I was worried to start with, of cause, and had my local dealer look at it twice. On both occasions they charged me £75ish for looking but said they could not diagnose anything because I had a non standard exhaust or non standard air filter.. (see old posts).
Anyway, I bought Carsoft (from Ebay) and ran it. It came back with '171 fuel trim too lean', great!! what does that mean??
There is masses of 'stuff' on the www on fault 171... boat loads, but a lot does not make sense and even less is specifically related to a Z3 1.8/1.9, to a layman its hard to know where to start.
In layman's terms this is what a 171 means.
The oxygen sensor in the exhaust is detecting too much unburnt oxygen.
The three main culprits are:
O2 sensor (sensing oxygen when it should not)
Vacuum leak (allowing more O2 in thats not measured by the MAS)
Mass Air Sensor (sensing less air that is actually entering the engine)
O2 sensors are a favourite of garages but they are relatively robust these days and besides, my thinking went, there are lots of other fault codes that relate directly to the O2 sensors, surely one of these should be showing as well if it was an O2 problem.
I found out that a good test for leaking vacuum leads, apart from expensive smoke testers, is to spray them with carb cleaner. Spray liberally at joints. Do it with the car running and if there is a leak then some of the clearer will get into the engine and you will hear the revs increase. [Danger!! Carb cleaner is inflammable .. obviously].
My car appeared to be OK on the vacuum front.
Mass air sensor. These get dirty. I guess its the poor air quality in Blighty but its amazing the cr*p that gets past the air filter. Oil impregnated air filters are renowned for creating problems too.
You may have problems with what feels like a sticky accelerator pedal or not idling automatically when you start the car cold until you blip the accelerator a couple of times. These are due a sticky butterfly valve in the throttle housing and sticky idle control valve caused by a build up of the same muck.
You can clean the MAS (mass air sensor) by spraying it with electrical contact cleaner (I got mine from Halfords), touching the wires is not advisable as they are apparently very delicate.
To remove the sensor first disconnect the cable, push the sides in and pull.
Then undo the two screws (you will need a security bit for this, DIY store or Screwfix). Once these are removed you can pull the sensor gently out of its housing.
This is not easy as there is a gasket on the sensor that is a tight fit, but keep going, little by little, wiggle it out... gently. When its out you will immediately be able to see a wire and a small resistor looking thing. The first time I only sprayed these, without looking for anything else and this did not cure the problem... If you look carefully there is a metal plate inside and in a section at the end of the sensor are some more wires. These also need to be liberally sprayed with contact cleaner.
Once well and truely sprayed LET IT DRY!! before installing it.
You will need to reset the Check Engine light, it will not go out on its own.
With my car, after resetting it always came on again after 6 driving cycles, after clearing the MAS properly it has not come on again.
Final pic with MAS part numbers, as fitted to my 2000(X) 1.8
