MOT fail 6% CO. Rich mixture fault. HELP.

[GTIJulian 0]

Hi all,

 

my 8v 87 1.9 phase 1 failed its MOT due to 6% CO (all else ok). It normally sails through the emissions test producing 3ish % (3.5% limit).  The tester revved the engine loads (which I didn't enjoy...) to clear it out to no avail.

Upon inspection the mixture adjustment allen keyed bolt on the AFM has no affect... It's usually set at 5/6 turns anticlockwise from the bottom, a little rich which helps with the low end driveability.

Had the plastic top off the AFM to see if anything obvious wrong, which there wasn't - no blockage in the mixture adjustment bypass channel route, for example. A part of the resistor track does look warn so performing the spindle readjustment so the wiper touches a slightly different place might help the the corresponding throttle position driveability. AFM no. is 0 280 202 109 so the correct one. LE Jetronic.

The car is definitely running rich but not excessively - I couldn't induce pinking with the AFM mixture adjustment at, like, 9 turns, by inappropriate gear selection/throttle demand, in fact its running well.

I think this fault has happened gradually - I've not noticed any step changes. The car is used regularly. With town driving the economy has always been poor - 25pmg, long trips maybe 35pmg.

With the AFM disconnected the engine won't run. The exhaust might have small leaks. The air hose between the filter and (BEFORE) the AFM is badly split so drawing in hotter air - I need to fix this, but I don't think it would cause 6% CO at tickover. The other hose after the AFM is ok. 

The SAD works but tends to close off too early before the engine is warm. So after the initial SAD higher revs they die back a bit too much til the engine is warmer. I'll see if it needs cleaning etc.

I've set the throttle body petrol switch so it operates the second the throttle moves. All things that affect carburetion which have been established for years haven't been changed deliberately. 

 

Sooo, What to try next? I think my dad has an old AFM in his garage so I'll try this. Before I take it to bits any ideas? My plan - check wiring to coolant temp sensor under distributor - (is it 0280 130 056 pt no? where can I get one?).  What about the Lambda sensor - I'm not familiar with this - where is it. Where can I get one?

 

Many Thanks in advance for any advise!

 

[estland 3]

Carbon track has clearly worn to ceramic layer. Bending the wiper, so that the contacts touch on the upper or lower edge can help.

Edited August 11, 2018 by estland

[GTIJulian 0]

AFAIK the bipass adjustment screw on the AFM only really affects tick over mixture (as in the MOT).

The warn part of the track doesn't come into play until more demand is placed on the engine resulting in higher air flow.

[Tom Fenton 1,542]

I’d test the coolant temp sensor first of all.

[estland 3]

On 8/12/2018 at 2:51 PM, GTIJulian said:

AFAIK the bipass adjustment screw on the AFM only really affects tick over mixture (as in the MOT).

The warn part of the track doesn't come into play until more demand is placed on the engine resulting in higher air flow.

On my car the AFM wiper stayed in that worn area during idling. Bad AFM caused an emission test failure. ( At least it was like that with XU9JAZ engine)

Edited August 14, 2018 by estland

[nixonmi16 5]

I agree check the cts or the wiring first if it still is high try a wd40 straw inserted into the air flow meter boot to intake manifold should lower the co to pass 

[DamirGTI 342]

Can temporally wound up the AFM spring tension to make it run lean , if you just want to pass the sniff test ..

 

Also , that AFM potentiometer track doesn't look too good .. try to start it up and you'll see that the wiper arm will be just above worn out areas on the potentiometer track while idling ..

Dunno if there's enough space left for retracking but do check the signal afterwards with an analogue multimeter to be sure .

 

D

Edited August 24, 2018 by DamirGTI