The immobiliser on my Sigma did fail several years ago - had an auto electrician diagnose it - the red LED was not doing what it normally did which prompted me to investigate this. From memory however the car would not turn over - completely dead so probably not the immobiliser!
The other problem I had once which behave in a similar way to what you describe is a damaged wire to the crank sensor?
Could it be the fuel pump? It sounds like the immobiliser is working as the pump primes and the engine turns over which it won't if the immobiliser hasn't de-activated itself.
I don't know but guess there is a pressure switch in the pump that causes it to run when needed to boost the pressure back up again and it sounds like that isn't happening.
Just a thought.....
I'm not familiar with the Sigma engine or ECU (I'm a K and D man), but does this ECU log the standard error codes? If so, would a code scanner not indicate the problem?
Also, when trying to start and after the pump has been primed, what tells this ECU that the crank is turning and to turn the pump on? I've always understood that the crank sensor did this (it does in the K). If so, your symptoms certainly suggest a problem with the crank sensor signal. Either it's not being sent, or not being received.
Many thanks for all the suggestions, I've ordered a new crank sensor this afternoon so will give that a try. Very frustrating as I'm in Wales and the lockdown seems to have been embraced. Hopefully it'll arrive early next week.
As far as I'm aware the Sigma can't be read by the usual fault scanner as it's a locked MBE unit. Tried when I first bought it years ago. I've had to use a generic one on my previous TT as this was plagued by emission code errors.
I've got the Easimap software as the 150 needs throttle body balancing and TPS setting on a regular basis. This gives a live reading of engine parameters but no codes appear stored (I think). Can't say I'm that familiar with the detailed use of it though
If it runs for a few seconds, there should be some data coming from the MBE which shows the reason.
If you connect the EasyMap software, add all the data panels so everything possible is being logged, try and start it, then save the data. The data files could then be shared.
I still have some data files from my 150 which I could compare your car with, happy to look at the data to see if there is anything obvious.
My detailed build blog
With Easimap, a few panels that could be added if not already on the logging page, that could give a clue to the crank sensor state would be:
- Cam Sensor Setup - Engine Synch Status
- Cam Sensor Setup - Sync Window
- Fault Conditions - Fault List
- Development - Crank Count
- Development - Bad Missing Tooth Count
Just to update things, new lambda, new TPS and harness, new crankcase sensor and plugs. Fuel pump is OK.
Still no joy, now waiting for Eric from Millwood to collect it - I give up, a perfect end to 2020.
Will update on the eventual solution
That's a shame, was sure one of those would be the culprit. How long will you have to wait for the car to be collected with the restrictions etc.?