Greetings and salutations all.

I have what seems to be a common problem with the 89 240's.

I've been told its the ECU but would like to get as many opions or analysis of this problem as possible.

First the stuff that doesnt need to be said, the car is 1989 240 Volvo with 2.4 Jetronic, EI and B230 engine.

The problem. The car starts with a little difficulty, it idles very very rough when cold and has a tough time holding idle once it warms up. It seems that the car is running rich because when I pulled out the replacement plugs I installed they were dark and powdery.
I bought the car a few months ago in a worse condition than as described above. Before it would hardly start if at all. Now it atleast starts.
When I crack open the throttle it isnt as responsive as I'd expect it to be. The idle smooths out a little at higher RPM's but the exhaust has somewhat excessive amounts of white smoke that comes out of the tailpipe.

I have the Bently manual and have performed the test it describes to check the connectivity from the ECU portion of the harness. That checked out fine.

I'm aware of the MAF test (disconnecting it from the ECU when the car is not running, paying attention to its idle characteristics), it checks out fine.

Are there any other handy tests like that, I've noted a post by someone on how to check the idle control valve but there werent any details.

Things I've done or replaced.
Replaced:
Spark Plugs (NGK) and wires (Bougicord)
02 sensor (Bosch)
Crank sensor (Volvo)
Temp sensor the correct one for the ECU on the drivers side of the block (Volvo)
Check valve in the fuel pump underneath the car (Volvo)
Fuel filter.
Timing belt (installed properly, all the marks line up)(Continental)
Checked harness connectivity as per the Bentley manual

I know the car runs rich. I've pulled the plugs a few times and they turned dark pretty quick. They are brand new NGK's, I put them in about a month ago.

If it is the ECU what causes them to fail? Is the 951 ECU the desirable replacement/upgrade?

Let me know what ya'll think

Thanks,
Steve