Hi all,

Got this VW Golf, failed MOT on CO% and HC.

CO% around 0.8
HC around 400
Lambda around 1.015

No faults stored and live data looks good. No exhaust or inlet leaks, had a new cat a few months ago as it was rattling (that was probably an indication of the problem starting then), performed engine service including oil flush, still the same. Customer has mentioned she has had to top the oil up a bit, tried an oil treatment for worn engines, HC's down to about 350 and CO% still around 0.8, have managed to extract from her the amount of oil she has been using, at least 5L in the past year. Performed compression check, all quite high although 1 cylinder was a little lower than the rest. Vehicle has just gone over the 100k miles and drives really nice, for a VW

I'm suspecting worn engine and the oil usage is the problem, but ive had vehicles burning the same amount of oil in a month and still pass the emissions easily.

Any other ideas?

Hi Ian,
Whats 02 sensor doing ?

Regards
Dave

O2 sensor is doing its thing, up and down nice and fast.

Have you tried resetting addaptions

I would suspect the cat even though it's fairly new, the hc's are a bit high though which could indicate slight misfire.

Brian

No i havent reset adaptions, ive not done/fitted anything that requires the adaptions to be reset back to factory.

Anyway, if the ECU has adapted itself to a limit then surely something has caused it to do that and resetting the adaptions will only hide the problem for a short time.

There was nothing to indicate that it was working at the extent of its adaptions.

Why would YOU reset adaptions?

Hi Ian

can you get a pre cat gas test ( difficult if cat is down pipe) would give more to go on.

Cheap cats can give poor reading after short time. Agree HC is high And needs checking out.

Aidan

I would agree with the CAT if it wasnt fairly new, i can normally get even a poor CAT to work if i get it hot enough, and i got this thing glowing, but there is still the HC problem. I would also agree that the HC's indicate a slight misfire, but power balance is good, and the plugs looked ok, and the engine runs smooth.

I will add though, the gas readings i gave are for fast idle, but they were only a little lower on idle.

I'm looking at the high HC's causing the high CO.

This wasnt a cheap CAT, i do sometimes fit cheap CAT's if asked for them and they struggle a year.

Its not a downpipe CAT, but i dont think i can to a pre CAT gas check unless i drill a small hole for the gas probe.

Hi Ian,
If 02 sensor is switching ok, and it's not a "false switch" (system has adapted) then CAT failure.
If system has adapted, then by clearing them, you will "see" a reaction in the 02 sensor.
But you should be able to look at fuel trim figures to get an idea if its a false switch or not.

You can also get an idea of "pre cat" gas figures by checking the emissions before the cat lights up.

Regards
Dave

Hi Ian,had a simular problem once on VW,and turned out cambelt timing was out one tooth,it might have had this problem since then(I presume its been changed)and thats the one thing code reading cant tell us and I've been caught enough times not thinking mechanical problems just a thought.Martin

Hi

As there is some question with cat. I would think worth drilling a 6mm test port, use a bit of 6mm pipe in stead of normal probe make sure probe well in of seal test point. weld hole up after.

Cheers Aidan

Hi Ian

From what I can see from all the info is that the big clue is the slightly high Lambda reading. I would normally expect it to be low if the cat is ok. I have learned from esteemed colleagues on these forums that a slightly high reading indicates a faulty cat. Assume it is a rubbish aftermarket cat. Could try a second hand VW cat and see what happens. Obviously it should go without saying you always take a risk with 2nd hand stuff and could confuse the issue.

Regards

Pete M

Hi all.

Thanks for all your input. As i think we are seeing more of now, the vehicle has been traded in, for a 57 fiesta, she got £1000 on the VW.

I was gonna try a small hole before the CAT and sample the emissions. As for the cam timing, yes ive had that before but usually seen the emissions a long way out of spec, with like you said no electrical faults showing, it did have a cam belt about 2 years ago, i would have checked it though. A second hand dealer CAT isnt possible as it was quite an unusual one which would be why it wasnt available in cheapy form. The lambda was only slightly high but still well within spec, more likely to be the tiny drain hole in the new rear exhaust box it had a while ago.

Blimey, how quickly time can disappear when you're investigating a fault.

My attention was focusing on the oil consumption and the high HC's. The customer was getting a bit fed up with the amount of oil it was getting through anyway, and the rate it was consuming it was increasing too.

Still, we will never know now. On to the next headache.

Thanks again all.