Hi,
I kinda have the same problem as you. I had my engine (b230ft) replaced like 3 years ago by a mechanic who was just a part changer, and gave it to me back running but rough as hell, and wouldn't start again after warming it up. It had to be cooled down for a couple of hours to start it. I had to park it to save enough money to bring it to a shop that only fix Volvos. I finally had it towed to the shop. I spoke to the mechanic telling him I was sending my 1985 760 turbo to him. At this point i never saw the mechanic who's name is Norris. after two days i finally went to the shop and saw Norris and we spoke for a while he let me know that the car was POORLY KEPT (my brother and i raced that car HARD) and that i had to change my engine harness before he could do any work (on my way there i knew he was going to mention the engine harness) as it was in bad shape. Norris told me when the tow truck guy showed up with the car and look under the hood he told the tow truck guy to take it back to the owner cause the car look like a major headache, but when he saw me he said I reminded him of his son. Norris and his partner work hard on my car to get it to run great. they found out that I had the wrong computers in the car (THAT PART CHANGER MECHANIC MESS UP BOTH MY COMPUTER AND REPLACED THEM WITH NON-TURBO COMPUTERS) which he replaced. He even gave me some FREE parts like fuel pump, fuel vapor canister, glove box, and other stuff. GOD BLESS THAT MAN. he gave it back to me not before pressure washing the engine. i drove the car to the gas station a couple of blocks away to fill her up. when i tried to start it up it wouldn't Norris's partner towed me back to the shop to find out that the used fuel pump that they gave me decided to go. they order me a new Volvo brand pump. i also reminded him that the first mechanic messed with the air mass sensor adjustment screw which he replaced.there was one last annoying problem. when i first start the car it starts normal, but after like 15-30 sec it slowly looses power until it shuts off. you have to keep you foot on the gas to keep it idling. after it warms up its fine. while driving for 15 min it would do that again and I have to rev it to like 2,000 rpm to keep it alive. Unfortunately Norris closed his shop and sold the land so some guy can build houses on it before he could fix this problem. after getting such good care from Norris I couldn't trust another mechanic unless he got the same Volvo training that Norris and his partner got when they work at the Volvo dealership. So I decided I would be my own mechanic. I brought Volvo factory service manual and educated myself on 700 models i even brought Volvo special tool #9995280 "test diode" for like $11.00. when i connected it to the ignition system, the test diode flash code 4. test conditions: idling or speed above 3150 rpm (the LED light flashed 4 times) which means a couple of thing in the manual. symptom: Max. timing retardation at high load. loss of power and drivability cause: knock sensor disconnected or improperly tightened. open-ciruit in wiring between knock sensor and control unit. action: check connection/tighten knock sensor. check wiring as described on page 178. if wring is intact, fault is in knock sensor or control unit. to check the knock sensor wiring, undo knock sensor connector. use a jumper wire to bridge the knock sensor's connector's terminal then remove the panel between the left-hand side of the instrument panel and the peddles you will see the ignition computer box. release catch and remove the ignition computer connector. undo the cover and remove the sealing washers (the sealing washers look like small tooth combs) of the connector. where the small tooth of the sealing washer goes in the connector is where you use an ohmmeter and connect it between terminal 12 (brown) an 13 (green) of the ignition computer to check if the wires are not opened. if you are wondering where is terminal 12 and 13, the ignition computer has 25 terminals. 13 terminals on the top row and 12 terminals on the bottom. terminal 12 (brown) and 13 (green) are on the top row to the right (the last two terminals) if the wiring are good then check if the knock sensor is torque right. There are two type of knock sensor the early type and the late type. the early type is round and has two flat surface with the male socket leading way from the engine at kinda like a 45 degree angle. The bolt has to be torque with a torque wrench to 11Nm (8ft LB) . Volvo don't sell the early type knock sensor because it was sensitive if you didn't torque it to spec. the late type knock sensor, which replaces the early type, look like an ice cream cone bitten at the bottom. there is on side that has a flat round side then taper to a small round flat surface with the male socket leading way from the engine at kinda like a 45 degree angle.the bolt has to be torque with a torque wrench to 20Nm (14.5ft LB). the late type is less sensitive to tightening torque than the early type. this design lets the torque to vary from 15 to 25Nm (11 to 18ft LB). when the knock sensor give a signal to the ignition computer, the ignition computer retards timing 2-3 degrees at a time (the knock sensor can make the ignition retard as much as 10-16 degree of timing from the basic setting which should be 12 degrees, 2-3 degrees at a time) and sends a signal to the fuel computer to spray more fuel into the engine until there is no knock signal. also make sure that connectors for the knock sensor and coolant temperature sensor have not been interchanged. knock sensor=black cover
coolant temp. sensor=blue cover. my car is park for now waiting for me to buy a torque wrench to torque the knock sensor bolt ( don't use a torque to loosen bolts, you will mess up the torque wrench calbration. ). i think i have the early type and i doubt that the knock sensor wiring is broken because its only been a year since i had my engine harness change. if all fells then I'm going to replace the fuel computer because after 5 months since Norris closed up his shop i had smoke coming out of the front passenger footwell where the fuel computer is, but then again i had the knock problem before the smoke incident anyway good luck, and post an update