MOT fails emissions test on HC only?

[DuncanM]

Hello everyone,

Took the Cerb for an MOT on Saturday and the HC was through the roof at 600 at 3000rpm.

test pass is 200 max.

Does anyone know what could be causing this as the car is running very nicely and the Lambda and O2 emissions are fine?

Car also failed on H/brake despite it being the best I have ever seen it since freeing up/adjusting and new pads etc.

The car is in Rushden/Northants area and I will need a friendly MOT station if I can't resolve this.



Regards

Duncan

[Gazzab]

Have you got cats?

[DuncanM]

Hello,

Yes, cats are on. They are old and maybe not in the best of condition.

We were told the cats are ok because the lambda and CO readings passed?

It's all a bit frustrating tbh.

Duncan

[jasonpmr2]

My understanding is that the cats only benefit the Co2 readings, not HC's.
I would have thought its down to the valve overlap, and aggressive cams?

[Rollcage]

High HC readings are usually down to unburnt fuel exiting the tailpipe, so start with stuff like plugs and leads.

[Gazzab]

would it be better if the car was very warm?

[Rollcage]

Gazzab said:

would it be better if the car was very warm?

Cats only work properly when "lit", ie hot. It can be interesting to watch the emissions of a car on a test - some need holding at quite high revs for some time to get the cats up to temperature, and then the CO falls like a stone!

Incidentally, high HC readings can affect cats - they dont generally like unburnt fuel.

[DuncanM]

So does anyone think new cats might fix this? The cats are old and knackered I think, needed welding up as they were blowing.

Other than that, anyone know a TVR MOT friendly garage around the Rushden/Northants/Bedford area?

Duncan

[julian64]

High HC tend to be unburnt fuel in the exhaust as said earlier. The problem is that with a Cerb, the pops and bangs you hear while holding the car at a high rpm on no load are unburnt fuel going into the exhaust.

So in the MOT try to find the sweet spot where there are no pops and bangs regardless (apoorx) of where the MOT tester says you should hold the revs.

If the MOT tester does it he knows those pops and bangs will fail you on emissions. Annoy him by protesting and he'll probably let you try.

Or possibly get a ECU chip just for the MOT which reduces that right down. ECU can be designed to exclude the pops and bangs although the car will probably not run ,power wise, very well.

[Boosted Cerb]

They can be a pain to get through (not all of them) Julian is right, I sometimes use a bit of card placed under the throttle stop to hold the rev's rather than your foot on the pedal, even the slightest movement and the ecu senses the change and adjusts the fueling eg pops and bangs.

You don't necessarily need a TVR friendly tester but you do need a tester who knows how to work the machine. Lots of settings can be bypassed/assumed within the gas analyser. This isn't a case of "I'll pass it for you nipper don't you worry" but more a tester realising the crudity of the TVR emissions control system and dealing with as such.

As stated already, it could be the CATs as part of their job is to break down HC as well as other stuff. The CATs sit relatively quite far from the engine and therefore do need a good amount of time to get hot(minutes), just keep an eye on the temp and oil pressure.

Its not uncommon to see the exhaust manifolds glowing red hot during the test. Its actually another sign the O2's are working well. A good MOT tester will be able to tell when the CAT's start working as the numbers will drop. You could also try some higher rev's as the cam will become more efficient. The rpm range can be omitted from the CAT test.

In the VOSA book (every MOT station has one) the Cerby does have wider allowable limits for the CAT test but IIRC its only wider on the lambda reading.

Also worth considering the valve clearances/cam time as with these high lift on overlap engines it plays a big part in how much unburnt fuel goes in and out, straight down the exhaust especially at low rpm. This wouldn't necessarily mean a bad running car, but a data log with the software should raise the issue.

And if all that fails remember......the Cerby exhaust's (with sports pipes) are isolated from each other so if it fails on one side you've got a second chance with the other

Rich.

[2mad]

As someone said earlier make sure the car is well warmed up and change the air filter before the test, to lean the fuel mixture.