Calibrating the AFM / adjusting mixture - Mk1 (old-style gate AFM)

[Rich_M]

Yes, the O2 sensor is working perfectly. I should make it clear that prior to trying this out, I had methodically tested every possible culprit. Anybody with the same problem should do the same before trying this. This means doing/checking/correcting:

Full service (incl. air filter, fuel filter, plugs, HT leads, base idle, bottle of redex), base timing & timing belt alignment, temp sensors (bench test), O2 sensor (rich and lean resistances and reaction), AFM sensor (bench

 

Hi Jon, Thanks, I’m fine with it now, it was you saying in your earlier post that you checked resistances (above) rather than voltages that threw me. I know the standard type produce voltage and some new ones change resistance and was just trying to decide how to test ours. Your later post clarified you checked voltage not resistance which now makes sense.  

[Roadster_Robbie]

All very interesting. Maybe it is psychosomatic but I think I may have a similar problem, although not as bad as yours.
Sounds well worth an experiment.
Where did you get the automotive grade sealant?

Andy, your 1.8 has a different type of air flow meter which can’t be “adjusted” in this way.

[rhino666]

 Hi guys

This may interest you…

http://www.autodiagnosticsandpublishing.com/feature/o2-sensor-testing.htm

The sensor should cycle between high and low voltage at least once a second.  If slower the signal is lagged and the ECU will overfuel to manage the situation.

MK1 and MK2 only have a precat lambda sensor.  MK2.5 onwards with onboard diagnostics have the two sensors - precat and post cat.

Post cat is diagnostic for the cat.  Precat is the only sensor that affects engine running. 

I am hoping that the AFM work you have done will not be unhinged by replacing a tired lambda sensor.  Very interesting reading - please keep us informed of your new MPG. 

   

 

 

 

[victormeldrew]

Andy, your 1.8 has a different type of air flow meter

Thanks I missed that!

[victormeldrew]

…

http://www.autodiagnosticsandpublishing.com/feature/o2-sensor-testing.htm

The sensor should cycle between high and low voltage at least once a second.  

It might be my brain, but I’m finding that a little tough to follow and contradictory in places. I followed the Mazda workshop manual’s test procedure, which involves ensuring the sensor reaches >0.9V and <0.1V in the correct conditions, and reacts quickly, which it does.

please keep us informed of your new MPG.

I have ‘retweaked’ since as I had a mild flat-spot around 3k, so I’ve gone a click richer and refilled the tank. I notched up 29vmpg (vigorous miles per gallon) on the quarter-tank I used, which seems more reasonable. I’ll drive like my mum for a quarter tank now and see…

[victormeldrew]

…

http://www.autodiagnosticsandpublishing.com/feature/o2-sensor-testing.htm

The sensor should cycle between high and low voltage at least once a second.  

It might be my brain, but I’m finding that a little tough to follow and contradictory in places. I followed the Mazda workshop manual’s test procedure, which involves ensuring the sensor reaches >0.9V and <0.1V in the correct conditions, and reacts quickly, which it does.

please keep us informed of your new MPG.

I have ‘retweaked’ since as I had a mild flat-spot around 3k, so I’ve gone a click richer and refilled the tank. I notched up 29vmpg (vigorous miles per gallon) on the quarter-tank I used, which seems more reasonable. I’ll drive like my mum for a quarter tank now and see…

[rhino666]

 The trouble with these articles is they try to be all things to all men.  This is one of the best I have read and it seems to have been viewed a few times.  Not necessarily proof of worth but indicative given the boring, specialist nature of the beast here.

If you have checked the sensor on an oscilloscope and it cycles between high and low voltage at least once a second, you are probably OK.  These sensors are very cheap, even direct fit.  In your position, it may be a good idea just to try another working sensor to be sure.

Your MPG is now the same as mine:-) 28 - 34 MPG but sill got a black end to my tailpipe

   

    

[Andy]

All very interesting. Maybe it is psychosomatic but I think I may have a similar problem, although not as bad as yours.
Sounds well worth an experiment.
Where did you get the automotive grade sealant?

Andy, your 1.8 has a different type of air flow meter which can’t be “adjusted” in this way.

Thanks Robbie, I hadn’t looked in the manual to check. Is it a hot wire type?

[victormeldrew]

Is it a hot wire type?

Yes, all but the Mk1 1.6 have hot wires… Pretty much every car since the mid-90s does as they are far cheaper and easier to produce and in most cases are more accurate.

[Dunnaz]

Sorry to dig up an old thread but do you think this adjustment is worthwhile, the issue I am experiencing is posted here - http://www.mx5oc.co.uk/forum/forums/t/51301.aspx

I shall be checking out the lambda sensor tomorrow but as the hunting issue happens before the lambda has even warmed up, I am thinking this is not at fault? (if I am understanding the role of lambda sensor correctly)

 

Regards - Chris

[victormeldrew]

Personally I wouldn’t bother unless you a) have ruled everything else out, and b) have a gas analyser or colortune handy. At first I tried judging it using experience and guesswork, and didn’t get it far wrong, but it was too lean where I left it, so I’m glad I popped down to my mate’s lockup and checked it out. Much better too rich than too lean!

But if the mixture is still out after a new lambda (and if the timing, air temp thermistor, and air valve check out ok) then it’s worth a pop. It was faltering and hanging back, particularly at around 1/4 throttle and low revs, but clearing up at about 4k+ and at full throttle. I think the issue was it was dumping in too much fuel for the air, until the ECU started ignoring the sensors and did its own thing at high revs. ‘Adjusting’ the AFM cured it straight away, so either that was the problem, or I corrected the symptoms of another (unknown) problem by mistake…either way she’s much happier now.

Just remember to mark the ratchet, and to be careful with the connector…and seal it up properly when you’re done, you don’t want the insides oxidising,

[Nedski]

To my knowledge the Air Mass flow and Lamda inputs are both used in calculating the quantity of fuel to be injected. The air mass is the first level rough calculation, take it as an open loop one, so if the AMF is inacurate your engine management will be a long way off. The Lamda sensor is then used to fine tune the system to get the optimal fuel to air ratio. So for a system to work correctly both need to be within a level of accuracy and a certain level of AMF error can be compensated for by the Lamda control loop.

But when there are transients, say when you plant your foot on the throttle, the AMF loop dominates.

The CAT can be damaged by too low a fuel to air ratio so in event of error the systems are set to err on the side of rich. 

Oh and what is called the Oxygen  sensor is a Lamda sensor I believe and it should be mounted on the down pipe prior to the CAT. It is on mine (1990 car)

 

Spot on . And yes oxygen or lamda are the same thing just going by different names. Later cars had one after the cat, purely to check the cat is working. 

 

 

 

[Dunnaz]

Thanks for all the input, I actually work at a Vauxhall dealership (I do IT for a living though) so we are going to test it on the MOT gas analyser later today to see what the readings are. Hopefully that will prove or disprove the running rich issue, and should make adjusting the AFM a lot easier if need be. I will post up my findings.

Cheers - Chris

[victormeldrew]

Make sure you run it with TEN and GND bridged so the lambda doesn’t just correct and hide any issues.

[Dunnaz]

Whoops! No it was in normal running state… will re-test it with GND and TEN bridged later this week