Technical It won't start randomly

[eolonano]

My Seicento has a problem that goes back four years.
Every now and then, it doesn't start. And no way you can do anything.
If you leave the car untouched for some days, even a week or two, you can try again and it starts again. At first, I thought that the weather was involved (especially rain/wet air); but it was not the case; that problem has shown itself in the hottest days of the year.
In the past I've tried to read the ecu, but no clue there.
Yesterday, the problem has reappeared; I've tried to start insistently, nothing.
Today I've tried again, no luck (either with or without the air filter box).
Then I put the battery under charge (I do that every time I insist on starting the stubborn engine).
I doubt the battery is involved (it is a bigger model than the original, and the engine spins at fast pace, no power-down effect); in the past, I've tried to fully recharge the battery and then try to start the vehicle, but no luck.

The only difference this time is that I've sprayed carburetor cleaner inside the butterfly flap body/throttle body; I've immediately tried to start the engine but no luck.
I've tried after 20 minutes: wroom wroom!!!
It was a different outcome: usually, when it refuse to start, it remains that way for at least few days......


Can it be something dirt/gunk related? It make sense dismount the throttle body an clean it?
It's strange, because after the car starts, all work fine for a long period of time.......

Any idea ??

 

[dadandlad]

Sounds electrical to me
When you turn ignition on can you hear the fuel pump priming
When the car will not start have you checked to see if you have a spark at the plugs

 

[rallycinq]

when it fails to start, whip a plug out quickly. Wet or dry?

Then check for spark depending on answer.

Crank sensor?

D

 

[bargepower]

I had a similar problem a couple of years ago - it turned out to be an rfid door access fob on my keyring confusing the immobiliser. As soon as I took it off the same keyring as the car key no further issues.

 

[eolonano]

thanks for the precious advices. Yes, when I turn the key the fuel pump works correctly, and no, i have nothing that can interfere with the immobilizer. I have tried swapping the keys, but it makes no difference at all. A more detailed explanation: When the car starts, it runs without problem, long or short distances, in the traffic, at high speed. When you stop the car it always restart, either with hot or cooled down engine. When the car sits for two or three days, it CAN refuse to start, but not so frequently. On the other hand, the car can sits there for a week and starts just fine. It's not weather related as I thought it was. It can refuse to start after two sunny days in June, and starts just fine after a week in cold and wet environment. Or it can go the other way around. The last strange thing is: if it sits there for three days and refuse to start, if you wait another week without touching it, it starts.

 

[GPz750]

Condensation, HT wires buggered, coil or coils packing up. Lose LT wire. I have a car years ago (Vauxhall) thet when the HT leads got hot the car would conk out and i had to sit till it cooled down. Took weeks to notice cracking in the HT. Also condensation could be getting into the plug caps, has it still got the rubber tops in good condition on each one?