Starting bulls in Megane 1.9 TDI 95 hp

I've been having trouble starting my Megane 1.9 TDI for a week now. After much trial and error, I discovered that by squeezing the priming bulb before engaging the starter motor, it would start on the first try.

The injection system in this car has a control unit (ECU), Lucas ECU DCN R04080001C, which takes data from:
crankshaft position sensor,
injector lift sensor #1,
water temperature sensor,
vehicle speed sensor,
atmospheric pressure
sensor, and throttle position sensor.

It calculates the necessary advance and sends the correct voltage to the ADVANCE CORRECTOR. This is a solenoid valve integrated into the injection pump that, depending on the voltage supplied to it, reduces the pressure inside the injection pump to retard the injection timing if it is not powered, or advance the injection timing if it is powered (more advance with higher voltage, at least according to the workshop manual).

The explanation for why squeezing the priming bulb helps with ignition is that by increasing the pressure inside the pump, we advance the injection timing.

I also noticed that when I turned the ignition on, the solenoid valve (timing advance regulator) would cycle on and off rapidly (like a machine gun) for 8 seconds, exactly the same amount of time the glow plug indicator light was on, and then it would go off completely. I thought it was a fault and started checking the readings of all the sensors without finding a solution.

I connected a voltmeter to the solenoid valve wires to measure the voltage and see what happened during starting. I finally discovered that the starting and stopping problems weren't faults at all; it was simply because it was missing the signal from the lift sensor for injector number 1 (which makes sense, since the engine wasn't moving).

When the starter motor is engaged, the injection process begins, but otherwise, there's no data from that sensor. After the first eight seconds, the power supply to the advance valve is completely cut off. Power is restored when the control unit detects movement at the injector (by then, we've already lost a couple of crankshaft revolutions or more).

Therefore, just as priming the primer bulb before engaging the starter motor facilitates starting, creating some pressure in the circuit and advancing the timing, so does turning the ignition key to the "on" position, and once the glow plug light goes out, immediately turning the key back to the "off" position and then back to the "on" position and starting the engine.

The explanation is simple: we return to the cycle where the timing advance system is alternately powered (until the glow plug light goes out). We take advantage of the advance being generated by the engine control unit (ECU) without needing to take data from the lift sensor of injector number 1, and we start the engine. In this way, when injection begins, the pump is already further advanced without having to wait for at least a couple of crankshaft revolutions, facilitating starting. Furthermore, the glow plugs haven't cooled down because the back-and-forth motion of turning the key was very quick, and they reactivate as soon as the key returns to the "on" position.

By the way, for those who don't know, the glow plugs don't turn off once the preheating light on the dashboard goes out. They then enter a post-heating phase and continue working for a while even with the engine running (up to 30 seconds if it's very cold or the coolant temperature sensor is faulty).

So, here are some things that can help start the Megane's 1.9 TDI engine when it's cold:

- Lift the hood and pump the priming bulb two or three times until you feel it get hard.
- Go inside the car, turn the ignition on, and when the preheating light goes out, turn the key to the "off" position all the way, then immediately turn it back to the "off" position and start the engine.

However, I still think something is wrong with the ECU; a sensor might be faulty, or something else is wrong. But anyway, in case of starting problems, and until it's properly fixed, this might be helpful.

Greetings from a new member.

Well, the things you mentioned are correct. The advance solenoid valve works with 12 volts, and the ground is generated by the CPU through pulses. When the ignition is on, the CPU activates the solenoid valve for self-check. If the solenoid doesn't work after starting, it's very possible that (there are no signals from the crankshaft position sensor), (there is no signal from the instrumented injector, which has an inductive sensor similar to the crankshaft position sensor), (the coolant temperature sensor), (or there is some fault in the solenoid's power supply wires). If you have any of the faults mentioned, the car's timing is probably advanced to the maximum because the solenoid won't work when the engine is idling or accelerating.
This is because the DPCN pump has the hydraulic advance at its maximum, and only the solenoid valve regulates the advance. In some cases, mechanics who don't have a scanner retard the pump timing. The gearing itself, to make the car less noisy, is not recommended.
Regards.

I forgot to mention, it happened to me once when a mechanic cut the negative wire to the solenoid and connected it to the engine ground, correcting the pump gear advance. However, the check engine light stayed on, so the customer brought it to my shop. The solution was to replace the crankshaft position sensor and connect the ECU ground to the solenoid valve. Regards.