Most Hyundai engines with a rubber timing belt require belt changes every 100,000km. Stretching this interval is pushing your luck as a failed belt will destroy the rest of the engine. With that in mind, you’re already over that limit by about 10,000km, so a change sooner rather than later is in order.

The car will, indeed, drive perfectly even with an old timing belt. Until it doesn’t. At which point you have an engine bay full of junk and a big engine replacement bill coming your way.

That’s a pretty comprehensive rebuild of the fuel system, so it’s possible that something that was removed at the time has gone back on not quite the way it was. A shuddering as you slow down suggests that the idle speed is perhaps a touch low. You should be able to adjust the ide speed fairly simply, so the first thing to try is to creep it up, say, 100rpm and see if the shudder goes away.

A mechanic will be able to do this quickly and easily. If that’s not the problem, you need to go back and double check every fitting and fastener to make sure there’s no air or fuel leak that is causing the engine to develop the staggers. Were the new injector seals treated to a dollop of rubber grease before they were installed? If not, there’s a chance that fitting them may have put a small tear in the rubber seal which is now leaking.

It sounds very much like there’s a blow-by problem with this engine. The pressure created by the combustion process is somehow getting past the pistons and into the crankcase, causing the dipstick to leave its tube and blowing oil all around the engine bay.

Blow-by is often caused by worn or broken piston rings or a damaged piston itself. But modern turbo-diesels are also prone to problems with their crankcase ventilation and emissions-control systems which can become full of gunk and not allow the crankcase to breathe properly. At which point, the same symptoms can occur.

A mechanic will be able to do some tests and determine what’s causing the pressure build-up inside the crankcase. If it’s internal wear, a rebuild may be required. But if it’s a build-up of crud inside the emissions-control systems, a manual disassembly and clean might be required. Either way, your car’s warranty might cover you for this, so the Hyundai dealer is your first port of call.