Isuzu Ute Range

What you’re experiencing is the front and rear axles fighting each other. Us old-timers call it axle wind-up and, as the name suggests, it’s what happens when the various wheels need to turn at different speeds, but can’t.

When you’re in 4H, you’re effectively driving the front and rear axles at precisely the same speed. Logic says this shouldn’t matter, but, depending on what lumps and bumps they’re covering at the time, there are times when the front and rear wheels need to travel at slightly different speeds. Throw in full steering lock, and you’ve just made things very difficult for the driveline with all sorts of competing forces being fed around the various tyres, axles and driveshafts.

This is far from limited to the Isuzu, and many four-wheel-drives without a centre differential (such as your D-Max) when driven at full lock in 4H will exhibit the same shunting and apparent loss of power. The power loss, meanwhile, is simply those front and rear tyres fighting each other; nothing to do with the engine at all.

When you shift back to two-wheel-drive (2H) the front and rear axles are suddenly free to turn at different speeds, and the sensation goes away. This whole situation is why you can’t use 4H on dry bitumen in a four-wheel-drive vehicle without a centre differential. Meantime, there’s no fix for it other than learning when and when not to engage 4H.

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.