Follow-up to my 156 emissions fail....

In news:1hvfxu8.1o3cqylu97ewnN% snipped-for-privacy@italiancar.co.uk, SteveH wittered on forthwith;

Could be a nasty MOT man.

Nope, it's not - I've looked at the car with our mechanic (who's not the MOT man) - and witnessed the drop in revs. - I sat in the car with all electrics off (aircon etc.) and seen the drop myself.

It's definitely some kind of sensor kicking out a faulty reading - but I suppose we won't know until we've had it hooked up to diagnostics.

I'm definitely getting the Alfa equivalent of VAG-COM after these hassles.

I was tempted to take the car to an alternative MOT station, but I trust my mechanic and don't really want to run the risk of a fail for something else at another MOT station.

Neither.

Get a scope / DVM / live data on the o2 sensor. I think you'll find it switching slowly, when the mixture 'over'-swings lean, thats causing your drop in revs and emissions spike.

Tim..

Oooh, that's interesting.

I'll suggest it.

Is this the kind of thing an OBD-II reader can report on?

Do the revs hold steady to achieve the required CO & HC readings outside the permitted RPM range? If so, and given that you've already fitted a new cat and the emissions are ok under normal driving conditions, I suppose another MoT bod may have cut you some slack and used a subjective estimate for the RPM.

Yes, they do and no, you can't do that, 'cos it's hooked up to a tester with an rpm sensor.

Take the O2 sensor(s) out and give it/them a right good blast with a blow torch, especially in the vents.

That'll sort the bugger.

Si

In situations where they don't use their own RPM probe it's noted on the emissions test that engine speed measurement was by-passed. They can then use the vehicles own tacho as a reference or their own judgement. The results will say: Engine speed: 2500 - 3000rpm / Manual Check / Pass.

Indeed.

Tim..

I'd say not the idle control valve, as it isn't idling, maybe the TPS.

Somebody tell SteveH "yes," because he has me in his sin bin (or whatever it's called)...

My tame tester very rarely uses any RPM measuring device apart from his ear. Quite handy as his ear keeps a close eye on the emissions readings.

Shouldn't he be using his nose for the emissions?

The ear is much better, not much fails when an ear is on duty.

Errr why?! Typical italian electrics! You mind it doesnt self-combustulate itself next week!!!

tim..

Well, the answer is some slightly odd wiring in the climate control system.

Apparently, even with the compressor turned off at the switch on the dash, it will still trigger the relay to engage the clutch on the pump. Pulling the fuse / relay has stopped this strange behaviour and we're confident of a pass when it goes back tomorrow.

Asking around, it's a 'design feature'.

SteveH ( snipped-for-privacy@italiancar.co.uk) gurgled happily, sounding much like they were saying :

There is, apparently, some kind of function to disable this 'feature' - involving holding buttons whilst turning on the ignition. Can't remember what it is, though, so pulling the fuse until after the test seems sensible.

Still very odd, though.