Engine Problem - driving me nuts!

[handpaper]

This started on the way to the 'Ring early this month; I tried a couple of local garages as well as a very helpful BMW dealer whilst out there and have racked my brains ever since I've been back, but can't think what it might be.

Setup/mods:
320i with 325i throttle body (manifold modified), DigiTechnik chip, stainless exhaust. Everything else on the engine is standard.

Symptoms:
At WOT in low gear, engine will cut *hard* at c.4000 rpm, coming back in again 1/2 a second later only to repeat after another half second.
If the throttle is left open, the car may 'bunny hop' up to between 5 and 5.5Krpm, from where it will then pull cleanly through to the limiter at 6950.
If the throttle is backed off enough, the bunny-hopping stops; WOT can be used again after about 5Krpm.
Cruising in top gear is fine below 4Krpm, above this engine speed everything is OK down to c.17mpg on the fun gauge - at this point it will cut out for the same half-second as in the lower gears, but will (usually) pull again for a few seconds before cutting out once more. If the car can be eased past 5500rpm in top without going below 17mpg (about 115mph - you try it!) all is rosy again and 140mph (indicated) comes up eventually.*
There is never a backfire when cutting out or in, and the Tachometer dies at the same time.

I've tried:
Replacing : Distributor cap and rotor arm (both heavily worn), Crankshaft position sensor (wire frayed from contact with water pump pulley)
Swapping out : coil and ecu**
Defeating, disconnecting and removing the immobiliser.
Disconnecting (and blocking) the vacuum hose from the fuel pressure regulator.
Defeating the fuel pressure regulator by clamping the return hose.
Disconnecting the AFM (I like the way Bosch think - you can limp home at 2-3000rpm, or you can use the WOT switch on the TB and race home )
Disconnecting the blue temperature sensor.
Disconnecting the throttle position switch.


I'm desperate to sort this before the next 'Ring trip on 22/6 as it spoiled most of my track time; an effective 4000rpm limit.
On the last day there, I did partially overcome it by ragging the poor thing senseless and never letting it drop below 5Krpm (dodgy - have to use 2nd at the Karussell and Adenaur Forst; two places I'd really prefer to cruise through in 3rd)

*Note: This happened in Germany, on the Autobahn, where such speeds are legal. I would never dream of travelling so quickly on UK motorways, which are so much narrower and twistier and are frequently interrupted by roundabouts etc. </sarcasm>

**Huge thanks here to Ralf Radermacher of Baum BMW in Bad Neuenahr, who let me use:
A spare bay/ramp in their workshop
A spare fitter (thanks, Tomas!)
Some tools I didn't have myself (soldering iron, multimeter)
HIS OWN, IMMACULATE, E30 320i to borrow parts from!

[Martinaston]

If the tacho dies at the same time i'd start there, a rough guess would be to check the earth points for the ECU and loom and make sure no wires are pinched anywhere.

[Brianmoooore]

As Martinaston says, the dying tacho is the key.
The ECU produces the tacho pulses by processing the information from the crank position sensor, so the problem is with one of these, or any relevant wiring and connectors.
The ECU tacho output isn't used for any essential engine functions, so ignore the connection between ECU and tacho.

[jone23]

Mine used to do exactly the same thing except it was intermitent. However, when it was doing it, it would always cut out at 4000 rpm as well and always on WOT. If i backed off again slightly it would pull fine again??

One thing i did notice though was that the tacho didn't just die and fall back to zero, it just fell suddenly with engine revs. This led me to believe it wasn't an ignition based problem.

Never solved it as such but it stopped doing it when i took the AFM off once to check the throttle position sensor. Thinking about it, i think i tried 2 different AFM's and 2 different ECU's and it did it with both. I just put it down to a bad connection on either the AFM or TPS. Perhaps the ECU not getting WOT signal? You can always check at the ECU plug if you have a multimeter and find the relavant pin numbers.

Sorry, not really much use really...

[jone23]

Just read your post again...

Are you saying if you disconnect the AFM you can then drive on WOT without it cutting at 4000rpm?

[handpaper]

As Martinaston says, the dying tacho is the key.
The ECU produces the tacho pulses by processing the information from the crank position sensor, so the problem is with one of these, or any relevant wiring and connectors.
The ECU tacho output isn't used for any essential engine functions, so ignore the connection between ECU and tacho.

Agreed, the hard cut, lack of backfire and tacho failure all point to the ecu, but since swapping it out didn't solve the problem it's either a lack of information (sensors) or power causing it.
All sensors appear to be fine; the cars starts and drives perfectly under light load.
Does the ecu draw more power under higher engine load? Where are the ecu power and earth connections?
(DME relay has been changed already)

jone23 - I can drive with the AFM disconnected at WOT, but it will still cut out at 4Krpm