High CO at MOT (3.1)

[zcacogp]

Chaps,

My 944 has just passed it's MOT. Very good news. I'm well chuffed. No rust anywhere, and only a very minor leak when it was up on the ramps (around the oil pressure sender), which I think I can live with.

The only advisory point was that the CO figure on the emissions was 3.1 (it was actually about 2.9 but spiked up to 3.1 at one stage, and therefore 3.1 was taken as the reading for some reason.) This wasn't a problem for the test (limit is 3.5) but I was advised by the MOT chap that the engine 'needs tuning, as this should be lower.'

I therefore dropped into a local indie on the way home (John at Tower Porsche, by Tower Bridge), who said that the engine is designed to run at 3.0, and therefore this is quite normal, and it shouldn't be tampered with.

I don't doubt Tower Porsche (who seemed exceedingly knowledgable, and very much on their game) but is this the case?

Thanks for your input. Oh, it's an '89 S2, 119k miles.


Oli.

 

[sawood12]

Congratulations - mine passed it's MOT earlier this week. Why is it we always assume they are going to fail and put ourselves though half an hour of agony reading alsorts into the face of the examiner during the test. Apart from a bit of adjustment of the n/s headlight the car passed with flying colours. My CO2 reading was 1.99% which seems to by quite lean if the cars are designed to run at around 3% as you've been informed.

I've got my tax coming up in Oct so how does all this work with CO2 emmissions? Do they go off the readings from your last MOT?

 

[zcacogp]

I always assume my car will fail, and always do a pre-check myself. I guess I am a gloomy type ... the pre-check is always a good idea. Nothing difficult, do it on your drive, check lights, tyres, simple things like CV boots, windscreen wipers, all the seatbelts work etc etc etc. Things that you should know about all the time if you an enthusiast, but worth a look. (I discovered that the windscreen wash pump connection was loose - happens quite a bit on mine - so pushed it home again and it works fine - much better than looking a wally when actually at the test centre!

I also walk around the car with the tester when it is being done (ask him nicely - he can say 'no'), as his off-the-cuff comments are useful. Things that are not test-critical are often commented upon, and worth knowing about. For instance, the small oil leak on the pressure sender - not a fail, but he pointed it out, and now I know about it. (And also know where to look when I come to fix it - much more helpful than being told but not having it pointed out, if you see what I mean!) Oh, and I don't have a 4-post lift, so the opportunity to have a good look around under my car in comfort is not to be sneezed at either!

Anyway, back to the CO. If yours is 1.99, I can't explain it. I wa told by my indie (not the guy who did the test) that they can be adjusted, but he doesn't recommend it. Perhaps someone has adjusted yours. Perhaps my indie is wrong. Anyone else heard anything about this?

I don't think they go by the last reading - I didn't take the sheet from last year with me (I have only owned the car for 4 months.) They take a reading then and there with an exhaust gas analyser - I watched him do it (fascination with toys - that's me!)

Glad yours passed tho'. It's a good feeling! (And even better when he had a row of much newer cars there which had just failed!)


Oli.

ETA: Just noticed, yours is a Turbo, mine an S2. Could be relevant?

 

[sawood12]

There is a screw on the top of the AFM that adjusts the mixture but obviously you need the equipment to know how much to adjust. The workshop manual states that you should disable the Idle Stabilisation Valve whilst adjusting this and tells you how to make a jumper to connect to the socket that's behind the nearside suspension tower if you havn't got the correct plug, so it's not something i'd like to do blindly.

 

[ross255]

I dont know what your indie garage is like, but if some people dont understand a problem, or know the solution to it, its easier to say "its supposed to be like that."
Of course it can be adjusted, and whether he recommends it or not, the reason it is there is SO it can be adjusted into range.
If you have a later car, it may have a o2 sensor that it failed, this could give a rich mixture. I know there were none as far as '87 as mine doesnt have one.


At idle with the mixture at 14.7-1 i.e. lambda
my co was 1.44%

so you should be aiming for thise figures, but 1.5- 2% is near enough.

 

[sawood12]

Isn't 14.5 lambda Stoichiometric - i.e. as lean as you can get? Wouldn't the temps at this mixture be too high?

 

[Fen]

14.7 AFR is 1.0 lambda which is stoichiometric; the most efficient mixture for petrol/air to burn. At idle that's fine, at higher revs and under load you want a bit of extra fuel to keep cylinder temps down. My Turbo would drop to low 15's:1 at idle quite happily.