My 745 Turbo recently began to lag severely when starting out from a dead stop. For the first 2-4 seconds after depressing the pedal the car would barely move and then it would take off like a "bat out of h..." The idle was also a little rough during this period. However, the car started well and had no accel problems until fully warmed up in hot weather. I went through all the diagnostic checks detailed in the Volvo manuals and found nothing - fuel pressure, sensor inputs, etc. I even tried substituting my 3 (used) spare AMM's and a spare ECU - all with the same results. I then realized that my symptoms appeared as soon as the electric cooling fan came on. It had stopped working last year and I had recently returned it to service (blown fuse and failed radiator coolant temp sensor). When the fan came on, I disconnected it at the fan connector and did not cure the problem, so the fan itself is not involved. I then found that the coolant sensor, in addition to turning on the fan, also advances the timing 13 degrees, from 12 to around 25 degrees. I verified these numbers with a timing light. When I unplug one of the wires at the sensor itself, my symptoms go away, so I can only assume that for some reason my car cannot tolerate the ignition advance, but why?
As the car is presently not overheating without the fan, even with the AC on, I suppose I could leave it this way for now. I guess I could also track down the wire for the sensor input into the ignition module and disconnect it so the fan only would be triggered by the sensor. But not understanding what is going on is really bugging me! Any thoughts would be appreciated.