Hi, my 90 model 760gle b280f engined car is driving me nuts! It has an irregular miss which is throughout the rev range and not load related, does it at idle, same at full noise. Sometimes it runs pig rich on startup, sometimes fine. Usually within about a minute of starting it gets really boggy to the point it will barely move. This clears up but occasionally presents itself at random again. Also when it is hot and has run for a while the idle starts to fluctuate wildly between over 2000rpm to 0 (it will stop the engine) which carries over into throttle so its not just an idle issue. This is awful in traffic.

Engine has been rebuilt, replaced crank angle sensor, coolant sensor, knock sensors, maf sensor, plugs, leads, cap, rotor, fuel pressure reg.

I made up the test light for lh2.2 and it blinks wildly with no apparent pause.

Any ideas?

so there are no air leaks & replacing ecus with known good ones did nothing...

I don't know the B280F that well, but your description of such huge rpm surging makes me think that the computer is not receiving a good rpm signal. On some other makes I've repaired, a bad high resolution pulse can cause such things. Many makes use a lower res pulse at higher rpm. I'm not sure if Bosch does this. I read that you replaced the crank angle sensor - and it should be fine - but if not, for whatever reason, that may be problem. I know that others have found that damaged tone rings on the flywheel will make for misfires, just how the B280 determines crank angle I don't know, but it should be investigated.
On one of the makes I've had experience with, disconnecting the idle air control valve "calmed" the surging, but the engine could still be felt trying to surge as the ignition timing changed. You might try that. On a scan tool (OBD 1) the rpm signal could be seen jumping up and down in excess to what the engine was actually achieving - and that confirmed the diagnosis.
I hope this helps.
Rhys

Maybe replace the ECU? I did much work on my car this year, fully tuned-up, replaced 95% hoses and the car still had weird cold engine idles and shifting. Bought a used ECM and the problems went away.

Anyone happen to know whether ecus from the v6 are interchangable with 4cyl ones? It doesn't "appear" to have any air leaks anywhere.

Yes, see chart here: http://www.brickboard.com/FAQ/700-900/EngineFIComputer.htm
However, Turbo and non-turbo ECU aren't equal swaps like the chart suggests. Ask me how I know. They will work but I had cold-engine idle problems with non-turbo ECU in my turbo.

Also go to FCPGroton.com to find the matching ECU for your car to get it's ECU part number.

Ok thanks that's great, will see what I've got...

I'd look for a massive air leak between the MAF sensor and the cylinder heads. The surging idle is the computer attempting to control idle speed and mixture, so it means the IAC valve is just fine.