Emissions & fault-finding

[MikeT]

Would it be ok to bend the rules? (Jim ) I'm not saying this engine is non-french (not saying it isn't either 8) ) but I'm looking for hints and tips on tracing an emissions failure as per the UK MOT.

It's a 134Kmiles fuel-injected 2.0 petrol engine with EFI management and it failed on too high CO content. Both MOT testers immediately suspected the cat.

There is an exhaust manifold leak between it and the head where one of the studs have sheared flush with end of the retaining nut though I have tried to seal it with putty for now. Besides, it passed the previous MOT emissions like this and with good margins.

The car has since, covered almost 5000 miles and has new fuel, oil, air and crankcase filters and oil. All four spark plugs are like new - sandy to tan coloured with no visible signs of wear on either the earth strap or the electrode. All HT leads are passing current and the compression on each cylinder reads good, if maybe a touch high. The crankcase is breathing well and there is no EGR, thankfully.

I have pulled the codes from the ECU and they all pass. I individually tested the engine coolant temperature sensor and the air intake temperature sensor which both passed. I tried to test the Lambda but that proved inconclusive, either I didn't have good contact, chose the wrong sets or wires or failed to read the multimeter, I just couldn't determine repeatable readings within specifications.

So what are the causes of high CO? Most articles refer to over-fuelling or poor ignition while both testers vounteered the cat (of which there are two in series).

All suggestions gratefully received as I don't want to pay £80+vat for a cat if it isn't the cause of the failure. Ditto for any component actually. Can cat's be tested? Is it common to find a tester and does it cost much?

[DickieG]

Whether the cat is the source of the fault or the Lambda sensor, read the following link http://www.picotech.com/auto/lambda_sensor.html

Any leaks on the exhaust can play havoc with an analyser when the emissions are being read as they allow non combusted gases to enter the system. A leak prior to the Lambda sensor will cause the ECU to richen the fuel mixture in an effort to reach the preset level of Lambda.

[MikeT]

Thanks Dickie, I've read similar so I'll be having a go at testing the lambda sensor next. I've been avoiding the manifold leak due to the risk of corroded studs.

If anyone can gain any insight from it, here's the history of emissions test readings



CO................HC.........Lambda

Nov 06 (PASSED)
Fast Idle
0.15%..... .49ppm......1.01
Natural Idle
0.27%

Mar 07 (FAILED)
Fast Idle
0.312%......52ppm......1.104
0.873%......70ppm......1.039
Natural Idle
0.853%

10 Days Later (FAILED)
Fast Idle
0.827%......65ppm......1.044
0.870%......64ppm......1.037
Natural Idle
0.776%

[MikeT]

Any leaks on the exhaust can play havoc with an analyser when the emissions are being read as they allow non combusted gases to enter the system. A leak prior to the Lambda sensor will cause the ECU to richen the fuel mixture in an effort to reach the preset level of Lambda.

Following such helpful comments as yours Dickie, I bit the bullet and tackled the exhaust manifold today and it seems my fear of shearing studs was unfounded as they all loosened off without a struggle - except one that was seized and that gave in after a few whacks of an impact wrench. The pre-sheared stud then came out just as easily with the use of a speed wrench. 8)


This is the pair of ports where the leak was noticed. Out of the two studs missing, it was the lower one that was sheared. The other preferred to come out before the nut would undo.




This is gasket I found. It looks like it's been in a bonfire...

[MikeT]

I picked up the gaskets and studs this morning, induction and exhaust is all back together again. Damn manifold to exhaust joint was blowing badly and I had to do some serious over-tightening to shut it up. As the exhaust got hot, smoke began billowing out of the bay much to my horror but I soon realised it was the plus-gas saturated downpipe just drying off

Thanks Dickie, I didn't want to tackle the exhaust but you helped me along and it's done now. I've still got the obstruction of testing two more sensors before I take a chance on a new cat but all the jobs I've managed to do so far have made a very noticeable difference 8)