M42 coughs, spurts, won't run properly

[DocKeR]

Hi all,

Since my daily driver was stolen last week, I've taken my E30 out of the garage after a year of sleep. Before doing anything, I checked the fluids, flushed the engine oil, charged the battery. Everything seemed ok, so I cranked it and it started allmost immediatly.

Problem, it idles well and runs correctly for a minute or so but as soon as I drive it, the engine coughs, won't rev unless being really really gentle on the throttle or, on the contrary, going pedal to the metal which leads to a second of hesitation followed by its accelerating rather well. Anything in the middle and the engine makes weird noises, like bad combustion, and sometimes no combustion at all, revs are simply droping as if I had left my foot from the throttle.

At first I thought the petrol had turned bad following the year without running, so I emptied the petrol tank and put fresh petrol in it, to no avail. I also tried to unplug the AFM, things are still the same.

So I need the help of M42 gurus, what should I check?
Thanks!

[DesktopDave]

Throttle cable binding?  BMW issued a notice about the cable cracking at the adjuster on the throttle body.  I'd also check the throttle position sensor.

It could also be a stuck AFM flap.  They need a bit of cleaning every once in a while. When you unplugged the AFM, the car should only idle cleanly...give her any throttle and the car will choke.  That sounds very similar to your problem, and might explain why unplugging it made no difference.  I'd definitely check that out very closely.

Bentley suggests the following tests:
1. AFM harness pin 1 must have 5V when tested against a chassis ground when ignition is on.
2. AFM socket pins 1&2 should show a steady increase in resistance (ohms) as the door is pushed open.  No specs on actual values.
3. The integrated air temp temp sensor (IAT) on pins 4&5 should have over 2K ohms at 20degC.  The resistance should rise if the sensor is colder.  At -10degC it should be between 7K and 12K ohms.

While you're testing the IAT, I'd also test the coolant temp sensor in the head.  It has the same specifications as the IAT.  At operating temp (80degC) it can get down to 300 ohms or so.

I'd also test for proper fuel pressure.  You should see 2.75 bar or so when the pump is running.  If you're "dead-heading" the pump by plugging a pressure tester into the end of the fuel supply line, a healthy pump can easily put out 5bar!

Of course, I always check the crank & cam sensors for proper resistance.  680 ohms between pins 1&2 for the crank sensor, 1280 for the cam sensor.

[DocKeR]

Thanks a lot!

AFAIK the throttle cable is ok, I actuated it manually and I didn't notice anything wrong.
The AFM flap moves as it should, freely.
The fuel pump has been changed three years ago, it's a TRE340 unit, I think it's ok but it's worth checking it.

I unplugged the TPS and it didn't change anything, would that mean it's the culprit?

I'll check everything and see what's what.

Again, thanks