deanthomas88

Took my 06 STI in for it's MOT yesterday (a week early!) and it failed on CO Emissions in both tests... the guys suggested a bottle of Cataclean and an Italian tune up. 75 miles later (the car feels more responsive, but could be placebo) but still failed with the same issue.

Allowed range is up to 0.20 and mine is at 0.29 on the fast idle, identical to the test before the Cataclean. Lambda was in range so all good on that front both times.

I did a de-cat on the centre pipe about 8 years ago, so i'm assuming it must be the cat on the downpipe. Do you think i could put the centre pipe back in and that would bring it into range, or is it time to bite the bullet and replace (upgrade...) the downpipe?

Any thoughts/suggestions are welcome! Thanks!

ST-X

If you have a reasonably fresh cat to go in then it would certainly be worth trying it although how effective the second cats are I don't know. Having said that I've just swapped out a ten year old system from my car so I can imagine you may be swearing at some rusty bolts with yours if you go down that route! Is it the standard cat you've got fitted or aftermarket 100/200 CEL type?

deanthomas88

Yep standard from factory down pipe, never been touched. Done 108k miles.

im tempted to try and pick up a second hand one that’s done less than mine and put that on - seems like a bit of a job though. Although the engine was rebuilt not too long ago so bolts shouldn’t be too bad…

Smithys STI

iTrader: (1)

Clear ECU with suitable diagnostic tool even if no fault codes are stored .
I find this works quite a lot of the time so worth a shot.

ALi-B

iTrader: (1)

What's the HC and Lambda like?

Whilst it may pass on these CO is a side effect, the catalyst can only clean up so much, obviously older cats and sports cats struggle, but it can be an indicator something else isn't quite right. For example a weeping headgasket can pollute the cat with silicone rendering it ineffective (why was the engine rebuilt? Head gasket failure?)

High CO usually is not enough air during combustion (rich mixture) or another source of hydrocarbon is being burnt (oil).

So if lambda is dipping a little, say to 0.98 it'll pass MOT on Lambda but Cat could struggle clearing the excess CO. So I'd be checking fueling trims and oxygen sensor switching/response. If you got the Lambda up to 1 the CO will naturally be lower before the cat, so it has less work to do.

Ppm limit is really relaxed for Mot (200ppm iirc). A good engine with good cat will be less than 50ppm. The limit is 200ppm but anything close to 100ppm indicates an issue, again the cat can only clean up so much. Elevated ppm is either oil burning or unburnt fuel. So if you are having to do oil top ups, then that is probably your root issue.

Point I'm making is replacement downpipe cat may not fix it.

Anyhoo, as a quick fix refitting the centre section cat will bring the emissions down, but tbh, it should pass with just the downpipe cat.

deanthomas88

See results below, Lambda seems OK? Spoke to EKS and they suggested the CAT is probably fine and could be something else going on. Currently trying to source a centre cat pipe that i can swap back in for the test.

Fast Idle Test
CO 0.28
HC 15
Lambda 0.994

Second Fast Idle Test
CO 0.33
HC 7
Lambda 0.991

Natural Idle Test
CO 0.29

I also have a Tactrix Open Port ready to log, so if there's anything you think i should log that might help, please let me know!

ALi-B

iTrader: (1)

That is so close. Centre cat will get it through
. If you're looking at live data: Short term and long term fuel trims (sometimes called additive and multiplative).

RobsyUK

iTrader: (2)

Remove the air filter for the test. It'll help but remember to put it back after