VW Polo 1.2 2002 - Polo failed MOT on high CO

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Wed 7 Sep 2011 23:08
VW Polo 1.2 2002 - Polo failed MOT on high CO - PeteSmith

Hi all.

Got a problem with my wife's car, and thought I'd run it past you.

The car's a 2002, 52 plate 1.2 65bph 3cylinder VW Polo.

A few years back the coolant sensor failed, leading to the engine managment light (MIL) to come on.

This was fixed, and it was fine until about 2 years ago.

The light came on, with the same "Catalyst low efficiency" code.

I cleared the code (ELM327 box + a laptop), replaced the sensor and it was fine for a while. Then the MIL came on again.

When it went in for its MOT/Service, the garage said they'd put their Bosch reader on it, and said the ECU was occasionally reporting a misreading airflow, which he thought was throwing out the fuel mix, which was confusing the lambda sensor, putting the MIL on.

This year, it's failed its MOT.

The Hydrocarbons were OK (a little high, but within spec). The fast idle CO was too high though (0.5% compared to 0.3%).

The garage (a different garage) put their reader on it, and it said there was probably a problem with an air leak, or the lambda sensor. They tried to check for air leaks with a can of gas, but couldn't find anything.

At this point, rather than start swapping bits, they just said that there was nothing more they could do with it, and to take it to an expert diagnostics place. (We've got the details of one in Warrington, Camco, who's an expert). They said the lambda sensor works sometimes, then seems to cut out. This could imply an ECU fault.

All we know is it's probably running a bit rich.

The engine's (according to the garage) "a bit flat", and we've noticed it stalls *very* easily.

I've looked on the forums, and other places, and found that this problem might be to do with...

  • Dirty air filter (unlikely as it's just been serviced).
  • Faulty MAF
  • Timing chain skipped a tooth (not uncommon on this engine apparently)
  • Air leak (tested for and not found)
  • Faulty Cat
  • Faulty Lambda sensor
  • Faulty MAP sensor.

What's everyone's opinion?

Regarding the timing chain. I drove the car on Monday, and noticed that the engine was noisier than usual when cold and at moderate (3-4k revs). It sounded quite "rushy". It sounds a bit like a slight blow in the exhaust, but this wasn't noted in the MOT, so I'm now wondering if it's something else, like the cat breaking up, a timing problem etc.

I'm just wondering if the timing chain has skipped a single tooth - not enough to throw all the sensors out, but enough to make it run badly, noisily and rich.

If anyone can make any suggestions before we take it to Warrington (Camco), I'd appreciate it. I'm going to give them a ring tomorrow to see what they say, but I always like a 2nd, 3rd and 4th opinion.

Thanks.

Pete.

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Wed 7 Sep 2011 23:50
VW Polo 1.2 2002 - Polo failed MOT on high CO - unthrottled

Dirty air filters don't affect the air/fuel mixture in fuel injected cars.

If the timing chain skipped a single tooth, you wouldn't notice the difference (you can move the cam position +/-2 degrees without noticing much difference), unless piston/valve interference became an issue-then you would definitely know about it!

The cat could be faulty. What is the lambda reading? If lambda is >1 and you have a high CO reading, then (assuming there are no exhaust leaks) the cat is probably faulty.

To check the exhaust for leaks, with the engine running, block the tail pipe with a rag and listen for hissing at the engine end.

However, given the lacklustre engine performance and a stalling tendency would tend to indicate a dirty/faulty MAF.

Try disconnecting the MAF and see if the performance improves.

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Wed 7 Sep 2011 23:54
VW Polo 1.2 2002 - Polo failed MOT on high CO - PeteSmith

Hi.

Thanks for the feedback.

Lambda is spot-on 1.0x, so no air leak in the exhaust. One thing I didn't mention was that the CO was OK (0.45 compared to the limit of 0.5) at normal idle, but the 2 fast idles it was way over.

I'll take a look at the MAF as soon as I can. Assuming I can find it! My car (Mondeo), it's obvious. On the Polo, the filter's integrated into the engine cover, and everything else seems to be moulded into the head & block!

Thanks again.

Pete.

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Fri 16 Sep 2011 12:47
VW Polo 1.2 2002 - Polo failed MOT on high CO - PeteSmith

Hi.

Just got the car back from Camco in Warrington.

It turned out that the fault code was actually correct (P0420 - catalyst below efficiency).

The reason for this is that the cat matrix had disintegrated, and the whole cat box was completely empty.

Camco swear blind that this coudn't have happened by accident or by theft on the driveway, and that one of the garages we've taken it to has jacked the car up, removed the cat and manifold, removed the cat contents and then flawlessly re-fitted the whole thing and returned the car to us.

Given the car's not been to a garage since its last MOT and service, I think it's unlikely.

What I think is most likely is that when the car was hot, and was driven into a huge pothole full of water on a rain night (enough to get the underneath of the bonnet covered in mud!), that the thermal and physical shock has been enough to damage the cat, and it's currently in little bits in the exhaust.

Hopefully it'll now pass its MOT (readings were down from 0.7% CO to 0.03% CO.

Pete.

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