E51 Blocked cats nissan elgrand e51

Lpgc

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Premium Member
#26
The only way an LPG conversion could cause premature cat failure is if it provides too rich a mixture. If that were the case it'd most likely be due to the installer not calibrating the system properly / could also be due to injectors on the affected cylinder bank dosing different cylinders with different amounts on fuel (LPG), which would most likely be due to a problem with an LPG injector or a crimped gas pipe (if a gas pipe to an injector on a cylinder bank is crimped, that cylinder gets less fuel than the other cylinders, which means the petrol ECU detects an average lean mixture, which it compensates for by pulsing injectors for longer, which means the none affected cylinders run too rich).

> Hmmm - was that article subject to peer review I wonder?
> I note the bit about lack of lubrication from the vapourised LPG - hence the common use of 'flash lube' type additives.
> If my memory serves me well, (often it doesn't :(), Elgrand LPG users on this forum have not experienced such problems on their vehicles (comments please?) - is that because most Elgrands here in the UK are older than 2006?
> What are Simon's (@Lpgc ) views on this I wonder?
Surely not many people will think there's going to be much liquid (petrol) left over after a cylinder has fired... So how is petrol supposed to lubricate the exhaust valve on a modern engine that doesn't feature much valve overlap? If there was enough liquid petrol left over after a cylinder had fired to lube the exhaust valve the vehicle would never pass an HC emssions test, engines with a lot of valve overlap also probably wouldn't pass an HC emissions test.

Sure petrol in liquid form can lube/cool the inlet valve on it's way into the cylinder, but after the cylinder has fired all that will remain is hot gasses and, crucially, a bit of soot. The lube fluid lubes the inlet valve on it's way in, burns to form a bit of soot, the soot protects both the inlet and exhaust valve.

All engines wear regardless of what fuel you run on. For sure lubes are needed on some engines, Fords and Japs in-particular. Since the mid 2000's some other manufacturer's engines also need lube systems but it's far from it being the case that all engines produced since the mid 2000's need a lube. Hydraulic valves are generally a positive being self adjusting, arguments for or against hydraulic valves don't really tie in with the subject of LPG unless the engine did need a lube fitting but didn't have one fitted, in which case we might expect faster VSR, in which case hydraulic valves would be even more of a positive because at least they'd self compensate and prevent the valves being burned out even quicker due to blow-by.
 
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Alan Morgan

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E51 Owner
#27
Thanks Simon @Lpgc - it's nice to hear another opinion, especially against opinions given on the internet without any obvious authority.

When looking to go on a cycle ride (I'm a fair weather cyclist), I search the weather forecasts until I find one that I like and then go for it (if you get my drift:grinning:).
 

stevemen

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E51 Owner
#29
Yep its on lpg. But doubt very much that this is anything to do with the lpg as the lpg was only fitted a few months back and looking at the state of disintegration of the front cat this issue has accumulated over a long time.
Just got the paper in for said motor and its only done a couple of thousand miles since lpg was fitted by autogas a few months back.
As i said almost certainly nothing whatsoever to do with the lpg.
Seen this syndrome loads of times now and yes this may of been the worst case iv seen as front cat has totally disintergrated but have a feeling that the situation was made worse by the owner "force" driving it when it was blocked and down on power.
If you feel the need to attach blame id attach it to nissan and all the other car manufacturers who adopt the 4 cat system in order to apease all the greenies. Shoving the front cat nearer to the engine to make it more efficient is much like stepping in the ring with sugar ray and shoving your chin out.
 

stevemen

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E51 Owner
#30
Just to add whilst looking about yesterday throu the pile of E51 exhausts we have we have noticed that quite a few have had the front cats removed.
The give away sign that front cats have been removed are that the outershieds which are originally spot welded together have been removed and gas welded back together on both ends of the join. They have all been very professionally done and all used exactly the same method , which leads us to believe they where done by nissan in japan??? I wonder.....well the would never publicly admit it would they???
Before removing your rear cats id look for the telltale welding on the front cat heat shields and then check with a digital thermometer as it may be that your front cats have already been removed in japan?
Steve
Tojomotorsuk
 

Alan Morgan

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Premium Member
E51 Owner
#31
Just to add whilst looking about yesterday throu the pile of E51 exhausts we have we have noticed that quite a few have had the front cats removed.
The give away sign that front cats have been removed are that the outershieds which are originally spot welded together have been removed and gas welded back together on both ends of the join. They have all been very professionally done and all used exactly the same method , which leads us to believe they where done by nissan in japan??? I wonder.....well the would never publicly admit it would they???
Before removing your rear cats id look for the telltale welding on the front cat heat shields and then check with a digital thermometer as it may be that your front cats have already been removed in japan?
Steve
Tojomotorsuk
That's a very interesting observation about the front cat removal. To help those of us who wouldn't recognise the difference in weld types, I don't suppose you could post a picture of both sorts (using actual E51 front cats) could you? This would certainly make a visual inspection by the uninitiated that much easier!
 

Lpgc

Well-Known Member
Premium Member
#33
It does seem very likely but is it definite that if the welds are as pictured the front cats will have been removed?
Also, is the rear cat problem always caused by break up of the front cats?
 

Karl

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E50 Owner
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E50 Expert
#34
It does seem very likely but is it definite that if the welds are as pictured the front cats will have been removed?
Also, is the rear cat problem always caused by break up of the front cats?
Yes. The ONLY issue with the rear cats is that the fronts start to disintegrate over time and the debris gets caught in and blocks the rear cat. Hence de-coring them removes the restriction and the debris is just blown out the back.

I did have something else to say, but my mind has gone blank, so I'm going to sit in the corner and have a little think.
 

malcolmyz85

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E50 Owner
Premium Member
#36
So would decoring the front cats be a better solution (though I'm assuming more difficult)
It would be a gd system to only have one cat on it not two ,but if u decore front it alot more work ,and u also have mess on with sensors packing them ,only prob with decoring rear cats ,is when the front cats break up and fly out your exhaust u have no cats at all on your exhaust system
 
#37
It would be a gd system to only have one cat on it not two ,but if u decore front it alot more work ,and u also have mess on with sensors packing them ,only prob with decoring rear cats ,is when the front cats break up and fly out your exhaust u have no cats at all on your exhaust system
That's my only concern about the usual dear coring procedure is that in the end you end up with no cats at all and maybe an MOT failure, and I can't imagine finding replacement cats will be easy.

Would having only the rear cats in place be enough to pass emissions tests do we know?
 

Karl

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E50 Expert
#38
No. The rear cats are for COLD START UP EMISSIONS ONLY. If you remove the front, and leave the back, not only are you going to throw a fault code due to O2 sensors (which are located on the front Cats), you will also fail an emissions test come MOT time.

The ONLY reason we can get away with removing the rear cats on these in the UK is because we do not test cold start up emissions. All emissions testing is done when the engine is at running temperature.

Obviously we remove the rear cats to allow the debris from the front cats to flow out of the back of the exhaust, and prevent blockage. The next problem is going to be... what happens when the fronts have finished disintegrating and clearing themselves out. You will effectively had a cat free system, which in turn means failed MOT. At this point, you need to replace the front cats. You can either go OEM, or get a new system made up by an exhaust manufacturer.

At this point, it would be easy enough to bring the downpipes together, and merge them BEFORE the first cat... then only have 1 cat in the system. This will save the expense of having to buy 2 cats.

Or, just get LPG, as the emissions are a lot lower, and you don't need any cats.
 
#39
No. The rear cats are for COLD START UP EMISSIONS ONLY. If you remove the front, and leave the back, not only are you going to throw a fault code due to O2 sensors (which are located on the front Cats), you will also fail an emissions test come MOT time.

The ONLY reason we can get away with removing the rear cats on these in the UK is because we do not test cold start up emissions. All emissions testing is done when the engine is at running temperature.

Obviously we remove the rear cats to allow the debris from the front cats to flow out of the back of the exhaust, and prevent blockage. The next problem is going to be... what happens when the fronts have finished disintegrating and clearing themselves out. You will effectively had a cat free system, which in turn means failed MOT. At this point, you need to replace the front cats. You can either go OEM, or get a new system made up by an exhaust manufacturer.

At this point, it would be easy enough to bring the downpipes together, and merge them BEFORE the first cat... then only have 1 cat in the system. This will save the expense of having to buy 2 cats.

Or, just get LPG, as the emissions are a lot lower, and you don't need any cats.
Thank you, that's the information I was after. Looking at it long term I was concerned about what to do when the front cats give up the ghost, I tend to buy a car I like and keep it until it's no longer what I need or is ready for the scrappy.
 

malcolmyz85

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E50 Owner
Premium Member
#40
Karl is right ,I have a e50 with a 3.5 which has only one set of cats on it from factory,front ones but mine melted cats so decored them ,so my system is cat free ,car is on lpg ,it's passed mot no problem on lpg ,I have read of people saying they e51 has passed mot while been cat free but haven't seen proof ,u must be same as me about ,I always keep my main car for a long long time