Starting probs at middling temperatures

[Withnail]

Evenin’ All… My A4 20v 1.8 ADR starts instantly from cold (cold as you like, down to at least minus 13, no problem) It also starts instantly from full working temperature. In between those it’s not so good.. Say the car’s been driven far enough to get warm and then I leave it parked for half an hour or so - then I can predict I might have to give it a bit of throttle at the first sign of it firing, in order to coax it over an initial lumpiness which may last a second or two. I believe the thermostat is working properly (Feeling the bottom hose it stays stone cold until the gauge shows 89 degrees and the needle stays there) Given that the gauge seems to work alright, does that mean it’s nothing to do with the coolant temperature sensor? I have read (elsewhere on here, etc) that either there are two sensors; one for the dashboard gauge and a second one for the ECU, to control fuelling. And that the two sensors can be combined in one unit??? It’s a problem that I could live with but I think it may be an underlying problem which contributed to the car failing its MoT this morning. Figures for the record were: Fast Idle Test: CO=0.67; HC=77ppm; Lambda=1.01
Second Fast Idle Test: CO=0.51; HC=69ppm; Lambda= 1.01
Natural Idle: CO=0.55
So, my Lambda values scored a Pass. As did my HC levels (less than last year) The problem seems to be the Carbon Monoxide readings. I was told there were no fault codes logged a month ago when I took it for a “diagnostic brake bleed”. Not sure if the bloke mean NO codes or just nothing relevant to the brakes/ABS. The car only does 4 miles to and from work lately, which I’m guessing has something to do with it. No doubt needs the Italian tuneup treatment? Anyone come across the starting problems I described??? Thanks for reading…..

[Alan02]

You do have 2 sensors, and the one for the ECU could be faulty. A code read would confirm, but use a good reader as some dont pick up faults which are actually current.
Give your car a good long blast on motorway may help emissions.But it could all be down to sensor.

[Withnail]

Thanks Alan. Are the two sensors combined in one unit? Reason I ask is that there are a number of Youtube videos about locating & replacing the CTS (retained with a c-clip in a cranked plastic tube at the back of the cylinderhead underneath the air intake pipe, as I understand it) with no mention of a sensor at any other location.

[green A8]

I'm not too certain on the 1.8 ADR, but I think there is one four pin sensor connected as you describe it retained via a plastic clip. The one thing I would suggest is that you get a genuine OEM replacement part. Some of the aftermarket ones are a waste of time, effort and cash. Hopefully with a new sensor fitted your car will become a better starter when warm and maybe give beter MPG.

As the car is only driven 4 miles or so I would think that you need an 'Italian tune up' Some Forte fuel system cleaner or similar would do no harm. It's also vital that the engine is at full operating temp when emission tested. To be fair most MOT centres get the car up to temp but I have known one or two who don't seem to woory too much. Great if the car passes it's emmission test, not so good if it fails!

[Withnail]

I didn't stay around to witness the test and in hindsight I may possibly have done better to stick with my usual test centre this time round. I do remember the first time I took the car in to my preferred test centre. The emissions had been condemned at a branch of a well known national chain; so on the offchance of a lunchtime I took it to this place near where I work and the fella there put it straight on the tester. He planted a boot on the throttle for some minutes until the figures on the machine's display came good. I was favourably impressed... Thanks Green A8, for the info. As you suggest, I'll definitely go OEM for that. I've had some less than satisfactory experiences with pattern parts of late...