Failed MOT fast idle CO

[shanra]

Ls 400 1997 failed mot on fast idle test CO max 0.3%,  reading 0.39% and second fast idle reading 0.42%. Took it back having given it an italian tune up and some Wynns Extreme petrol emissions reducer for about 40 miles, cleaned air filter which wasn't really dirty, cleaned Mass air controller with proper cleaner, oil change and filter (Amsoil synthetic 5w30) cleaned pcv valve which was working ok, and it still failed. New iridium plugs 25,000 miles ago at the same time 2 upstream Lambda sensors Denso and good quality ignition wires. Perfect tickover and seems to be running well no misfires etc. Tested voltage on Lambda sensors this morning with multimeter through diagnostic E1 and 0X1 and 0X2 with warm engine both tickover and revving engine and voltage flew up and down as it should. 225,800 on the clock so just run in really! Stuck paper clip in diagnostics port and no codes. Last year it failed on the same thing but an italian tune up and some Wynns catalytic converter and lambda cleaner did the job (0.08% reading for fast idle) although I did run it for more like 90 miles before retest. I am at a loss to what to do next. All other emissions passed.

[J Henderson]

Are you using E10 or E5 fuel? 

[Malc1]

I’ve not had anything like this issue ( yet ) on my 95 Mk3 at 247k miles 

But I use semi synthetic engine oil and had the front-end exhaust section rebuilt a couple of years ago coz that was irreparable after several years of “ fixing “  

Maybe some of this could help ?!?🤔

And I always use E5 ….. usually Tesco or Shell V Power coz they’re 99 octane  

Hope you find the problem soon …… different MOT Tester maybe ! 

Malc 

[shanra]

E5 and the odd tank of Shell power. Oil and filter change every year and never over 6000 miles. It seems to be running as smooth as it could do.

[Malc1]

Might you give her an even longer good run just prior to a fresh MOT at a different Station 

OR MAYBE someone with technical knowledge might see this Post and more rationally suggest something …… 🤔

Malc 

[shanra]

I shall try running it a bit longer and who knows it may be the answer.

[steve2006]

I have 2 suggestions

Engine coolant temperature sensor, if this is faulty it could be telling the ECU the engine is cold even when warmed up so could result in over fuelling increasing CO levels due to unburnt fuel

Engine ECU capacitor problem this can cause numerous running problems but no CEL codes.

[shanra]

Thanks for your reply Steve. I replaced the coolant temperature sensor about two years ago and you replaced my capacitors on my ECU around about that time also and thanks again for that. This is why I am so puzzled by the problem and the car seems to be running so well when warm. It does take a while to warm up and come off 'choke' but I think that's normal for the Ls 400.

[hockeyedwards]

It does grab my attention that she runs well 'when the car is warm' that something is amiss and I would ask how long is a while? I'd say my fast idle drops off quite quickly after start up, has been a tad longer now the temp has dropped but not significant.

Any other 'odd' symptoms that the car has? It seems you've done a lot of fettling in the right areas with no luck. Could one or both of the cats be on their way out/ clogged/ partially clogged? Any smell from the exhaust, sluggish throttle response, dark smokey exhaust colour...

[shanra]

Car is running very well no problems at all. If I start the car and leave it to warm up in this weather it will take over five minutes to come down to a perfect tickover but it's always done that. I will run the car for another 40 odd miles or so to let the additive work and a bit more italian tuning, then take it back for a retest and if it fails again I shall get it tested elsewhere as there may be a problem with their test equipment. Cats maybe on the way out but again no codes and the downstream lambda sensors should have picked that up. I was talking to a mechanic mate of mine and it baffled him and he said maybe their equipment is not working properly. Thanks for the replies always open to another opinion.

[Newbie777]

Sorry I am not a mechanic or technical.

1. Does the smell from the exhaust smell lean?

2.Have you scanned for any  any codes?

I like your approach of elimination?

Also we as the read spot on last year if so what changes have you done in last 12 months?

Just guessing some exhaust temp?

 Good luck mate, hope it gets resolved quickly and without any further pain for your brilliant car.

 

[BigBoomer]

Since it was marginal last year and has failed this year it is almost certainly that the CATs are less efficient to the point where you are getting CO in the exhaust. The catalytic plating on the ceramic matrix in the CAT doesn't last forever. I think you will need new CATs.

[shanra]

It failed last year and I put in Wynns Catalytic and Lambda cleaner in and gave it an italian tune up and it then passed easily. I will give it a good run and take it back once more but unless their exhaust analyser is faulty (unlikely) it is pointing towards your view on the cats. If it is the cats I won't fit original as they are just too expensive even from Amayama. Any ideas on where to buy cats and what make etc?

[Malc1]

Maybe seek the appropriate Cats from a breakers yard ? 

Malc 

[shanra]

4 minutes ago, Malc1 said:

Maybe seek the appropriate Cats from a breakers yard ? 

Malc 

No way of knowing if they are any good and how long they will last though. They usually want silly money for the parts as well. Thanks for the suggestion it may end up I have to do just that.

[shanra]

Looking at the Cats underneath the car they are bolt on at the front and welded at the rear. Can't find any parts company here who seem to stock them, all 90-95 or 97-2000!

[shanra]

11 minutes ago, shanra said:

Looking at the Cats underneath the car they are bolt on at the front and welded at the rear. Can't find any parts company here who seem to stock them, all 90-95 or 97-2000!

Amayama discontinued part. So if getting a new one would have to get a specialist exhaust company to fit or put something together.

[hockeyedwards]

3 hours ago, shanra said:

It failed last year and I put in Wynns Catalytic and Lambda cleaner

Think that tells you what the problem is...

Exhaust wise, better off going to a fabricator and getting them to cut off and reweld new ones in.

Not sure if Rockauto stocks the OEM ones if you were that fussed, but unsure on £££ cost vs. UK exhaust shop.

Hope it gets sorted either way 🙂

[Littorio]

Just reading your post and thought of a company I came across recently but have not used although they have a good repetition. They do Cats and Sport Cats for most cars may be worth a phone call?

https://www.empperformance.com/services/36/Catalytic_Converters.html

[Spock66]

Another option may be refurbishment (no personal experience of this).

http://www.westmerciaradiators.co.uk/catalytic_converters.html

[shanra]

Thanks for all the replies. Rockauto seem to do the ones which bolt on both ends and there is one with an open end but not prepared to risk it being the wrong one at over £600 for a pair.

[VFR]

I would at least have a full tank of BP Ultimate or Shell V-Power in the week leading up to an MOT if I were having any doubts about emissions   

 

[Cs150]

I would try another MOT station, one with half decent reviews. There is a certain amount of leniency with the results. A few years past my old Ford failed one of the tests marginally related to emissions. The station still allowed it to pass but said they could not give me the print off from the emissions machine!

[Tinonline]

I’ve had issues like this in UK and Spain with a few vehicles.

Do use a Cat cleaner on 1/4 tank of fuel and get the engine hot. Use premium fuel. You can use oil anti emissions additive in an oil change. 
Get a high quality plug in check on diagnostics…something is not right…

[Malc1]

Just a Q ……. are the Cats in older Ls’s of a different quality / structure to more recent Ls’s ? 

I ask this coz Testing equipment at the/your MOT Station might be set-up possibly to reflect more modern Cat build structure and be intolerant of older designed Cats and unintentionally misread / give false readings ….. simply being less tolerant  and giving effective “ false” readings 

OR maybe I’m on a red herring path 😰

Malc 

( ignore me if it’s nonsensical )