Hello everyone.

My clio (1994 1.4) failed its MOT earlier.

Failed on backbox corroded and blowing, which now that I’ve been out and gone under for a look myself, is fair enough, it does have a hole in it.

However it also failed on “emissions not tested (due to engine overheating)”

That’s odd I thought, I’ve NEVER had even an inkling of a problem with this overheating.

I’ve had it sitting outside here idling for about 45 minutes and it’s happily sitting there with the fan cutting in and out, about ~1 minute off then ~2 minutes on.

If you prop the throttle open to hold it at >2000rpm, then yes you can make the needle creep VERY slowly higher and higher. i.e one of the smallest gauge divisions in about 5 minutes. To make it ‘overheat’ you’d need to hold it at >2000rpm for maybe 20mins+. Why would you need to do that on an MOT?

Let it drop back to natural idle and leave it and the needle creeps back down again until eventually it’s cool enough for the fan to switch off.

That’s exactly what you’d expect it to do isn’t it?

So can anyone tell me what the problem is? The guy on the desk at the place that did the test didn’t know the reason that the tester was saying it was overheating.

Was going to fail anyhow so the guy probably just made up an excuse not to do it so he could get home earlier

What funkymonkey said.

Chances are it would of failed the emission test due to the hole, so he just used an excuse to avoid testing it.

Repair or replace the backbox and wack it through again. Do they do a free retest? Some garages do. Its not overheating, although cars generally warm up more if stationary.

Most of the emissions tests should be carried out at 2500rpm (or half max if lower) for a period of 30 seconds. The engine may be preconditioned, ie warmed up at higher rpm for 3 to 5 minutes.

Some garages I’ve used have run the tests repeatedly (longer or at high rpm before commencing) to see if the car “will” pass when the readings have been marginal.

A rear silencer with o hole in it will invariably fail an emissons test on Lambda values. If the gas levels do not met the initial 30 second BET test then a full test must be carried out. A full test can mean that the engine will need to be run at a ‘fast idle speed’, the speed of which will depend on vehicle and engine type but would normally be between 3000 and 3500 RPM, for between six and ten minutes.
A vehicle, functioning correctly, should be able to regulate it’s engine temperature regardless of RPM even without being driven.
However, you should just ask to speak to the tester. Find out if the is an over-heating issue or the emissions failure was added to avoid the prolonged emissions test on a vehicle that was clearly going to fail.
To add the failure for this reason isn’t something VOSA would approve of so tread carefully if you want a straight answer.
A ’94 Clio only failing on a rear silencer leak is a very good result!

The MOT tester doesn’t want to blow your head gasket during an emissions test.

I’d see it as a chance to get the cooling system – water pump or rad – sorted before it becomes a problem in a traffic jam on a hot day.

The temp needle moving above “middle” or 90 degrees celcius in any car built in the last 20-30 years means that something is up with the cooling system.

Thanks for the replies chaps. Sounds like they were just saving themselves some time in light of a known failure. I’ll just change the back box and take it back for a retest.

one thing you dont want to do is challenge the guy. take him ginger biscuits if you want but dont face up and tell him how to do his job

Maybe in the bad old days MOT testers would give up on a car with a fialure, but not now.

They need to do the full list, because if they retest after failure, they just do that one item and issue a certificate. The reasons for failure are on the MOT database from the fail sheet, too, so they are not done lightly.

You need to ask the tester what the issue was, not a bunch of insomniac bike obsessives.

some great answers in this thread lol

the reason he did not test it is becuase it would have passed not failed lol.

mixing clean air in with the exhuast gases will dilute them meaning a car with high hydrocarbons and co will pass a test they should fail.

sbd16v – Nice work on posting a mocking about others and then talking utter rubbish!

“mixing clean air with exhaust gases will dilute them”?!

An exhuast leak will increase the oxygen content of the emissions being tested and will cause a FAILURE due to a lambda level outside of tolerated limits.

“lol”

milky

sorry but your wrong, can you explain what a lamba sensor measure’s ??

have you had anything to do with tuning cars, mapping engines etc ??

infact i cannot be bothered to get into this argument on here, i have years of experiance building, specing and maintaing mapping a 400bhp+ turbo engine, that was in my trackday car and know the effects of what a hole in the exuast will cause to the lamba readings when mapping a car.

but just for you here is the mot guide lines

http://www.motuk.co.uk/manual_730.htm

that state (section 5) A holed exhaust can allow air to be sucked in, causing artificially low readings.

low readings are what you want for a pass, so as i said above they fail cars with holes in the exhaust because they could pass the emmisions test when they should fail

Nice try sausageface but I have been in the motor trade for the last 14 years and an MOT tester for the last 8.

I’m very doubtful of your claimed experience if you don’t know what lambda is!

The link you posted is not to the VOSA vehicle testing guidelines which are the only rules that apply to an MOT test.

Milky is right.

A holed exhaust is likely to throw the lambda reading outside the emission limits.
Only cars pre-catalyst where lambda isn’t checked, would benefit from additional air being drawn into the exhaust.

Although it would be theoretically possible to have additional air maintain lambda within emmission limits, chances of managing it are slim. You’d need the pre-cat lambda to be out by enough to compensate for the additional air being injected, however you’re then likely to fail the test on CO/HC limits, due to the cat not working within ideal limits.

milky

seeing as i had 2 lamba sensors fitted this day

so i think i do know what they do.

just out of interest

An exhuast leak will increase the oxygen content of the emissions being tested and will cause a FAILURE due to a lambda level outside of tolerated limits.

what are the tollerated limits for lambda level ??

milky

your right the above link was not the vosa vehical testing guidelines the only rules that apply to an mot test

so just for you here is the vosa mot inspection manual (which being an mot tester im sure your very familier with)

http://www.motinfo.gov.uk/htdocs/m3i00000001.htm

section 7.3.5

http://www.motinfo.gov.uk/htdocs/m4s07000303.htm

” Holed exhaust: A holed exhaust can allow air to be sucked in, causing artificially low readings”

we can go around in circles for ages but i can be bothered

the fact is i was not wrong when i said it could cause a car that would fail due to high emissions to pass as it could do, you were also not wrong when you said that it could fail due to a low lambda reading so im not really sure where this is going, ive just read my first post and i ment to said could have passed not would have passed, if the emissions were only a little outside limits a small leak could cause a pass.

seeing as i had 2 lamba sensors fitted this day

It’ll take more than two lambda sensors to make a 20 year old Astra (or any Astra really) an interesting car….

Ha ha ha!

You mad bro?

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