Hi guys I have just fitted a magnex sports cat with a 3" stainless centre section, since putting it bk together it will sit and idle no problems bus as soon as I put my foot down it stutters and nearly stalls. I have checked hoses for air leaks and also checked to maf, could anybody shed a bit of light on what's happening as I am stumped like I said only the exhaust has been replaced.

Have you checked the o2 sensor and wiring/plug near the clutch slave cylinder ?

as you might have damaged it or have a loose connection when swapping the downpipe over

I can't see any visible damage to the sensor or the harness

Just thought that would be the only thing electrical you would have removed to fit the downpipe .

What about checking the vacuum pipes ?

Does it idle ok ?

If it's a classic have you done a code check by connecting the plugs under the dash ?

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Yes it idles fine I have done the code check and it doesn't have any so reset ecu but still the same , like you say it's the only electrical component I removed really has baffled me