The Mégane II Owners' Club

I own a 2005 Mégane 1.5dci. I am extremely proud of my car and like it running in top condition (which she usually does).
My problem:
For several months now my engine has been severely struggling to start. After all the stuff I have looked into on this forum and else where following directions from several people, I am almost at the point of tearing my hair out. nothing I seem to do works.
When the engine is cold, she will usually start no problem, like a normal diesel, slight vibration. If the engine is warm, or has been started recently. Meaning, it is in the summer and the outside temp is above roughly 10 degrees, the engine is up to temperature OR the engine has just been started then switched off, and started again. When starting the engine in these circumstances, once the start/stop button has been pressed, the engine will "set up", steering lock will disengage and the starter motor will turn the engine. The engine will then drop a lot of revs, and start violently juddering. After several seconds, the engine will calm down and the engine will hold revs at about 750 - 800 but I have no effect from the accelerator pedal for about 20 - 30 seconds.
I went and plugged a diagnostic computer into the OBD port. The two errors that come up are "Crankshaft Position/Camshaft Position, Bank 1 Sensor A - correlation" and "Preheater unit diagnostic - short to ground"
The things I have tried to fix this problem:
replaced the Camshaft sensor
replaced the Crankshaft sensor
replaced the glow plugs
replaced the fuel filter (as I read that it could be a blocked fuel filter)
Engine has been carbon cleaned
EGR valve has been cleaned several times

All of these had little to no effect on the problem.

Because this problem is a little difficult to explain the symptoms I have taken a video of the start up.
https://youtu.be/-NzEvDKjx1A

In the summer the problem is worse, it can take up to a full minute to start the engine and have throttle control. But as this video was recorded in winter, it didn't struggle as much, but it still demonstrated the problem.

Any help would be greatly appreciated.

This is a very interesting problem. As far as I could see, you have not given the recorded mileage of your car. This would indicate or rule out certain problems. I note all the sensors you have replaced, none of which worked. I recognise these as "desperation measures" as carried out in many garages at the customers' cost. If your rev counter is working, crank sensor cannot be at fault. Cam sensor would stop the car. This engine starts fine in the summer even though all 4 glow plugs are dud (which they usually are!) I listened to your video and my initial reaction to the knocking noises is that this is injector knock. You mentioned OBD. Most of the instruments on sale simply do not work with this car. Believe me, I have tried various instruments including ELM 327 interface on a laptop. If you could get hold of Renault CAN Clip, it would give you the answer - it certainly helped me. I tried using many year's experience to diagnose faults but it proved impossible.
With CLIP you can see what is happening all the time in relation to fuel rail pressure etc. It would also give information about all the sensors including the two-gang accelerator pedal. Not only is all the information available but some fascinating tests can be carried out such as checking high pressure pump maximum output. With CLIP the world is your oyster! Above 100k miles and you can expect problems with leaking injectors. Are you still using the originals? Have you tried removing, splitting and cleaning your injectors? It is not as hard as it sounds and can work well. I could write more but this will suffice for now. Will it help?

Hello, and thank you for your reply.
The millage of my car is 119000 miles. The dash takes a long time to display this after "oil level okay"
I replaced all the items myself like as you say, it was a desperation thing. Didn't want to put it to a garage and spend out alot of money for simple jobs.
I'll give the injectors a try, haven't gotten round to them yet as I usually tackle/try one thing at a time then run for a week and see if there is any difference.
Glow plugs must be fine as car started perfectly this morning from being at -4°c. The engine only does this if it is warm. Was going to try the "coolant temperature sensor" that could have stuck it in "warm up mode"
I got told this 2 days ago by my mechanic friend.
I'll definitely change the injectors as as far as I know, they have never been changed.
I'll phone my local Renault garage and ask if they can hook up their Renault diagnostic machine. Mine is a delphi.
Thank you so much for your help, I'll post back the outcome when I get round to changing items.

If you have got to more than 100k miles without injector problems you are very lucky. Unfortunately Delphi injectors are expensive and most of those on ebay are rubbish - "car was running fine when it came into the yard mate...." If your car is 80bhp you could have EJBR01801Z injectors and these are getting as rare as rocking horse s**t! Rather than go to a Renault dealer and apply for a second mortgage, have you got a fuel injection specialist near you? A new set from such a place might set you back say £500, possibly good value as your car is not worth a lot and evidently you like it.
You could try the CTS - easy to get to and fit. Would you feel confident enough to remove and split an injector yourself? You have already done more than most. If you changed the fuel filter yourself, you deserve a medal!
Have you seen any warnings on the dash display? Level 1 = check injection. Level 2 = STOP, injection fault.

As an illustration re injector overhaul, since I wrote the above post I have popped out to the garage, removed injectors 1 and 2 (flywheel end), split them, stripped them, cleaned in cellulose thinner, blasted with high pressure airline and refitted to the car. I was looking for swarf from inside the HP pump possibly blocking the control valve (its like a little wheel and does sometimes stick). I found none. Particular attention to the 5 miniscule injection holes which can get blocked. Tested by blasting thinner through them and counting the little streams. These were in good nick but injectors with stubborn deposits can be soaked overnight. This procedure has worked well for me for a number of years on the Megane and my own Mondeo TDCI - similar Delphi DDCR to the Megane. Why not give it a go? Just a couple of small items that can drop out and get lost!

I have been lucky with having no injector faults so far. I bought the car at 80,000 miles. I love my car, I'd vertually try and do anything to keep her on the road as she's my first car.
The fuel filter, that was relitivly easy, bumper off, washer bottle out pipes off, pipes on etc. That needed to be changed though. Was still the original that came with the car. That was changed about 110,000 miles.
£500 for injectors, omg. That's extortionate. I'll definitely look into stripping the injectors but although I am confident in doing some things, maybe not that. But I'll see though.
Thank you so much for your help and giving my a nudge in the right direction.
Now I need to find the time to take my engine to pieces again.