Noisy tappets (also known as hydraulic lifters) are relatively common on engines that have done their fair share of work. By using oil pressure generated from the engine’s oil pump, the hydraulic lifter can adjust to maintain the perfect gap between itself and the engine’s intake and exhaust valves. This improves efficiency and reduces engine wear, so it’s an important job.

Over time, the lifters may become a bit lazy (or weak) and may not be able to maintain that correct gap. At which point the extra clearance causes the tapping sound we associate with worn lifters. Replacement is the solution, and best practice says replace them all at once on the basis that if one lifter is already weak, the rest won’t be too clever either.

However, there’s one thing you can try before that, and that is to give the engine an oil change with a high-detergent (as most diesel engine oils are) content oil and a clean filter. Sometimes, fresh oil will clear out a small piece of grit or rubbish that is blocking the oil flow to a lifter and restore it to full health.

By connecting the Ranger to a diagnostic computer, you should be able to work out what's constricting the engine power. Modern engines constantly monitor everything from boost pressure to fuel pressure, and transmission temperature to alternator output. Somewhere in those hundreds of parameters will be the problem and it should be fairly obvious to somebody who knows how to interpret the codes the car's computer will spit out.

Meantime, it could be that the engine's installation skipped hooking up a sensor or piece of wiring and that has caused the engine to go into limp-home mode which limits the power the car will produce in the interests of self preservation. On a really simple level, have you checked that all the turbocharger's plumbing is intact and the joints are tight post engine installation? Loose plumbing joints and cracked or split plastic tubing are a common way to lose turbo-boost pressure.

Normally this would be an issue with the glow plugs, but the 2.5 turbo-diesel Rangers seem to occasionally have an issue with the assembly around the fuel filter (which includes the water trap and primer pump) sucking in air. This can be fixed by replacing the unit as a whole. Similarly Ranger owners have reported trouble cold-starting their 2.5 TD when the valve stopping fuel flowing back into the tank from the pump fails. Try priming your fuel system a couple of times before starting and if it starts up fine then replace that valve near the fuel pump.