CO reading very low - What's this all about then?

[PeteE30Tourer]

Whilst I had my 325 in the garage yesterday to get the clutch sorted, i asked them to check and adjust the CO level as my car basically stinks! No-one ever gets too close to my rear end in case they get fumigated (stop sniggering at the back there - pay attention!!).

Anyway, they read the CO level at 0.1 which is incredibly low i understand, so they tried to adjust it but could only get it up to 0.8. They reckoned it should be above 1 and previous advice i'd had on here suggested it should be 1.5. So how i cant get mine up (I told you - no sniggering - any more and you're out!). I have a standard air-box, BBTB, ported head, 282/272 cam, 6-branch & "Ant special" chip.

I have to say that the car felt a little more responsive this morning after the adjustment they did, but perhaps this is just a placebo effect.

What i want to know is - what's this CO reading all about? I know it's carbon monoxide (i did pay some attention in chemistry class apart from lighting up the bench gas taps), but what does it indicate if mine is so low? The adjustment is only done at idle i believe, but what exactly is adjusted and how does this effect the CO?

I just like to understand these things.

[12345kevin]

Just for info, mine went through MOT yesterday and the CO reading is 1.4.

1988 325SE with 156,000 miles.

[johnono]

its the amout of fuel being burnt in the fuel / air mixture that gives a higer or lower reading

not sure what the limit is, but 1.5 is ok. i heard .85 - 1.0 is also a good range for m20 with dme.

[E30BeemerLad]

I don't know if adjustment of the CO is achieved by fiddling with the hex key on the AFM,but prob best for Brianmooore to take over this class. Please sir, can I go outside for a fag?

[Ant]

The Co on its own will tell you nothing Pete, the Hydrocarbon content is where the info is at.

you stink is a result of the lean mix at idle,ie not enough fuel going in to combust correctly hence high HC reading and a smelly exhaust, sweet and sickly for those following

the Co/HC relationship is a balancing act, 1.5 Co% @ idle with >400 HC is about spot on for a stock engine.

add in a long duration cam with lots of overlap and the normal rules dont apply anymore, the car is tuned to run best @ top end, so the idle mix will need to be tweaked for the best compromise. 1.5 and c 650 HC normally works well on the m20

the chip does not affect the idle mix as such, thats the air bleeds pupose, the ECU sees the idle from the TPS, dumps the pre set fuel in and the mix is altered by bleeding in outside air, just like the air screw on a carb.

HTH

[Paul325i]

Low idle CO like that, i'd look for a vacuum leak somewhere mate.

I always kept my 325i dead on 1.3% CO @ idle.

[Demlotcrew]

Low idle CO like that, i'd look for a vacuum leak somewhere mate.

I always kept my 325i dead on 1.3% CO @ idle.

Yeah that!

[Andy335Touring]

Bollox, i had mine on a meter at the garage the other day and i didn't look at the HC just the CO

Before i changed the injectors the CO was at 6 % , now it's 8.5 %

It does seem to be running just as well now as before i changed the injectors ?

I think i need to put it on the rollers to see how the fueling is through out the rev range, i know the fecker is supping plenty of juice !

[PeteE30Tourer]

Many thanks for all the responses on this. Someone else suggested it may be a faulty AFM, so i'm going to swap this over at the weekend, as well as looking for air-leaks.

So does the adjustment on the AFM only affect the mixture at idle or is it throughout the rev range? I know the car stinks at idle, but it's not exactly pleasantly fragrant at speed either. As i said before, i believe the car is more responsive to drive since the CO was adjusted up from 0.1 to 0.8.

I'm also going to be changing my injectors soon for an identical set that's been ultrasonically cleaned - perhaps this could be part of the problem too.

I'll keep you all posted on events.

[Brianmoooore]

I don't know if adjustment of the CO is achieved by fiddling with the hex key on the AFM,but prob best for Brianmooore to take over this class. Please sir, can I go outside for a fag?

Electrics are my department! There's others on here who know far more about this kind of thing than me.

[Brianmoooore]

Thought AFM was ignored at idle. Or rather, closed throttle.

[Paul325i]

Yes the AFM has little function at idle on the M20, the screw to adjust it is for idle CO setting only.

In a lean case i'd check for vacuum leaks, or a faulty ICV which may also go with a rough or uneven idle.

Idle CO isn't too important to the car's over all running but it can vastly improve the way the engine picks up and returns to idle so i'd always set it correctly. Watch out as often the Allen key adjuster screw will be loose, a small dab of loctite once you've set it will solve that.

If you're all fine there and still can't pull the CO up then i'd look at the way the car runs over the entire rev range, if it's lumpy or feels hesitant i'd consider having the injectors cleaned.

[racecar]

ah - this helpful - have an almost identical set up to pete 284/2.7/bbtb/antchip/headwork/full btb system etc etc and my car runs really well but will not directly return to idle - stalls unless progressively brought down - and then when warm it is v.lumpy - mine is exacerbated by v.light flywheel - looks like that bloody allen key adjuster will have to be forced loose - think it has been loctited up -