CO high...

[tref]

Gentlefolk,

Preparing for MOT, the CO on my 944 S2 is up at 5.9%, (3.5% is the |MOT limit, but I seem to recall it should be much lower than that). Can someone point me in the direction of a "troubleshooting guide" to overcome this... I am going to check the Motronic temperature sensor, because that caused a similar problem before on my Turbo, but that is hardly a logical fault finding and correction method!

TIA,

Tref.

 

[tref]

Well... it may not be the right course of diagnosis, but sure enough, the resistance is just out of range for either a 2.5NA or Turbo... unless the resistance should be different for an S2? Anyone any ideas?

For information... the cold resistance was 1578ohm, hot 405ohms (lowest) according to the Haynes book of lies, it should be 360 or 390ohms max for normally aspirated or turbo respectively, but I can't fid a figure for an S2.

TIA,

Tref.

 

[zcacogp]

Tref,

I had the same problem at MOT time last year. I didn't check the sensor as I didn't have the resistance tables, but a BMW part was interchangeable and only cost £10 so I bought and fitted one of those. If you want, I can look up the part number but I think it's on the Alternative Parts Fitment thread.

I also changed the Fuel Pressure Regulator which was a bit more expensive - £80 from memory, although I had suspected this was on the way out for a while. And I put a load of fuel system cleaner in the tank as well. And re-adjusted the wiper arm on the air flow meter. And took the car for a good, hard, fast run just before the test. I think the CO came down to just over 2.0 (from nearly 6) as a result of these things and it did run noticeably better.

Yes, I will be the first to admit that my method wasn't scientific and I admire your more sophisticated approach. But it did get the required result.


Oli.

 

[tref]

Thanks Oli,

I hadn't thought of the FPR, but do have the test equipment to check it It was replaced a year or so ago, but no harm in checking...

Other thing I can look at is the oxygen sensor... since it is unchanged in at least 130k miles, I think that has an excuse if it has died!

That has given me the idea of comparing part numbers for the temperature sensor, maybe it is the same as either of the other two, in which case I can be sure it is out of range (it doesn't seem a lot, but I almost discounted this as not being the problem on my turbo before because "it is only a little bit out of range"... it made a big difference.

 

[Paul 290T]

tref,

I stand to be corrected, but if your car has a Lambda sensor then it operates in closed loop mode for the air/fuel ratio. Your S2 is running rich, and the closed loop control either can't pull it back enough, or has stopped functioning.

The temp sensor if its reading high will be richening the mixture. But there are others things that'll make it rich, you could run through those. Google will I am sure provide an extensive check-list, then I could give you.

I think the AFM is not involved in idle. So if that's true you can rule it out.

Leaks after the AFM would make it run lean, so forget those.

The vacuum line to the FPR ensures the fuel pressure is always a set value above manifold pressure. If the vacuum line was leaking then the fuel pressure would I think be higher than it should be, this would increase richness.

Injector gummed up and stuck open?

I'm sorry some of my pointers are a bit vague, its been many years since i looked into this kind of problem and my knowledge of the system is rusty.

Have you tried the factory manual, I seem to remember they gave the kind of diagnostic check-list you originally asked for, and resistance values for the temp sensor:

0C 4.4-6.8kOhm
15-30C 1.4-3.6kOhm
40C 1.0-1.3kOhm
80C 250-390Ohm
100C 160-210Ohm

Good luck

 

[tref]

Many thanks Paul... some good points there... haven't used injector cleaner on the current lump, and it has been stood about, so that is certainly well worth a shot (because it is so easy!).