on the later 740 (and all 940) turbos, there's a electrical connection to a fitting on the fresh air intake hose that goes between the MAF and the turbo, this fitting has a heater element in it, and is connected to the crankcase ventilation via a 1"(?) hose/pipe.

 

Thanks Pierce for all your help!
Unfortunately, the problem has returned. Putting the new MAF kept the CE light off for about a day. The gas pedal wasn't acting very responsive so I decided to clean the MAF. After putting it back in the CE light came back. I kind of believe it was a fluke that the CE stayed off for the day. It didn't stay off long enough for me to gauge whether or not my milage improved.
I finally buckled and took it to a volvo specialist for a diagnosis. I guess he tested my o2, fpr, and maf, and told me they're all functioning properly.
He also stated that since my i don't have a proper catylitic converter that he was incapable of running certian tests.
He said his next step would be to try a different ECU but he didn't have one on hand that's compatible with my 740.
According to him, I have a "federal" car and not a "california" car.
I took a look at my Ecu and here's the model
Bosch
0 280 000 563
3 517 370

He led me to believe that I need to find an ECU where all the numbers are identical.
Is that true? Does the Botch Model model number need to be exactly the same? I think he said something about the last 3 numbers but I'm still in the dark.

P.S. - I wasn't charged for this information since he wasn't able to provide me any definative answers. Though it sucks that my car is still taking up so much gas I feel a little better about myself having spent so much time on trying to fix it. Even someone who is a volvo specialist is perplexed so I can live with all my failed attempts at fixing this issue.

 

Engine ECU suggests that a -563 was used on model year 1990-91 turbos. there's a list on that table of supposedly compataible ECUs (they are all 0 280 000 xxx, so only the last 3 digits distinguish them)

re: california vs federal, there should be an emissions control sticker under the hood that says whether its 49 state or 50 state (California approved).

does your car have an EGR valve? afaik, california turbos all have EGR valves, so you'd see the EGR pipe coming off the exhaust side of the turbo, and going around behind the block, also see the EGR vacuum modulator, near the power steering reservoir.

 

Oh damn! according to that list there's a ton of compatible ECUS for my 740. I'm pretty sure mine's the 49 state version. I'm not sure about the EGR pipes, I'll take pictures and post in the morning. Why do you ask? Would having EGR valves or pipes factor into which ECU I need or why I'm running rich?

 

I run the newer ECU from the 940 on my 91 740 turbo.

Its a 939. It has a fast processer in it and I feel it runs slightly better.

 

That's good to know!
I'm sure I'll find something at pick and pull tomorrow since the list of substitutes is relatively huge.

 

The ECU was the problem the whole time! The only Turbo at the Junk Yard was a 91 940. Just like TIPSP I popped in a 939 ECU. Finally, no more god damn error codes! I pulled the MAF plug and started it just to make sure the CE capable of coming on if something was wrong. sure enough it did! The car sounds like a dream now!