Desperately seeking help with diagnosing over fueling.

Desperately seeking some advice here. I have a 197, with a over fueling issue, that I am really struggling to find a solution to, and is causing MOT fail, with lamdas reading 0.95-1.36-1.028.

The car has new cat, new bosch lamdas, new NGK plugs, new pcv. Has cambelt done last year, newish NGK coils, fresh oil, water and fuel pumps changed last year. It is running a milltek cat back, with a AS homologated CAT. MAP is a year old, two O-rings and tie wrapped in place. New thermostat last summer, always fresh oil.

When I brought this car, it had no cat. I fitted one, and the milltek for MOT and it just passed, last time. This time, I brought a new cat, the same as before and no changes to the readings.

I'm wondering, if the car has been mapped for no cat, group N exhaust prior to me, whether this would be causing these issues, and would be why we see no changes when swapping out these parts.

Live readings go 0.9 0.9 on WOT, jump around from 0.7 - 0.9 and down.

I collected it from a motorsport garage were I had the exhaust checked for leaks, vacuum leak checked, new plugs, and lamdas fitted. Could find nothing wrong, mechanically perfect, no misfires and runs really strong, no hickups, flatspots pulls very hard.

When I collected it, had the OB2 connected, ran through the emission cycle and all green. Next start, when I took it to the MOT, reports cat issue (which is brand new, under 500M)

I'm lost with what next steps to take, and having a real issue diagnosing this, any help or knowledge really appreciated.

RSRowe said:

What exhaust manifold does the car have?

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Manifold is standard, asked the last garage to check the exhaust, they build customs and said it is OK (but i do not know the internals of the manifold cat, the 2nd is new)

RSRowe said:

I feel like the manifold cat would solve your issues for the least amount of money.

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Thanks, I think you might be right, but do you think that would account for the high lamdas? same readings, on the previous lamdas, i changed them both. Previous, and new both bosch, but no changes to the lamdas on the road, live readings more or less the same. Same with the cat swap (2nd).

The car had no 2nd cat when I got it couple of years ago or so. No matter what, the lamdas have always more or less, read the same, with, without 2nd cat, and all new parts mostly.

VeiRoNv2 said:

The first lambda is before the first cat isnt it? then i dont see how having cats or not is going to make a difference to the air fuel ratio.

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Yes it is before the first cat in the manifold,

VeiRoNv2 said:

The first lambda is before the first cat isnt it? then i dont see how having cats or not is going to make a difference to the air fuel ratio.

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This is the part I am confused by, the fuel ratio's don't appear to change with any cat changes, or anything else for that matter.

Not blindly changing parts, Renault reported the cat as gone, which I replaced. Rest is service items getting it were I want and lambdas to be sure. The car had belts done, fuel pump, thermostat, etc at Renault. So presume this is all correct, and pointing out everything is in good condition, serviced, etc,

VeiRoNv2 said:

Because you need to look somewhere else,I feel like going to somebody like Engine dynamics or EFI where they can see exactly what's going on in the parameters it's your best bet instead of blindly keep replacing parts.

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Wish I could, I am English but over in Spain and this is not easy in another language

Air filter is a KN, clean. Same figures more or less with a Pipercross, checked the inlet and connections, sealed them.