewired so it runs without the lambda sensors or cat, and was working beautifully. It was rewired by an engine builder in Chester- a guy who really knows his stuff.

Its running rough on both petrol and LPG, I'm not sure if they adjusted the mixture- I'll check. The LPG system has yet to be properly tuned (it will be at 1000 miles when everything's bedded in)

I think I'll have to take everything apart this weekend! I'm wondering if the LPG company have touched the injector wiring. . .

Thanks again,
Steve

Most multi-point lpg systems use the engine sensors for mixture etc., if you haven't got those fitted, how does the engine management control what is being put in?

This is pretty important as the weather changes and mixtures are adjusted for the denser air in the cooler temperatures.

There must be some means of controlling that, if there are no Lambdas, what is used?

Multi-point LPG usually just switches the injector drives to the gas valves and uses the existing sensors.

Peter

My 3.9 auto 90 developed a misfire I spent days if not weeks trying to sort it,turned out the cheapie tin head gasket had failed a compression test showed nothing but I didn't do a compression leak test that would have shown the fault, on inspection one pot on each bank had leaky h gasket. Mebbe worth a check skip.

Thanks for the replies chaps! I'm pretty sure i've narrowed it down to a faulty Air Mass Meter. If she's running rough, i disconnect the AMM for a few seconds, then plug it back in and she runs like a swiss watch again.
Does this sound like a reasonable diagnosis?

The part's inly a few montsh old, so going to try and get a replacement for free. . .

director_steve said:

Thanks for the replies chaps! I'm pretty sure i've narrowed it down to a faulty Air Mass Meter. If she's running rough, i disconnect the AMM for a few seconds, then plug it back in and she runs like a swiss watch again.
Does this sound like a reasonable diagnosis?

Click to expand...

Possibly but because you have a custom ECU and re-wire etc normal suspects hard to advise on. If it is a standard Lucas hotwire air flow meter it should be possible to test it with a multi meter (do a search). If it was a standard ECU I would suggest testing by substitution, the ECU could be the fault.

Also, if it is a standard Lucas air flow meter, have a careful look at the connector plug and make sure all the wires are connecting properly.

HTH

Thanks for the advice- i will do.

S

Surely, all you would need to do to allow running without cats and lambdas on the 14cux is change the Tune resistor to the pre 92 or Saudi spec one, no rewiring or anything that fancy!
I have found the pins on the connector to the AFM can become loose fitting and corroded, might be worth checking them and carefully closing them slightly.
Also might be worth disconnecting battery for a bit to allow ECU to clear fault codes.