[PeteChappers]

My volvo v50 2011 drive is misfiring. what can I check?
I'm an engineer so I should be able to do quite a bit. Given where to look.

Basically my car ran fine but the start/stop never worked and is the original reason i joined this forum. I changed the battery etc but it never worked but the car ran fine so ignored it.

Later the car went in to limp mode. I took it too a garage and they found the partical filter presure sensor pipes had perished and some other small issues they replaced the parts and did a force regeneration. £110.

Later the car went back in to limp mode so I took it back they ran some more tests and said the DPF had failed completely blocked which is unusual for this car. So they replaced that £595.

As soon as I got the car back. in there car park I let the engine warm slightly and revved the engine but it got stuck around 3000 - 3200 rpm and would just bounce around there and kicked out a rich smoke. I went straight back to the guy and he said the sensors might need time to recalibrate etc.

So drove it for a week. Any ways car still struggle at 3000 rpm. Kicking out smoke and while driving I hit a flat spot which is like left foot breaking at 3000rpm in lower gears.
I took the car back to the garage they had it for two days then said the internal timing chain(cam chain) sound like it's out (no cost) and is why the engine is misfiring but they do not have the tools to do it. but if they can get it, it will be about 400quid.

So basically before I go down this path is there anything else it might be that I can check? would the EGU cause it?
I just find it a little strange that the car ran fine then after the dpf replace it misfires. It might be why the dpf blocked but why was there no misfiring before at 3000rpm?
Can it be done my self?
Any advice at all or knowlege