intermitting starting problem

[henry]

Hi all ,
I have a 2005 megan 1.5 dci that has devolped some starting trouble . sometimes it starts first time ,sometimes i will be sitting there for 10 minutes pushing the start button before the car starts .When the car does not start it just does nothing ,i hear the steering lock disengage and then nothing . thank goodness its has never not started (15 minutes the longest it has taken ) . But i would prefer to get the problem fixed before the car decides to stop starting altogther ,anyone got any ideas of what the problem might be ? ? ?
thanks

p.s. great site

[tomalamix]

no messages on the dash?

[henry]

hi
no message on the dash, i just got the car diagnostic scan done today and the only fault showing is cam position sensor .Is this type of starting problem usually associated with cam sensor ? does anyone know ? Is it a simple job to change (diy) ?

[chewbacker]

cam sensor would not stop the engine turning over as the sensor does not work until it turns over.
what is the red light on the dials doing when it won't start and if the problem happens when the engine is still warm does the engine temp gauge work or does it look like it's not powering up.

[tomalamix]

im sorry to disagree but the cam sensor works while firing up the car, it must work...you can test to disconnect the TDC sensor and try to fire it up.

[chewbacker]

you could be right but when trying to elimnate possible causes for my own car not cranking several different tech's told me to ignore all engine sensors as they would only stop the car from starting and they have no use until engine starts cranking.
the starting order for a megane 2 is
foot on brake /clutch press start
uch scans sends signal to card reader for code, reader sends code back to uch
uch sends code to steering lock, if code is same as stored in lock steering lock disengages and sends signal back to uch confirming it's unlocked
uch sends code to ecu and if code is the same it sends signal back to uch and stays powered up
uch sends command to start to the engine bay fusebox which activates the starter motor

the ecu staying powered up means there are live feeds to the sensors but untill the starter rotates the engine the sensors do not pick up there required data and then relay that back to the ecu which controls fuelling and timing to start and run the engine.

the orange wire that everyone mentions breaking is the ignition live to the ecu from the engine bay fuse box
the fusebox has a constant 12v supply but contains a module that works with the uch using the canbus system to control varies components.

this website is very usefull, 87G is the chapter
https://janit.iki.fi/sh1t/megane/MR364MEGANE8.pdf
hope this helps

[tomalamix]

the tdc must be there for a correct starting sequence, the ECU needs that info. in its absence the ECU will try to fire up anyway using the mass wheel rpm and position, that's why disconnecting it the tdc sensor it will be ignored instead of delivering wrong data.

the bottom line here is that a failing tdc sensor will not prevent the engine from sniping, even if it doesn't fire up. i think its worth a shot, its simple.

[henry]

hi all,
I bought a new cam sensor at the weekend ,will hopefully get it fitted over the next few days. I will see then if it has cured the starting problem,will keep yous updated .
thanks all