* Coolant temperature sensor (CTS) failure. The CTS is responsible for sending the engine coolant temperature to the engine control module (ECM). If the CTS is faulty, the ECM may not be able to properly control the fuel injection and ignition timing, which can lead to a hard start when cold.
* Fuel pressure problems. Low fuel pressure can cause a hard start when cold, as the fuel injectors may not be able to deliver enough fuel to the cylinders. This can be caused by a faulty fuel pump, a clogged fuel filter, or a leaky injector.
* Ignition problems. A weak spark can also cause a hard start when cold, as the spark plugs may not be able to ignite the fuel mixture. This can be caused by a faulty ignition coil, spark plugs, or ignition wires.
* Vacuum leaks. A vacuum leak can cause a hard start when cold, as it can lean out the fuel mixture and make it more difficult for the engine to start. This can be caused by a cracked or loose hose, or a faulty vacuum pump.
To diagnose the cause of a hard start when cold in a 1991 Volvo 740 turbo, it is recommended to start by checking the coolant temperature sensor, fuel pressure, ignition system, and vacuum system. If any of these components are found to be faulty, they should be repaired or replaced.
