Another M59 1.6 petrol cranks but no start - how to diagnose?

[shedpete]

Hi all,

My long serving 2003 1.6 petrol was ignominiously recovered on the back of a lorry from a B&Q car park last weekend... she'd been going fine for ages but once we came out of the shop and tried to start her up to go home, she just wouldn't start. Cranked fine but just wouldn't catch.

Definitely not the battery. The old one seemed fine but it was a bit old so I got a brand new one from a car parts place nearby before calling recovery. No change - still cranked but no start.

Recovery guy turned up. He squirted a bit of petrol spray into the air intake and she started up fine for a few seconds. He thought it must be:

• A key chip/immobiliser fault (but I don't think so, there would be a key symbol on the dash)
  OR
  
• the inertial fuel cut-off switch which had wrongly triggered by some bump and was not resetting even after the button was pressed
  OR
  
• the crankshaft position sensor (computer won't let the car start unless it gets sensible readings, and seems they sometimes go bad, especially when hot).
  

Anyway, got her home. Next day I thought I'd try her and she started up instantly just as normal. Maybe suggests a crankshaft sensor that is OK again when cool? 

Plugged in my ELM327 and used car scanner on my phone to look for fault codes. Nothing showed up.

For background: about 2 years ago the fan failed and engine got hot, but didn't fail, and soon after that she did something similar (cranked but wouldn't start, after a 3 hour drive when she was seemingly working perfectly). A burnt wiring harness was replaced along with the crankshaft sensor, which seemed to fix it. 

She is now at over 150k miles and I'd had her well over a decade so she owes me nothing, but I think there's still life in her yet apart from this. But equally I don't want to pump masses of money into her at this stage of her life. Last MOT was just a few months ago and passed OK but mentioned the dreaded corrosion... 

I may be massively overthinking it but I'm not sure which way to turn now:

• Because she starts perfectly every time on my driveway I can't easily test while the fault condition is actually happening. If I could get it to happen again for test purposes I could presumably e.g. try bypassing the inertia switch and/or replacing the crankshaft sensor so see if that fixes it. But that seems to be out.
  
  
• Only have the one key so not sure how to easily test the immobiliser theory even if I could reproduce the fault. 
  

• I could drive her to a garage for diagnostics but if Car Scanner is showing nothing, would that have to be a main dealer with Diagbox & Lexia to have a chance at finding anything useful? Or do non-Citroen garages generally have it too? And if it was the inertia switch or crankshaft sensor, would this even be clear from stored fault codes?
  

• I could just replace both inertia switch and crankshaft sensor and hope for the best that this fixes it. But I'd much rather know for sure what it was first, so as to have some real confidence that it's definitely fixed not just waiting to reoccur possibly somewhere less benign than in a B&Q car park. Also not sure where to get the parts.
  
  
• I could buy 'full chip' Diagbox/Lexia off the Ebay link from a recent thread here and try to install it and then try to make sense of the readings. TBH not something I'm all that confident doing and if no good I'm £140ish down.
  

Any thoughts on how best to proceed? All ideas much appreciated!

PS Located between Luton and St Albans, in case relevant...

[Sol]

Diagbox is needed to get anything more than basic codes. It's possible the crank sensor or camshaft sensor went bad (they have to sync) and failing hot is common for them. Back street garage is unlikely to have Diagbox.

BUT... more likely to have been a fuel airlock since it started with a squirt of petrol...with no working crank sensor or cam sensor then it would not have done so - logical thinking!

I'd start with the fuel system and get it checked.

Blocked evap cannister causing tank line airlock from cooling down maybe? (Unable to vent air into the tank so fuel was pulled right back when the air above the fuel cools)

Either way it'll take some mech / tech savvy to trace the cause. And you may never find anything definitive.

But a decent mechanic should be able to trouble shoot a no start which worked when petrol was sprayed in surely. Check basics. Fuel lines, filters, evap cannister, pump, don't over think it.

[shedpete]

Many thanks Zion, that's a great steer. I'll have a look myself but more likely see what a local garage can do :-)