My non-cat S4 failed an emissions test today with very high CO (7%) and Lambda of 0.9, so it's running very rich. TBH I expected this as strong smell of gas in the exhaust!

This set me off checking things like the O2 sensor. I'd never looked for this before or checked it.

Was surprised to find I didn't have a sensor, or at least the connector by the relay panel just has a loop of wire on one side bridging the 1 and 3 pins! It does look like a factory installed loop though bridging a proper plug.

I do have the idle/CO adjust pot mounted on one side of the ABS control unit.

I'm very curious why my car (MY 88, UK spec) would not have had an O2 sensor installed?

Also does anyone have any theories for the (very) rich mixture? I have used the JDS Spanner which reported no LH faults. I do have a warm start problem which makes me think I may have leaking injector(s) (have fitted a new check valve at the fuel pump). A new MAF was fitted in 2004 - can they go bad that fast? The car passed its idle emission test a year ago (just before I bought it), and the idle adjustment screw has not been touched since while I've had the car - never even knew it was there till today!

Idle is pretty steady with maybe slight hunting between about 600-750rpm. Haven't noticed any flat spots under throttle.

Appreciate that ROW cars without O2 may be less well known here, but thanks in advance for any advice.

 

I could be completely wrong here, but from what I understand, this is a sign of a failing MAF in our cars.
I thought the CO was adjusted from the MAF itself though, i'm not familiar with a seperate adjusting pot. Then again, my car is an S2!

Every car i've ever had without a cat has never had an O2 sensor installed. The emmision requirements for our cars weren't and still are not very strict, so we don't really need an O2 sensor.

 

Thanks Ben - looks like I need to get the MAF checked out.

That makes sense about the lax emissions requirement. For a non-Cat in the UK I think it's 3 or 3.5%, whereas the cat limit is something like 0.3% CO!

If it is the MAF I wonder what has caused it to fail so soon (it's 3 years old). I haven't subjected it to any fancy K&N air filters.

 

I think the threshold is '87, prior to that we are allowed 4.5% CO!
Cats weren't mandatory until late 1992.

I bought a 'Gunsons Gastester' Great tool for quickly checking your CO levels prior to an MOT.

 

You've got me thinking now.... have you got vacuum at your fuel pressure regulator??

 

I need to check for vacuum there Ben (can't work on it again till the weekend).

I'm beginning to think it is more likely to be something to do with FPR or leaky injector, as the workshop manual says I should get fault codes reported for MAF or bad LH when I test with the JDS Spanner diagnostic tool. The tool says LH and EZK are OK as well as all the other components tested.

 

Test the resistance across pins 1 and 2 (Check these pin N0.s only two wires go to the plug make sure I have the right pair) of the CO adjustment pot - with an in spec MAF it should be around 330 Ohms

 

A diagnostic tester will only detect gross MAF failure - like no output.

If your CO has gone up dramatically from last years MOT then you may have

1) MAF failure
2) Vac hose off FPR
3) Leaky injector(s)