Rough idle when cold (compare AFM settting between M40's?)

[DaniTD]

1) Never touch the adjustments on an E30 engine. They were set at the factory to their one and only correct position.
2) Never buy a car that' been owned by an idiot.

As you say Brianmoooore, E30's (and I think all cars) are delivered with the best settings for a normal usage from factory.
Buying a car from an idiot well... you never know maybe the PO seems no idiot but the car was mantained by some idiot mechanics.
Anyway the two Golden rules are well suited and I am with you on them.

One question, did your AFM lid look like it had been opened before? Mine didn't, the whole car seemed untouched...

Yes, the AFM lid seemed unopened, but I was doubting of the AFM because I think I took a look to every sensor and leak. So I opened it and discovered that, although it was hard to take off, it hadn't sealant all around it. Then I discovered the marks on the toothed Wheel and the two deep stripes that indicates a worn pot. Will try to bend the wiper arm to see if flat points at acceleration and shimmy idle goes by.

unlikely though as the last owner (from 16 to 6 years ago...me since) was an elderly gent, unlikely to be, as Brian puts it an idiot!

You never know! Maybe PO wasn't an idiot, but his workshop people could be! Instead of diagnosing why mixture was so lean that can't be adjusted with the CO pot (air leaks), they could went through the easy way softening the spring tensión, being not aware of other problems that this could cause (overfueling on cold starts and on fast throttle opening).

In my humble opinion, get the ECU reset, then get the CO set properly and drive it for a couple/few days and see how goes!

I'd be tempted to do a full ECU reset by disconnecting the battery and joining the 2 battery cables together and turning the ignition to position 2. Leave for 10 mins.

Tomorrow I will test it without resetting it. I will drive it to my university and will stay parked there for about 9 hours with a temperatura of aprox. 5ºC. If then cold idle Works right with high idle speed and without throttle lag, then I will confirm that cold starts were from a messed up spring tensión setting of the AFM.
Then, I will do an ECU reset to see if flat spots and shimmy idle are gone (flat spots can be because of stored fueling data about part throttle being not proper for correct AFM setting). If after resetting and doing a few kilometers flat spots are still present,I would bend the wiper arm of AFM to see if that helps (flat spots seems to be a fueling problem, because if it was an ignition problem I think the flat spots would be more violent and seeming like cut-outs).

I will post results here, can be useful for people with this kind of problems.

You now know a hell of a lot more about how engine management systems work than you did when you started

I can say that, being an industrial engineer student, focused on the automotive specialty, I'm learning more about engine management systems with my E30 than on any engine electronics subject

Thank you all for your help!

[soul4t]

If after resetting and doing a few kilometers flat spots are still present,I would bend the wiper arm of AFM

In general, with the voltage constant...yea Idealy proved with an analogue meter as the voltage meters don't tend to react that quickly, you shouldn't need to do it...having said that I wanted to do it to eliminate it, so if you do do it, I remember you mentioning that you didn't see a way to remove the arm assembly? If you pop out the little black rubber half way along the black wire, that is attached to the back of the arm, then you can mark the position of the arm retaining screw, then remove it...the arm assembly should pull off the shaft, you can bend it carefully from there...I did mine with bull nose pliers...worked well...

Your retaining screw looks like it has the original glue on it so you know that's not been tampered with!

I got hold of a replacement AFM before I did the procedure in case I messed it up!

[DaniTD]

I would like to not mess with the retaining screw from the wiper arm if it's not necessary. I read somewhere that you can bend it without removing the wiper, just putting a piece of paper between the wiper contact and the pot track and the pressing with care the wiper arm to get it reading on the inner side. The problem is, I'm afraid of mess it up and make it not work anymore, as I don't have a spare one!
Will try first with an ECU reset though...

[DaniTD]

Well, car was standing 4 hours at 2°C (sure it couldn't cool down enough, will try another day for more hours) and car started right up to 1000rpm for 2 seconds and then would keep at 900rpm which is fine. Today I didn't notice flat spots when accelerating although idle is still shimmy, it feels like if you hold a water line and it doesn't have constant pressure so you feel vibrations on it. Other than that, car pulls off from low rpm a lot better, no more dangerous situations entering a roundabout thinking you have power and then car refused to pull enough (maybe flooding)
Maybe next thing to do that makes sense is resetting ECU.

[soul4t]

Yea, thats interesting, will be good to see what the reset does.

[DaniTD]

Well, ATM I can say the engine runs fairly better on acceleration, more low end torque. Cold starts are improved, getting a somewhat high idle for some seconds (900rpm or so).
Although one time I started the car being mid-hot (off the blue zone and before the first line) the engine went to 1200rpm, then drop to 700 and stay there a couple of seconds, then move to 800 and finally to 750. And I still have a lumpy idle after ECU reset. Even though, car has improved.

One thing I must say is, the story tells that AFM are laser calibrated. Well, that could be true for the prototyping one, so with a proper kind of spring they could achieve the flow calibration they wanted for that AFM. And after then, the production AFM would be set with the same spring and the same nº of tooth adjustment.
I saw a photo of another M40 AFM that had more marks so it's clear that was adjust more than one time. And the first mark iot has, is 2 tooth leaner than mine. Maybe mine was adjusted first time moving 2 tooth and they didn't mark the original position? Maybe I'll try to set it 2 tooth leaner and see if it improves more.
Could we, M40 users, take a photo of our opened AFM to see the actual adjust we have (taking by reference the printed numbers on the wheel) and see if there is a clear pattern of factory adjustment and so we can determine if our AFM's had been messed or not with some PO.