Thanks for the information.
If it's 40 degrees the inlet heater should stay on. The ECM will leave the heater on until the sum of the coolant temp and intake air temp exceed 120 degrees.
But to be honest. The engine should not have any trouble starting in 40 degrees. The inlet heater is not there to start the engine but to eliminate smoke at start up. It's an emissions thing.
I would suspect you have another problem.
The first is a low or no fuel pressure condition. The engine needs at least 25 psi to start. If it is low, the injectors will not get filled and it will not start. If the engine has a hand priming pump, and it gets stiff when operating it, the engine has enough pressure to start. If there is no hand priming pump, I would connect a psi gauge and measure the pressure. If the pressure is low, I would change the fuel filters first if you haven’t already. If it is still low, I would suspect a failed fuel pressure regulator on the back of the head or a weak fuel transfer pump. The pump is located on the back of the HEUI pump.
Second is a low injection actuation pressure. This is the high pressure oil system that activates the injectors. The engine needs at least 870 psi to start. If it is low, you have either a failed Injection actuation pressure control valve in the HEUI pump, a weak HEUI pump, a high pressure oil leak at an injector, or a faulty Injection actuation pressure sensor. To test the sensor, just unplug it. If the engine starts, the sensor is bad. It has 3 wires and is located on top of the engine near the front and screwed into the head.
Third is a failed engine speed timing sensor. Your engine has two. They are located on the driver’s side of the engine on the back of the engines front cover under the HEUI pump. The bottom sensor is the primary start sensor. If it has failed, the engine will be hard to start. The top one is the primary run sensor. They come as a pair.
Next is low battery voltage to the ECM while cranking. The ECM must have at least 7.5 volts DC to operate. I have seen weak batteries or a poor connection cause low voltage. You could have a poor connection at the batteries or starter.
Next is a faulty engine ECM. The ECM has an injector driver module inside. This is what powers the injectors. If it fails, the ECM will log injector codes for all the injectors.
Lastly are worn injectors. Over time the injectors can wear if they have a lot of hours on them. To verify, I like to check AC to the injectors while cranking. You should have between 20-40 AC while cranking. If you do, and you have good fuel pressure, the injectors are worn.
Most often when these things are happening there will be diagnostic codes in the ECM. If your engine has cruise control switches, you can pull the codes. If you have never done this, let me know. I can get you the procedure.
Please reply back if you need more help or have more questions. I am happy to assist.