Misfire on idle

[DaniTD]

Well, replaced the AFM on my M40 with a non tampered one (its the redesigned one BMW sells now, with a different pot housing)
Now the engine sounds healthier, and it feels like with more power although now I can feel some flat spots when accelerating at about 1800rpm. I suspected the AFM to be worn so I did the following:
-this AFM although used has the wiper pot factory sealed and the CO pot has the antitampering plug. So, applied 5 volts and tested the output, gently moving the flap: No dead zones or irregular voltage.
-rechecked TPS, adjusted and working.

The thing that concerns me most is that at idle it still feels like a miss every 8 seconds or so as you feel a stronger vibration. Also, when cranking cold it starts as a new car with a few turns, but when engine is warm it starts with the starter even making one turn? Its kind of strange, you barely hear the starter

I though of clogged/leaky injectors as the car has some years and the PO didnt take much care. Maybe the idle thing is because of fuel buildup on one cylinder and flat spots because of delivering less fuel than expected?
Also it backfires when coasting below 1500rpm...

[Brianmoooore]

What age is your engine?
Does it have a cat/lambda sensor?
Are you sure your replacement AFM actually has a CO pot fitted? Late ones didn't.
Any E30 should start on the first firing stroke after the 'missing' tooth on the cranks sensor disc passes the sensor.

[DaniTD]

Hi Brian, engine has 272000km and 24 years. The new AFM has the CO pot, and checled its output voltage on the connector pin. My car is non-cat.
One strange thing is, I read that idle valve and fuel pump should work on position 2 of the key, even before cranking. Mine doesnt do that, but the fuel pump and idle valve works when the engine is running. Could it be that working before ignition was on previous Jetronic and motronic 1.1, or indeed I have a fault?
Also, I noticed that when switching off the engine, 3 or 4 seconds later I can hear the idle valve releasing with an audible sound. Is that normal?
Thank you

[Brianmoooore]

Fuel pump only works when the crankshaft is rotating, and only the M20 idle valve becomes 'active' when the ignition is on - it's of a completely different design to the M40 one.
If your engine is still on the original injectors, they are way overdue for a rebuild, and will definitely be affecting performance. Injectortune, in our traders' section offers a very good and cheap service, but I suspect you might not be in the UK, so may not be of use to you.
May be an idea to check out the condition of the distributor cap and rotor arm as well, since the M40 is sensitive to failure of these.

[DaniTD]

Distributor cap and rotor are OK, they were checked like 800 miles ago as the cambelt and water pump were replaced. Sounds like the way to go is injectors then...
Putting in good order a a car that was not properly maintained by the previous owner is a pain in the...
So much things had been touched to mask problems... Sounds like PO leaned the mixture touching the AFM and introducing leaks to mask the fueling problem

[DaniTD]

One more question: I have a misfire on idle, more noticeable when the engine is warm, you can notice it every 7 seconds or so. When it misfires, you can feel the car shaking and the rpm go down a Little bit, then restores to normal. I tried to disconnect the ICV to not compensate this, and every 7 seconds yo can hear a pop off the exhaust. Can this confirm a fueling problem or maybe ignition? Sparkplugs are new and the recommended ones, and the cables were tested and are OK.

[Brianmoooore]

A weak mixture is difficult to ignite, and when it fails to do so, it will pass straight through the affected cylinder and into the exhaust system, where gasses from one of the other cylinders may just be hot enough to ignite the unburnt fuel, giving a popping sound from the exhaust.

[DaniTD]

All said then, as you said Thank you very much

[Brianmoooore]

No guarantees that rebuilt injectors will cure your problems, but for an engine to work properly, EVERYTHING has to be right, and 1/4 of a million km old injectors certainly won't be 'right'.