Any time a diesel engine produces black smoke, it’s doing so because too much fuel is entering the engine. The danger then is that the engine will start to run hot and that can cause all sorts of meltdowns. But combined with a knocking noise, black smoke can also signal serious internal engine damage.

The best advice is to not drive it any farther and get a diesel specialist to look at it. Diesel engines often make a knocking noise as a normal part of their operation, but if this noise has changed, then something’s wrong.

There’s a bit to unpack here. The first is that the D-Max is rated to safely tow 3.5 tonnes, so it’s not as if you constantly have the vehicle overloaded in terms of what it’s towing.

But if you also have the tray full of gear as well as 2.3 tonnes of caravan on the hitch, you might find that you’ve been inadvertently running overloaded for most of the time. It happens more than you think. At which point, the Pedders specialist is right; you need a suspension upgrade.

Even then, however, you still need to stay within the manufacturer’s maximum GCM (Gross Combination Mass) and the GVM (Gross Vehicle Mass) to stay legal. You can sometimes officially upgrade the GVM with a suspension upgrade that’s been properly engineered, tested and approved (which Pedders is a specialist in) but the GCM is generally not able to be upgraded according to the law in each state and territory.

But none of this explains why a vehicle designed to two 3.5 tonnes is wearing out its rear springs with 2.3 tonnes hitched up. Is the tow-ball down-weight within the legal limit (usually 10 per cent of the weight of the towed load)? Incorrect loading in this manner could affect the springs. Perhaps a fully-loaded visit to an accredited weighbridge might net more answers.

However, if it’s a simple case of the springs wearing out prematurely, then it’s possibly Isuzu’s problem to fix under warranty. Perhaps there was a bad batch of springs fitted at the factory that is affecting your vehicle and others built at the same time. Stranger things have happened.

It sounds very much like the vehicle has gone into limp-home mode. This is a self-preservation system whereby the engine and drastically reduces the power it’s producing, limits top speed and often locks the transmission into a low gear. Something has happened that has caused this, and a common reason is that something in the engine, fuel system, or (more likely) the transmission has started to get too hot with the strain of towing.

With luck, full operation will be restored once the vehicle has been allowed to cool down. But you may need to visit a dealership to have things reset. It’s always a good idea to do a full engine and transmission fluid and filter change after any overheating event anyway.