1.4 TSI ACT Failed Emissions - Help

[Thriftmeister]

Hi
Our 2013 1.4 TSI ACT has just failed its MOT on emissions. There are no warning lights and no fault codes.

VW have reached a dead end (after about an hour and £100) and want to start changing the CAT & lambda sensor despite their diagnostics saying they are both fine. £££££

It drives fine and economy is the same as ever but emissions are way out. Idle CO is 1.69, limit 0.3, Fast idle CO is 1.03, limit 0.2, Lambda up and down, in and out of limits mainly out. HC all OK.

My local chap now has it and so far found nothing either, and tested it on his machine with the same results.

It sailed through every other MOT. The only work done since last year was a new cam belt. VW did suggest it may be a tooth out but wouldn't that throw up codes, run rough and or drink fuel?

Any ideas from any teck people?

[Crasher]

If the Co is 1.69 at idle, the post cat lambda sensor would this up, store a code and turn on the MIL. If the cam belt was a tooth out it would run like **** and again store a code BUT it could be a little out which would cause problems, the CPTA engine has a very complex cam belt system and requires a set of special tools to change, if it was done by the Tipex and pray method you will need to get someone with the correct tool set to re-time it; ask your chap if he has a T10504. It doesn't make sense. Based on what you say I would fit a new pre cat lambda sensor but it still does not add up.

[Thriftmeister]

Thanks for that. I'm not sure I understand how a cam belt can be a little out and not at least a whole tooth on one of the cams. The garage used a proprietary camshaft tool AFAIK as I've seen it in their garage. It was red!!
I will ask them about a T10504. I agree and both garages agree, it does not make sense. No codes and no lights and it runs fine at all speeds and throttle positions.
Could it be contaminated fuel?

[Crasher]

If it is not converting the combusted fuel into Co2, water vapour and little else then the after cat lambda sensor should alert you to this no matter what so even if it was the fuel, the light would come on. I will email you the instructions for the CPTA engine, it is a very fussy setup.

[Crasher]

Whilst stuck in traffic today I was mulling over this and came to the conclusion it simply can’t happen to an unmolested car. The post cat Oxygen sensor is there to monitor and add to what is going on and is FAR more fussy than MOT equipment, this has to be a problem with the systems being used to test it.

[Thriftmeister]

HELP!! Unmolested car? What does that mean?

It's been to 2 MOT stations and both gave the same readings so I can't see it being the testing equipment.

I understand what you're saying about the post CAT sensor and my mechanic said the same. It isn't even trying to lean it off which is what he'd expect it to do and then throw up a fault too.

We've had it since Jan 2015 and it has sailed through every MOT until last Weds. It hasn't had any work outside annual services apart from the Cam belt 2 weeks ago. It's done 45,000 miles.

The exhaust smells lightly of ammonia which shows the CAT is working on the unburnt fuel but not totally successfully. There is a disconnect between what the car is doing and what it thinks it is doing so a sensor isn't sending the correct info and doesn't know it's not.

My mechanic's wife has a CPTA engined car and next Weds he's going to rob some bits to see if he can isolate a fault. He's a star. VW were going to start putting bits on at random on a charge the customer regardless basis.

I'll keep you updated.

[boosted_diesel]

I had just been doing some research when I came across this thread.
There seems to be an awful lot of misinformation within this thread which will unfortunately lead people astray who are trying to solve problems with their own cars.
There’s far too much to respond to it all but let me add this information as I am a qualified professional mechanic and this info will help folks.

Firstly, there are hundreds of faults that can occur without illuminating a warning lamp or setting fault codes. This is often but not always due to sensors creating a signal that’s within the normal range of operation but is in actual fact an incorrect signal/value.

Next the idea that a car fuel injection system and more so the lambda sensors are more technical or complex than an MOT station gas analyser is ridiculous and some what funny. A lambda sensor senses oxygen and nothing else. It cannot detect co/co2 etc. The MOT station gas analyser can detect co/co2/lambda and hc is detected in parts per million.

The most common reason why a garage cannot find a problem or cannot find a fault is because they don’t know what they are doing with diagnostic machines or how to interpret the live data, to put it simply they aren’t trained or experienced enough. Turning sockets and spanner’s is very easy and most can do this with a manual or some training. It takes a lot more brains to understand how it all works and how to interpret the systems sensor outputs.

Co is basically unburned fuel or oil.
In my experience the most common causes of high co emissions are the following;
Bad pre cat o2 sensor.
Ignition system misfires.
Bad pcv valve.
Excessive crank case pressure.
Boost leaks.
Leaky evap purge valve.
MAF sensor fault.
Catalyst deficiency. (Bad cat)
The list does go on, anything that can upset a/f ratio is also suspect.
Another common cause of cars failing the mot on co only is due to MOT testers not heating the engine and cat up enough for the cat to work properly.

When diagnosing a high co condition all the other info from the MOT emissions print out are taken into consideration as all gases are indicators to combustion quality. Other factors are included when making decisions too. For example if co is high and hc is high but within limits then you know you have excessive unburnt fuel or a lack of air. All the clues lead you in the diagnostic one step at a time.

I’m a member of a bunch of forums and have been for years. I don’t mind helping folks out when I have time by giving advice for free. I’ll pop back over here at some point and see if anyones stuck.

Cheers