Fitted a new cat and toyosport manifold at the end of last season. Cleaned the MAF and put in a new air filter. A few miles(200 ish) now issues. Just recently couple of backfires. P420 shows up.
I have changed the plugs. Old ones were a nice grey. Doesn't suggest running rich.
Took it round my local garage. To test immersion. @2k revs all ok. @4k revs looks like overfueling. My sense of smell is non existent but the garage owner said the exhaust smelt a little of petrol fumes.
I could put extenders on the upstream O2 sensors.
Any other suggestions?
Exhaust leaks? Crush and donut gasket gas tight?
Oem or toyo gasket at manifold?
Quote from: Ardent on August 7, 2026, 21:11Exhaust leaks? Crush and donut gasket gas tight?
Oem or toyo gasket at manifold?
Manifold gasket OE style steel ditched the toyo one.
Did you use OEM crush rings between the manifiold and cat?
Did you use any exhaust sealing paste?
Is it the original cat?
I suspect the cat is the problem...
Quote from: Carolyn on August 8, 2026, 08:32Did you use OEM crush rings between the manifiold and cat?yes
Did you use any exhaust sealing paste? Rhys did it so I would think so
Is it the original cat? CAT2u
I suspect the cat is the problem...did the emissions check at the garage co2 is ok
If no leaks, I'd suspect the maf.
Exhaust paste reaslly shouldn't be used upstream of the cat. But, as it passed emissions, Iit's not the cat.
I'm with Gaz on the MAF as a possibility. Maf problems don't throw a 'MAF code' but manifest in other codes.
From what I understand it's not uncommon for after market cats to throw P420. I fitted a new one and although passing emissions for the MOT I still get one every now and again, as I believe others have reported.
I occasionally get a smell of petrol fumes on start up and have been through just about everything and wonder that's just a fact that these cars are over fuelled on cold start.
If there is a leak I would expect it to be at the manifold to block interface so the upstream sensors would see too much O2 and add more fuel.
3 amigos would not effect fueling I believe.
Nope, the cat joint would do so. The manifold with a steel gasket very rarely gives problems.
Do I recall if you give a squib of egr or electrical contact cleaner around the manifold joint, if the revs pick up, you have a leak.
Quote from: Ardent on August 8, 2026, 20:14Do I recall if you give a squib of egr or electrical contact cleaner around the manifold joint, if the revs pick up, you have a leak.
No......the intake.
Trying to think what car i tried this on.
(https://i.ibb.co/Pv7DVW0G/20260810-100545.jpg) (https://ibb.co/Pv7DVW0G)
Took this from bank 1 sensor 2. First part of graph is with a 3k rpm. Second part at idle.
(https://i.ibb.co/JjL3cDYB/20260810-143924.jpg) (https://ibb.co/JjL3cDYB)
This is just at idle.
Looks wrong.
I'm not expert when it comes to sensor traces, but I do know that the post cat barely moves in comparison the manifold ones vary much more.
I'd think your fuel trims would be more useful.
@Carolyn @shnazzle
Agreed.
Was that open loop or closed loop?
Quote from: Gaz mr-s on August 10, 2026, 20:10I'm not expert when it comes to sensor traces, but I do know that the post cat barely moves in comparison the manifold ones vary much more.
I'd think your fuel trims would be more useful. @Carolyn @shnazzle
Trace is from the manifold O2 not the post cat one.
Quote from: Newbie57 on August 10, 2026, 20:35Trace is from the manifold O2 not the post cat one.
Bank 1 sensor2 is post-cat. Manifold r/h is 1/1, l/hand Bank 2/1
Quote from: Gaz mr-s on August 10, 2026, 21:04Bank 1 sensor2 is post-cat. Manifold r/h is 1/1, l/hand Bank 2/1
Thanks.
So, cat looks ok at idle but can't handle anything more
Quote from: Newbie57 on August 10, 2026, 22:14Thanks.
So, cat looks ok at idle but can't handle anything more
Wait until either of the others I notified state what they think is best way forward.
Just jotting down a few potentially helpful notes;
- car is in closed loop at (warm!) idle. So you'll see sensor activity at idle
- car is in open loop at higher loads (narrowband O2 sensors aren't fast/precise enough at those engine speeds)
- post-cat sensor is only there for micro-adjustments for ULEV compliance. Mr-s doesn't even have a post-cat. As per what Gaz said; it should barely move on trace.
- if manifold sensors say all is ok (sensors, not trims) but post-cat does not then you have a leak in the exhaust post-manifold. Jason's (Ardent) test using a spray would alter oxygen levels so if you spray that near the exhaust joints (my bet is crush gaskets).
I'll revisit tomorrow after sleep
Quote from: shnazzle on August 11, 2026, 00:14Jason's (Ardent) test using a spray would alter oxygen levels so if you spray that near the exhaust joints (my bet is crush gaskets).
He can't feel any leak. Is a combustible spray not just for the intake?
Quote from: Gaz mr-s on August 11, 2026, 07:43He can't feel any leak. Is a combustible spray not just for the intake?
Leaks can be tiny and o2 sensors are sensitive. It doesn't have to be combustible. Basically anything that dilutes (or enhances) oxygen in the air. Nitrogen works too :) If you have that handy.
Basically anything "the kids" huff to get high (as it depletes oxygen to the brain) will work.
I had the same issue once. The cause was me being rough with mounting the cat to the mani and one of the crush gaskets shifting slightly and one "corner" becoming flattened and letting in oxygen from the open air
@shnazzle .......ok, thanks.
Quote from: shnazzle on August 11, 2026, 08:09Leaks can be tiny and o2 sensors are sensitive. It doesn't have to be combustible. Basically anything that dilutes (or enhances) oxygen in the air. Nitrogen works too :) If you have that handy.
Basically anything "the kids" huff to get high (as it depletes oxygen to the brain) will work.
I had the same issue once. The cause was me being rough with mounting the cat to the mani and one of the crush gaskets shifting slightly and one "corner" becoming flattened and letting in oxygen from the open air
How about brake cleaner or MAF cleaner?
Quote from: Newbie57 on August 11, 2026, 10:28How about brake cleaner or MAF cleaner?
Yep, I'd think so. That's the sort of stuff used on intake testing anyway.
Quote from: Gaz mr-s on August 11, 2026, 10:36Yep, I'd think so. That's the sort of stuff used on intake testing anyway.
Just make sure you keep hte brake cleaner away from rubber lines and sparky bits :) It's super flammable
What am I expecting to happen?
Change in idle or a different graph output?
Quote from: Newbie57 on August 16, 2026, 08:13What am I expecting to happen?
Change in idle or a different graph output?
A very fair question. Apologies for leaving that out!
Yes, you should see an immediate response on the B1S2 reading toward rich. Entirely possible even that you'll notice the RPMs change.
Quote from: shnazzle on August 16, 2026, 08:58A very fair question. Apologies for leaving that out!
Yes, you should see an immediate response on the B1S2 reading toward rich. Entirely possible even that you'll notice the RPMs change.
So tried this. No response that I could see from B1S2. Post cat sensor.No change in idle revs.
Should I be doing this at a higher rpm?
I've previously suggested it could be the maf. You mentioned you cleaned it, can you clarify you know it's the component at the top inside it, rather than the visible blob?
If you take the maf out again check that the o ring is intact.
If no issue there maybe go to Rhys & see if he'll lend you a maf.
Quote from: Newbie57 on August 18, 2026, 20:17So tried this. No response that I could see from B1S2. Post cat sensor.No change in idle revs.
Should I be doing this at a higher rpm?
Nope it would appear there's no leak then. That's good news.
See gaz's post. MAF is another good culprit.
It's a bit special because the P0420 code suggests a problem with the O2 sensor, but the O2 sensor could be downstream from the actual cause.
Much like how a bad foot can lead to back complaints.
It makes the p0420 a bit tricky.
Especially, as Carolyn said, the MAF doesn't throw codes. Which is stupid in my opinion....
(https://i.ibb.co/5XcktRsR/20260820-103225.jpg) (https://ibb.co/5XcktRsR)
For completeness these are the old plugs I changed out
They look in good nic to me.
As in engine happy.
Where I am now
Released the maf. No difference
At idle B2S1 voltage reads 0.01V to 0.04V
From what I read this suggest a very lean condition. Maybe exhaust leak, although I can't find it.
Once I lift the revs to 2k B1S2 follows B1S1 pattern voltage which suggests bad cat. Cat is only 1 year old and done very little milage. Still under warrenty from Cat2u.
Will see if I can borrow a good cat from somewhere.
What do you mean by "released the MAf"?
I assume it's meant to be 'replaced'. What with, new or a known used one?
I would have replaced the post-cat sensor first.
Also, stupid question but...the wiring is definitely put back as it was?
Quote from: shnazzle on August 21, 2026, 19:54I would have replaced the post-cat sensor first.
Also, stupid question but...the wiring is definitely put back as it was?
Post cat sensor is a quite new denso one. Looking at the traces from all the sensors I would say they are working.
Wiring is ok.
Quote from: Carolyn on August 21, 2026, 18:56What do you mean by "released the MAf"?
typo cleaned it again
You've cleaned it, not changed it?
Quote from: Newbie57 on August 22, 2026, 09:14typo cleaned it again
I think it's time for a new one. It's the only way to know for sure. Get a Denso.
Quote from: Carolyn on August 22, 2026, 11:40I think it's time for a new one. It's the only way to know for sure. Get a Denso.
Or meet up with another owner with known good and swap
I've borrowed a maf. Also saw my airbox lid had popped open. I go for a test drive see what's happening
No real change unfortunately.
B1S2 readings are good for cat efficiency until about 3.5k revs then it mimics B1S1. It would probably pass an mot.
On the garage exhaust emmisson test the cat is not working at this point 3.5k rpm
I presume the maf lender is confident that it's good?
My partner has a 2zz-comp engine with a 200 cell cat. It will fail an mot test, with some (at least) testers saying 'you need a new cat' if Cataclean isn't added before the testing.
Putting it in for test this year she forgot to buy Cataclean. She then did so & did only 20 miles before re-test & it sailed through.
It's not cheap, but if you run the tank down to almost low-fuel light, then put half a bottle in, that'll probably do it.
Quote from: Gaz mr-s on August 24, 2026, 15:41I presume the maf lender is confident that it's good? Cars been off the road for a while but was ok before that.
My partner has a 2zz-comp engine with a 200 cell cat. It will fail an mot test, with some (at least) testers saying 'you need a new cat' if Cataclean isn't added before the testing.
Putting it in for test this year she forgot to buy Cataclean. She then did so & did only 20 miles before re-test & it sailed through.
It's not cheap, but if you run the tank down to almost low-fuel light, then put half a bottle in, that'll probably do it.
The cat was fitted at the end of last season, CAT2u homologated one. Done very few miles.
Quote from: Newbie57 on August 24, 2026, 20:55The cat was fitted at the end of last season, CAT2u homologated one. Done very few miles.
I know, I've been paying attention, but if there's some doubt that it's not producing the emissions the ECU wants to see, a clean by such a cleaner might change the result. If it doesn't does that mean the cat isn't up to it.....?
Carolyn's better qualified to answer that one.
Like Gaz, I had come down to 'it must be the cat'.
It's possible that the cat is defective. Cats2U are usually pretty good, but any mnufacturer can make the occasional dud!
I'd contact Cats2U and explain that you've ruled out other possibilities and see what they say.
I have contacted cats2u. I need to get a credit from my local garage. They will submit it to there supplier
Took some reading off the MAF
Air flow at idle 2.2 to 3 g/s
At 2.5k revs unloaded 8.3 g/s
More investigation tonight.
(https://i.ibb.co/GDSzzzn/20260825-193421.jpg) (https://ibb.co/GDSzzzn)
B2s1 shows consistent high reading a 4k revs. Not cycling as before
Long term fuel trim 10%
Short term at idle -2.7% & -4,3% at idle
Max min voltage on b1s1 b1s2 show fail on diagnostics
That looks ok, based on my vague memories.
But...just going back through the thread..I noticed something..
Would you be able to post your short and long term fuel trims for the 3k-4k range?
Quote from: Newbie57 on August 25, 2026, 20:13More investigation tonight.
(https://i.ibb.co/GDSzzzn/20260825-193421.jpg) (https://ibb.co/GDSzzzn)
B2s1 shows consistent high reading a 4k revs. Not cycling as before
Long term fuel trim 10%
Short term at idle -2.7% & -4,3% at idle
Max min voltage on b1s1 b1s2 show fail on diagnostics
Is that 10% positive ? Or negative
Basically I'm starting to suspect injector or something upstream and the cat is just struggling to fight the overfueling.
If it's constantly trimming fuel...then there's your answer. It IS rich. But why...?
Probably injector
Quote from: shnazzle on August 25, 2026, 20:37Is that 10% positive ? Or negative
Basically I'm starting to suspect injector or something upstream and the cat is just struggling to fight the overfueling.
If it's constantly trimming fuel...then there's your answer. It IS rich. But why...?
Probably injector
10% is positive
Quote from: Newbie57 on August 25, 2026, 21:1710% is positive
Damn.
That's that theory out the window.
Hmm...
Ok next theory. Air leak near throttle body.
Unmetered air causing lean condition, so it's dumping in more fuel.
Why is the trim negative at idle you ask? Well, at idle, vacuum is highest, and what you can see is that that vacuum sucks the leak shut. Now because your overall trim is positive, it has to trim fuel down at idle when the leak is shut.
So... Same test as before with the exhaust, spray EGR cleaner or MAF cleaner by the throttle body and the joins by the intake
I have little to contribute to these threads but love reading them. The ROC collective working to solve a problem they are miles away from.
I mentioned intake as an aside earlier on......I think in my mind I was believing it had already been done..... ???
Pull the sprk plugs nd see if one is obviously running rich. If one is, that isolates the issue to an injector.
Quote from: Carolyn on August 26, 2026, 08:05Pull the sprk plugs nd see if one is obviously running rich. If one is, that isolates the issue to an injector.
Which the OP already did earlier...all sparks looked fine.
So I guess back to Gaz:s intake leak
Quote from: shnazzle on August 25, 2026, 21:25Damn.
That's that theory out the window.
Hmm...
Ok next theory. Air leak near throttle body.
Unmetered air causing lean condition, so it's dumping in more fuel.
Why is the trim negative at idle you ask? Well, at idle, vacuum is highest, and what you can see is that that vacuum sucks the leak shut. Now because your overall trim is positive, it has to trim fuel down at idle when the leak is shut.
So... Same test as before with the exhaust, spray EGR cleaner or MAF cleaner by the throttle body and the joins by the intake
From memory I think short term at 4k revs was +14%
@ Newbie57
Did you fix it?
Quote from: Gaz mr-s on September 20, 2026, 12:16Newbie57
Did you fix it?
putting some miles on ok so far. We will see. Thanks for all the help.
Quote from: Newbie57 on October 4, 2026, 22:13putting some miles on ok so far. We will see. Thanks for all the help.
What was the fix. ?
Not sure it's actually fixed.
I tightened the 3 amigos.
The airbox had popped open. Sothat was put back together.
Done 100 miles or so with no pending codes