Deface Posted May 13, 2011 Share Posted May 13, 2011 hi there my dad has got a 1998 disco 1 3.9 efi v8 with lpg and has recently had a stainless steel exhaust made for it which means now it has no cats and ever since it has developed the tendency when he releases the throttle when driving it makes a loud bang. It tends to happen more often on lpg than petrol. Is this a tuning issue with unburt fuel going down the exhaust or just something that happens when the cats are removed? any ideas? cheers Quote Link to comment Share on other sites More sharing options...
mickeyw Posted May 13, 2011 Share Posted May 13, 2011 I'm not sure about how the change of exhaust has changed things, but back firing of that degree is usually more to do with the timing being set wrong. Quote Link to comment Share on other sites More sharing options...
ThreeSheds Posted May 14, 2011 Share Posted May 14, 2011 This can sometimes be caused by an air leak into the exhaust (usually the manifold in my experience), but I am also wondering about the removal of the cats, which begs a few questions: Have you told the (petrol) ECU that it has no cats now (tune resistor changed)? Does it still have original lambda sensors connected? Has the gas been set up to take account of the new free-er flowing exhaust? Does the gas system have it's own lambda sensor(s)? Also (of course) this could be coincidental and could be a symptom of something else entirely: Mine was recently exploding in the exhaust on over-run, which is why I looked at this post. ( I have a side exhaust and it always seemed to do it just as I was passing a car with the windows open :0 . Frankly I am surprised that some of them didn't crash with that thing going off in their ear! ) In my case t was a dodgy connector in the ignition circuit meaning that a sudden reversal of torque from on to off power would rock the engine and temporarily cut the ignition to two of the pots (I am on MegaJolt). After the initial rocking of the engine the spark returned and the exhaust ignited the unburned gas/air mix. Hope this helps Roger p.s. Does a 1998 car pass an MoT with no cats? Quote Link to comment Share on other sites More sharing options...
Deface Posted May 14, 2011 Author Share Posted May 14, 2011 Have you told the (petrol) ECU that it has no cats now (tune resistor changed)? Does it still have original lambda sensors connected? Has the gas been set up to take account of the new free-er flowing exhaust? Does the gas system have it's own lambda sensor(s)? we havent touched the ecu or had it remapped at all. original lambda sensors still connected. the gas has also not been touched. the gas has no lambda sensor of it own that we are aware of. if these things are an issue by what means can i change them?, ive been trying to get him to megasquirt it but he says he hasnt got the money for that. Im not sure i will be the same as yours as we have just changed all the plugs, leads, rotor arm and dizzy cap, although the coil has never been changed sincet weve had it, weve had no reason to change it as yet. so is this more than likely going to be a tuning issue? as the exhaust has been checked with no leaks, its got original manifolds and the pipe is the same diameter all that has been altered is that now is has no cats and one silencer. any ideas as to why it does it alot more on lpg rather than petrol? cheers Quote Link to comment Share on other sites More sharing options...
mickeyw Posted May 14, 2011 Share Posted May 14, 2011 LPG and petrol ideally want to run with different timing settings. Some people will set it optimised for one fuel or the other. This means it won't run as well as it should on the other fuel type. Gas needs more advance than petrol because it is a slower burning fuel. If the timing is not advanced enough it can result in the fuel igniting in the exhaust pipe, although in my experience the timing has to be quite a long way out for this to happen. If you have access to a strobe light and know how to use it, I would check the ignition timing. For your late 3.9 engine I think you need to be looking for at least 12 degrees BTDC at tickover when running on gas. Quote Link to comment Share on other sites More sharing options...
webbersdad Posted May 14, 2011 Share Posted May 14, 2011 I note that you've changed the spark plugs. Did you gap them to 0.7mm? If not do so. webbersdad Quote Link to comment Share on other sites More sharing options...
vougese39 Posted May 14, 2011 Share Posted May 14, 2011 the gas should be teed into one of the lambda sensor if you have kept the lambdas then you will not need to swap the tune resister. if it is backfiring on gas then it is overfuel get the emissions reset Quote Link to comment Share on other sites More sharing options...
Range Rover Blues Posted May 16, 2011 Share Posted May 16, 2011 It could be that the free flwoing exhaust just makes more noise that the cats used to absorb, or as said a fuel leak. LPG needs more ignition advance because it takes longer to burn. The EFi doesn't need to know you have removed the cats unless you also remove the lamdas, in which case it will have a fit. The default mode (no TSR fitted) is catalyst model with lambdas and closed loop self-learning. Quote Link to comment Share on other sites More sharing options...
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