Hi, I have a '90 740 turbo sedan, I bought it not running, the previous owner said it had a problem with stalling, it would run great and then just stall out and after some time it would start and run fine, it would take anywhere from a few minutes to a few hours before it would fire up. I am thinking it could be either the ecu or a fuel pump sensor, can someone tell me where the fuel sensor is located on this model?. I have an '86 240dl I use mostly as a parts car, will the sensor from this car work in the 740?.
also which ecu is the best one to run in this car, I have heard there are a couple different units that are used in this model. Any help would be greatly appreciated. Thanks, Bill

 

I think the first step would be to determine if its dying from lack of spark or lack of fuel delivery. After it won't start, pull a spark plug, is it dry or wet? Or, does the exhaust smell of gas fumes after cranking when it won't start?

I don't know any 'fuel pump sensor'... there are two fuel pumps in a 740 turbo, one in the gas tank (along with the fuel level sender which drives the fuel gauge on the dashboard), and another under the car. these pumps are controlled by a relay which is in the main relay panel behind the fuse panel and ash tray in the center console (remove the cigarette lighter + cubby to access the relays). I believe its relay "E" (middle row, leftmost) and its a double relay that also turns on the fuel injection ECU.

The fuel pumps get turned on for just a blip when you turn the ignition on, then when the ECU senses the engine is turning (via a magnetic sensor reading the flywheel) its turned on to stay.

AFAIK, a 740T pretty much has to use the ECU for a LH2.4 740 Turbo, not any other kind. There's one EGR for turbos with EGR and another without. I *think* its dark orange for no EGR, turqouise for EGR, but frankly, the ECU would be pretty far down on my troubleshooting list, the relay and the wiring are more likely culprits.

The relays can develop bad solder joints and/or the relay socket can be loose and not making good contact. start the car, reach in there and wiggle that relay around, see if it stalls. or, next time it stalls, wiggle the relay and see if it fixes it.

theres a fair number of differences between a 86 LH2.2 240 normally aspirated and a 1990 LH2.4 740 turbo such that very few parts are interchangable.

does the onboard diagnostics show any error codes in position 2 or 6? See Engine and OBD Diagnostic Codes
(you don't need a code reader for these).

there's a whole lot more troubleshooting here...
Volvo Maintenance Hints for 7xx/9xx