Search tells me it's time for a new cat, but I can't help but ask - sorryu

[shnazzle]

Trouble is with the new cat it looks so much better with the nappy removed. It's tempting to leave it off and have everything get mucky.

Mine's off. 

Quite like the look of the wrapped cat underneath. 
It's very "race car" ... Even though your average 1.2 Polo is quicker

[thetyrant]

Be careful with unshielded cat, get it wet when its hot and it can crack the matrix inside and i think thats why my cats2u started to struggle and i was getting po420 code, it was ok for a few weeks after fitting then it started throwing the code and had to get it seriously hot to get it through the mot.

I dont run a nappy and cat wasnt wrapped or shielded in anyway, im suspecting when jetwashing it with hot cat ive damaged it and why i had problem/codes, if i was you i would either refit the nappy or make a shield/wrap the cat body and protect it.

[MartinF]

With a bit of fiddling about the heatsheilds are back on the Cats2u cat. I was splashing through water today thinking about exactly the issue you raise so now I'm pleased they're back on

[Tomo70]

Ahhhh...good to know the whole loop business is based around the O2 sensors.

So if the O2 sensors in the manifold read lean then is the only possible leak in the manifold to block gasket?

Also I seem to remember some wise chap (possibly you Shnazzle?) pointing out that the MR2 uses the post-cat sensor as part of it's feedback loop as well, extending the potential location of leaks.

I am thinking I should do an extended fast idle test with Elmscan and look for the average voltage on the 2 pre-cat sensors - I guess if they don't both come out around .45v then I may see the issue.

Leak can also be between cat and manifold, and at a stretch, cat flexis (post-cat sensor fouling).
Remember that the voltages will swing. That's by design for the cat. Reset the ecu let it settle at idle. Warm the engine so as to allow for closed loop operation. Then rev it at 2500 for a bit,see how it responds. Give it a second to respond.

You're looking for lean readings (<0.45) and positive fuel trims at fast idle. I.e. "I'm reading too much oxygen, so I'm adding more fuel over and above what the map tells me".

Pay attention to which bank it's looking skewed on. Could indicate whether it's a leak on one side of the manifold or the other, or if an injector is leaking.

Fingers crossed it's an easy fix. Usually is.

I realise I am late to this problem but having read what Patrick noted thought I would mention problems I have had.
I take it you have a de-cat manifold. These have been known to warp near the head.
Out of interest, if you have put a de-cat which gaskets did you use.
Personally I would suggest using the stainless steel Toyota one on the cylinder head. The ones supplied with the De-cat ones tend to hold moisture and corrode the manifold flange.

[MartinF]

Hi Tomo
Thanks for the thoughts. Actually I am still on the stock manifold, but with the precats knocked out of them. I can't hear an obvious leak but I haven't yet got really stuck in looking for a minor blow.