Plug gaps, HT leads and lpg....
Discussion
As some of you know I run lpg on my Ro 6 litre vxr and although it drives fine - I've never had any actual problems, I frequently get the check engine light thing. Various attempts have been made to remedy it, so far without success.(NB it only does this on gas never on petrol)
I recently came across this info in the FAQ's of a lpg installers site and I'm wondering if it might be the answer to my issue, I will start by re-gapping th plugs as they suggestand if that doesn't help maybe replace the HT leads, but any comments and thoughts on this would be most welcome. Here's what they say....
'Misfiring', when reported in OBD terms, means something quite diffrerent to a failure to fire. It means that the cylinder the trouble code relates to has fired, but the result of the firing (combustion) was out of parameter. This is why we don't feel a reported misfire when driving. The cylinder involved hasn't failed to fire, although it may be firing out of parameter because the resulting combustion is incorrect.
The number 1 and 2 causes of true misfiring ( in the OBD sense as described above) are spark plugs and coils. This happens because incorrect ignition will often leave some fuel unburnt and the engine management system will see that cylinder as running too rich as a result. If this isn't the case, a reported misfire in OBD terms can also mean that the fuelling of the cylinder is out of parameter which could be due to incorrect amounts of fuel being injected. A cylinder that is running with fuel too rich or too lean is Misfiring.
The path to cure will be a process of elimination, but it is always best to begin with the cheapest and easiest things, the spark plugs.
'Gas/air mix is much harder to ignite than a petrol air mix.
As a result, the voltages reached within the HT components (just before the spark occurs) can become far higher when running on gas as opposed on petrol, typically 12-15 Kv's on petrol and up to 30 - 35 Kv's on gas.
This means that the insulation of HT components is at least twice as likely to break down when running on gas, often resulting in tracking, leakage, misfiring etc.
This is the point where we often hear the statement 'It doesn't misfire/backfire/judder/stall on petrol'.
In the light of the above, it wouldn't.
To help reduce the potential for problems when running on gas, set plug gaps to .030" (.75mm).'
To show this effect in action you need to see what a reduction in plug gaps does on a 'scope.
Too wide = High voltage.
Reduce gap = Reduced voltage.
No one says that reducing the plug gap will dramatically remedy backfiring, but it will certainly reduce the risk.
Another factor often missed is that once HT leads etc. have been exposed to higher than normal voltages for some time they will have had their insulation damaged to the extent that a later reduction in plug gaps cannot stop the leakage. Misfiring and the ensuing backfire will still occur. 'Tracking' or 'Shorting' is, after all, mini lightning. You'd expect something to be damaged by this if only by the inherent erosion. Such damage makes just the place where damp likes to dwell, making things even worse.
If that is the case, only replacement of the damaged components will cure the problem (if the backfiring is really due to ignition faults in the first instance).
I recently came across this info in the FAQ's of a lpg installers site and I'm wondering if it might be the answer to my issue, I will start by re-gapping th plugs as they suggestand if that doesn't help maybe replace the HT leads, but any comments and thoughts on this would be most welcome. Here's what they say....
'Misfiring', when reported in OBD terms, means something quite diffrerent to a failure to fire. It means that the cylinder the trouble code relates to has fired, but the result of the firing (combustion) was out of parameter. This is why we don't feel a reported misfire when driving. The cylinder involved hasn't failed to fire, although it may be firing out of parameter because the resulting combustion is incorrect.
The number 1 and 2 causes of true misfiring ( in the OBD sense as described above) are spark plugs and coils. This happens because incorrect ignition will often leave some fuel unburnt and the engine management system will see that cylinder as running too rich as a result. If this isn't the case, a reported misfire in OBD terms can also mean that the fuelling of the cylinder is out of parameter which could be due to incorrect amounts of fuel being injected. A cylinder that is running with fuel too rich or too lean is Misfiring.
The path to cure will be a process of elimination, but it is always best to begin with the cheapest and easiest things, the spark plugs.
'Gas/air mix is much harder to ignite than a petrol air mix.
As a result, the voltages reached within the HT components (just before the spark occurs) can become far higher when running on gas as opposed on petrol, typically 12-15 Kv's on petrol and up to 30 - 35 Kv's on gas.
This means that the insulation of HT components is at least twice as likely to break down when running on gas, often resulting in tracking, leakage, misfiring etc.
This is the point where we often hear the statement 'It doesn't misfire/backfire/judder/stall on petrol'.
In the light of the above, it wouldn't.
To help reduce the potential for problems when running on gas, set plug gaps to .030" (.75mm).'
To show this effect in action you need to see what a reduction in plug gaps does on a 'scope.
Too wide = High voltage.
Reduce gap = Reduced voltage.
No one says that reducing the plug gap will dramatically remedy backfiring, but it will certainly reduce the risk.
Another factor often missed is that once HT leads etc. have been exposed to higher than normal voltages for some time they will have had their insulation damaged to the extent that a later reduction in plug gaps cannot stop the leakage. Misfiring and the ensuing backfire will still occur. 'Tracking' or 'Shorting' is, after all, mini lightning. You'd expect something to be damaged by this if only by the inherent erosion. Such damage makes just the place where damp likes to dwell, making things even worse.
If that is the case, only replacement of the damaged components will cure the problem (if the backfiring is really due to ignition faults in the first instance).
Well, I tried altering the plug gap as detailed above. Result: MASSIVE IMPROVEMENT. Check Engine light warnings have all dried up. Car drives perfectly on gas or petrol with no issues. It DOES make a HUGE difference. I've now ordered a set or Irridium plugs and Magnecor KV85 leads to make sure I'm getting the zappiest spark possible.
TBH I'm fair dancing a jig over this discovery as the engine check has been driving me nuts. Over the last year I've remapped the car, gas fuel trims have been adjusted a million times all to no avail. Finally regap the plugs and problem solved.
So if you have lpg and have this issue, give it a try!
TBH I'm fair dancing a jig over this discovery as the engine check has been driving me nuts. Over the last year I've remapped the car, gas fuel trims have been adjusted a million times all to no avail. Finally regap the plugs and problem solved.
So if you have lpg and have this issue, give it a try!
I've actually yet to replace the plugs, but have a set of NGK irridiums ITR6F13 on order. I also have the Magnacore leads on order. All I've done is regap the bog standard plugs as specified and I've already had a big improvement. I may have to try a number of combinations until I get the best result, but at last I know I'm barking up the right tree with the ignition components.
I'll report further as my experiments progress.
NB: I'm not suggesting this will fix ALL lpg engine light issues, but on my car it certainly seems to be the culprit and if you have similar issues its worth a try. Certainly workling for me.
I'll report further as my experiments progress.
NB: I'm not suggesting this will fix ALL lpg engine light issues, but on my car it certainly seems to be the culprit and if you have similar issues its worth a try. Certainly workling for me.
wallsjay said:
who did your lpg conversion and would you recomend them ?
Capital Autogas did the work. Would I recomend them....Tricky. The install is neat and they were competitive priece wise. My experience was that they tried extremely hard to fix my check engine light issues. They had the car back on numerous occassions. However, they ultimately failed and never once were ignition components ever even considered as a possible cause of the trouble. Only my own researches and the random stumbling over the article above has set me on the right track.
However, they have since converted several other LS series V8's with varying degrees of success and so have a fair bit of experience with them. Mine was their first.
So in conclusion, they're ok, but you might do better and also worse.... If someone knows an LPGA installer whos a whizz with LS V8's I'd be very pleased to hear about them too.
Very interesting Glynn. Are you still getting the check engine light on lpg, or has it cured it?
This saturday I installed Magnecore 8.5mm HT leads and the car deffo runs smoother. I've still got the original plugs regapped as described. I'm getting the check engine light, but it seems pretty random. Some days I don't get it at all, other days I'll get several check engine signals.(Before messing around with the plugs I got check engine every day).
Regardless of signals the car always runs smoothly.The next step for me will be to install the irridium plugs and see if I get any further improvement...and yes you're right about access to the rear of the right hand bank of cylinders!
This saturday I installed Magnecore 8.5mm HT leads and the car deffo runs smoother. I've still got the original plugs regapped as described. I'm getting the check engine light, but it seems pretty random. Some days I don't get it at all, other days I'll get several check engine signals.(Before messing around with the plugs I got check engine every day).
Regardless of signals the car always runs smoothly.The next step for me will be to install the irridium plugs and see if I get any further improvement...and yes you're right about access to the rear of the right hand bank of cylinders!
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