MOT Fail - Emissions - Worse After Fitting Cat

[Erudite]

Hi,

Took my car in for testing running a decat pipe (fitted by previous owner) and it failed but came back with the following readings:

2490 rpm:

CO = 0.48
HC = 49
Lambda = 1.023

2545 rpm:

CO = 0.39
HC = 36
Lambda = 1.024

Notes:

Engine oil temperature measurement by-passed
Engine oil temperature check - cooling fan cut in
- just noticed these comments myself but the mechanic didn't point them out/mention them

I got the garage to get the original cat fitted and the post cat sensor was replaced (the wires had been cut by the previous owner). Then had it put through again for mot (no indication given as to the status of the engine on these readings):

CO = 3.67
HC = 136
CO2 level = 12.48
O2 level = 0.19
Lambda = 0.901

Anyone have any ideas why it's gotten significantly worse after the cat has been fitted?

The guy who fitted said it was because of the primary cat in the manifold (presume pre-cats?) but I explained about the issue with the pre cats and how people here pass no problem without them. Beyond that he didn't know why.

Could it be that the ecu wasn't reset after the cat was replaced or the emissions were tested around start up the second time (it strikes me as slightly odd that there's no engine status on the sole emissions reading that was done the second time)?

I have an ODB reader if any information from that would give any clues as to what may be going on.

[Petrus]

Looks like a cold cat issue.

Take the car out for a blast and make sure the cat is properly hot when tested.

[jonbill]

that's running very rich, the engine might be still warming up.

[Carolyn]

How long was that cat sitting off the car?  It could be full of crap picked up through the open pipes.

If the fitting of the cat has resulted in an upstream leak, that will have an effect.

It might be time for a Cats2U special!  With new OEM crush rings too.

[Erudite]

How long was that cat sitting off the car?  It could be full of crap picked up through the open pipes.

If the fitting of the cat has resulted in an upstream leak, that will have an effect.

It might be time for a Cats2U special!  With new OEM crush rings too.

I would say that it's been off for 10-12 months at least (guessing slightly as the previous owner took it off originally).

Any tips on checking for a leak? Is it best just to have a look round the area of the cat while running?

[Erudite]

Looks like a cold cat issue.

Take the car out for a blast and make sure the cat is properly hot when tested.

Ok thanks, I'll try to get the emissions tested again. As I say, the original emissions reading is a lot more thorough than the second.

[Carolyn]

How long was that cat sitting off the car?  It could be full of crap picked up through the open pipes.

If the fitting of the cat has resulted in an upstream leak, that will have an effect.

It might be time for a Cats2U special!  With new OEM crush rings too.

I would say that it's been off for 10-12 months at least (guessing slightly as the previous owner took it off originally).

Any tips on checking for a leak? Is it best just to have a look round the area of the cat while running?

Tape a strip of tissue to the end of a longish screwdriver, so it can flap like a flag.  Pass it around all the joints.  If there's a leak, it will flap.

[Gaz mr-s]

Running some Cataclean or similar through it might be helpful, but with no current MOT, limits the driving of it.

[Erudite]

Running some Cataclean or similar through it might be helpful, but with no current MOT, limits the driving of it.

The mot expires tomorrow. I'm going to check for any signs of an exhaust leak first thing and will pick up some cataclean and take it for a blast if no signs of a leak. Thanks for the suggestion 👍

[Erudite]

Just to update, I got some cataclean and took it to another testing place today. They did the emissions test for no charge. Flew through no problems.

Took it back to the original place and got them to pass it. Glad to have it back on the road just before the mot expired.

I have now noticed that I think I've got heatshield rattle on the cat. Any tips on this? I was going to try heat resistant silicone (proof to 310*C).

[Gaz mr-s]

I have now noticed that I think I've got heatshield rattle on the cat. Any tips on this? I was going to try heat resistant silicone (proof to 310*C).

Usual fix is a large jubilee clamp around it.

[Ardent]

Option 2.
Separate out the clam shell heat shield from the cat. New nuts n bolts will no doubt be required. The rattle is often a spacer that has come detached and is floating around. Not always, but fairly often.