I have a Punto 1.9 turbo Diesel with a Cat. converter and O2 sensor but non DFP equipped. I have been battling and very hard cold start problem for some time now. It can take up to 8.5 seconds from a cold start only and when it finally fires it blows a large cloud of bluey/white smoke, it doesn't misfire or have rough idle on start up purrs like kitten. After the initial start and run, you can start and stop the vehicle numerous times in sub 1 second, thinking of leaking injectors. I have replaced the battery a few months ago and have rechecked it an is in still good condition. I also have replaced cyl.2 glow plug a year ago due to a glow plug code and a amp check. I have done a relative comp. test and are all within 10 to 15% of each other unable to do a proper comp. test due to fittings . I have removed and thoroughly cleaned the EGR valve and t/body. I have even got a common rail Diesel specialist around to see the fault and was told to pulled the injectors as he deemed them to be the problem. Witch I thought too being if it was a petrol I would agree it had an injector/s faulty/leaking. The injectors have been done by the dealer some years prior. The injectors came back with no problems at all. Not what I wanted to here but it is what it is.I did find that cyl.1 and 2. copper sealing washers had not done their job and had some carbon leakage. I told the Diesel guy what I had found and was told 'I think you've found your problem' nope still the same. I have some scope patterns of my path I will try and post. The thing is we are good at petrol vehicles here but the later Diesels especially the Euro are a bit more difficult with limited info. The dealers here keep it secret. I have checked cam & crank, starter & all glow plug current, fuel press. sensor MAF, suction valve, injector voltage neg. & current. I hope I can post it. The lady that owns this car has an Alfa 159 5cyl Diesel with DPF with the same problem but not as bad hard start and then beautiful exactly the same as the Fiat. And I have another 159 client same car same problem just never have the cars long enough to test them properly. Thanks in advanced Dom
But now i have taken a closer look at it and there are definitely too large deviation in the rail pressure after the engine has started. At idle speed the rail pressure should relative quick stabilize at 1.3 volts which correspond to around 350 bar.
Deviation in the rail pressure can be caused by a sticking DRV or M-Prop - so it might be worth looking at those valves and there function.There are off course other diagnostic paths you may choose to take along your route to the final fault.
Strangely enough - despite the deviation in rail pressure at start up of the engine is the high pressure pump able of deliver even more pressure - according to this capture it goes as high as 1.691 volt at engine startup which correspond to around 500 bars of pressure - well enough to start this engine !!!!
I have looked at some of your picoscope captures you have made and took a closer look at your crankshaft position sensor and the signal its producing.
Seen with my eyes is there nothing wrong with shape of the signal but when i measured the cranking speed of the engine did I found some variations in the cranking speed from as low as 176 rpm to 271 rpm.
I don´t have the cranking speed specifications from Fiat, but 176 rpm seems very low for diesel engine.