Isuzu MU-X Engine Problems

It’s difficult to be definitive here without actually driving the vehicle, but there are at least two possible sources for this. The first is the simplest and involves either a wheel and tyre or driveshaft that is out of balance. That should be easy to diagnose and fix.

The second possibility is that the vehicle’s driveline (engine, gearbox and transfer-case) has what’s called a resonant frequency that, when reached at a particular speed range, will result in some weird harmonics and vibrations which is possibly what you’re feeling and hearing. This becomes worse when the torque converter locks up (to save fuel) and there’s no slip in the driveline to absorb these bad vibes. By shifting back to fifth gear, you’re raising the engine revs and moving it out of that harmonic patch.

I’ve heard that Isuzu has been remapping some ECUs (on-board computers) to try to move this harmonic zone into a different (higher) speed range (by manipulating the torque-converter lock-up point) where it won’t be a problem to most owners. The fact is the Isuzu diesel has never been the most refined one out there and some vibrations and harmonics are relatively common, However, they shouldn’t be bad enough to ruin your enjoyment of the car.

In the old days, a different exhaust was often enough to move the harmonics around or get rid of them altogether, but I can understand why that’s not really an option on a brand-new vehicle.

The car could be trying to trying to tell you that there’s something wrong with the key. So start with the basics. The most common cause for the key to not operate the ignition and security system (which is why the engine won’t start) is because the key’s battery has gone flat. Change it and see if that works. At the very least, try the second key (that should have come with the car) to see if it will operate things properly.

If not, then you have to start digging deeper, at which point the cause could be a glitch within the car’s body computer which is the brain that controls everything from the power windows to the central locking and immobiliser. If that’s the case, then an auto electrician is probably your best bet, but definitely check the key’s battery first.

Not being able to match a vehicle’s official fuel consumption number is not just common, it’s something just about every owner discovers the hard way. That’s because the test that determines the number is a laboratory test, not a real world one. The test figure serves as a comparison between different makes and models, not a realistic expectation of fuel consumption.

However, the additions you’ve made to your car will all increase fuel consumption. A nudge bar and tow bar both add wight and a set of roof bars can easily add a litre of fuel to every 100km travelled thanks to increased aerodynamic drag. Even the GVM upgrade might make a difference, especially if it sits the vehicle up higher, affecting aerodynamics even further. If you’ve also fitted bigger tyres, fuel consumption will rise even more. To be honest, I reckon 10 litres per 100km is about right for this vehicle and matches most other vehicles of this type. Don’t forget, either, that an Isuzu MU-X is a much bigger, heavier vehicle than a Subaru Forester.

A car that’s left to sit for a length of time will often develop a flat battery as things like the clock and security system slowly drain the battery. But this should not happen in just three days.

If there’s absolutely no drain on the battery (as checked by your electrician) then suspicion turns to the battery itself. It may have done this since it was brand-new, but the battery could have a manufacturing flaw or internal damage that means it won’t hold a charge beyond three days. Borrow a different battery and see if the problem goes away.

What you’re experiencing is the transmission calibration that many modern vehicles use to increase fuel economy. By always trying to be in a higher gear, the vehicle will record a better fuel economy number on the official test that compares it with its competitors.

The downside is that the constant upshifting means the vehicle will sometimes find itself in a gear that’s slightly too tall, and that’s when you’ll start to get the symptoms of feeling like the engine is labouring or lugging.

As you’ve noted, when the car shifts down a gear, the sensation goes away. The solution is to manually pull the car back a gear when this happens. If t really bothers you, you could ask Isuzu if there’s a software update that makes the transmission a little less eager to be in top gear.

The first question to ask is have you changed the way you drive the vehicle. Driving style has a huge bearing on fuel economy, and if you’ve started to drive the vehicle harder or even on roads that are more winding and hilly than before, a jump in fuel consumption is common. On that level, is it just you driving the vehicle or is somebody else using it lately who might have a heavier foot than you?

Have you added a roof rack or more weight in the form of a winch. Even a new set of more aggressive or larger diameter tyres can do terrible things to fuel economy. But if the answer to all that is no, then you need to have the car scanned to see if any fault codes are thrown up by the computer. And when was the engine last tuned? Is the air cleaner free of dust and are the fuel injectors working as efficiently as they should be?

If the new clutch helped the vehicle climb hills, then I dare say that was money well spent. As for noises from the engine under load, I’m afraid that’s a job for somebody on the spot who can listen to the vehicle and make an assessment. The noise could be major internal problems or it could be something as simple as a loose heat shield under the car. Even a loose or worn engine mount can create all sorts of mysterious and expensive-sounding noises and rattles.

Don’t forget, your vehicle came with a five-year factory warranty, so a major failure that is not the result of mis-use or general wear and tear (provided the vehicle has travelled less than 130,000km) is still covered by Isuzu.

I wouldn’t go on a trip without fixing the issue. I'm surprised to hear that there is no stock in Australia, but I would reach out to Isuzu Ute Australia head office if you've only tried dealers so far.

UPDATE 12/2/19: Greg advises that this part was replaced under warranty shortly after this response was published. 

What do you know about its history before you bought it? It could have occurred before you bought it, and just shown up now. It’s not something we hear about.