2008 sudden quit on road

[mustangSally2008]

my 2008 Sally suddenly started convulsing violently on hwy after just a few kms down road..check engine light came on, shut her off as too scary sounding and behavior. tried restarting after letting her settle awhile but she was not able to start without bucking and rattling sound under hood (open by now). hubby replaced ignition coil which didn't do the trick. now mechanic has replaced cam sensor and another major sensor but still same problem with metallic rattling sound when trying to start it up. mechanic thinking could be valve spring came off ??? anyone have any ideas on this one? 240k kms on engine.
many thanks!

 

[Bullitt95]

The first diagnostic step is to scan the ECU for fault codes before firing the parts cannon at the problem.

 

[mustangSally2008]

The first diagnostic step is to scan the ECU for fault codes before firing the parts cannon at the problem.

they did scan and then they put cps and cam sensor in..but what can rattling be when they attempt to start? they think it could be a spring came loose off valve.. I don't know..oulling my hair out!!

 

[Bullitt95]

Evidently the mechanics are clueless and just threw parts at the problem hoping they'd fix it.
The rattling upon start up could be a loose timing chain (failed tensioner, broken guide), and it's possible that chain has jumped a tooth or two causing the poor running.
If a valve spring retainer had come off, the corresponding valve would have dropped into the cylinder and destroyed the engine by now, so I think that's unlikely.
Find a mechanic who actually knows what he's doing and get a proper diagnosis.

 

[EmmJay]

Does your car have a V8 or V6 engine?