Mazda Problems

You say the battery is okay, and it’s possible for it to show 12 volts on a multi-meter, yet still be ready to be replaced. The trick is to load test the battery. That is, measure the voltage across the terminals while the battery is trying to crank the engine. Sometimes, a battery that seems okay with no load will turn to jelly once you apply the starter-motor load.

Cars (and batteries) don’t like sitting around doing nothing. The car’s alarm and even things like the clock in the dashboard will all continue to run, slowly draining the battery. A drained battery, if sat long enough, will often not recover even if you try to recharge it. Beyond that, an auto electrician will be your best first stop.

There have, indeed, been issues with this transmission with Mazda authorising a service bulletin to attend to some vehicles. However, it‘s generally thought that affected vehicles were built between 2011 and 2014, after which point, the problems were fixed. Which is not to say your 2015-built car doesn’t have the same problems.

The best way to proceed is to ignore the dealerships and go straight to Mazda Australia’s customer service department. Unfortunately, your car is now 10-years-old and well and truly outside its warranty, but I agree that 120,000km is not an acceptable lifespan for a component such as a modern automatic transmission to fail.

The fact that the fuel economy average seems to be creeping up can have a lot to do with the fact that you’re becoming more familiar with the vehicle and using more throttle as a result. But remember, too, that until you reset the average on the trip computer, what you’re seeing is the full history of your consumption from when the trip computer was last zeroed, not just the most recent trip you’ve done.

It's true that engines often start to use a little less fuel as they 'bed in’ after a few thousand kilometres. And for a modern turbo-diesel like yours, I reckon 10,000km or even 15,000km might be the magic number. However, don’t be too disappointed if it doesn’t come down a whole lot. I reckon for that vehicle, driven the way you’ve described, 10 or 11 litres per 100km isn’t a bad average to aim for.

Other factors play a big part, too. Does the vehicle have a bull-bar or roof racks? Both these add weight and aerodynamic drag respectively, and can easily make the car consume more fuel. A roof rack at 100km/h can be worth an extra litre per 100km.

A lot of modern transmissions don’t have a dipstick at all, particularly as manufacturers move towards transmissions that have very long service intervals and are considered sealed systems. Which is fine until it’s not. And that’s when you need to check the level and condition of the transmission fluid.

The good news is that the BT-50 in question does have a dipstick. The bad news is that’s mighty difficult to get to. You need to be under the car (so safety is the key here) and then locate a 19mm fitting on the driver’s side of the transmission. By unscrewing this fitting, you can withdraw the dipstick and check the level. The other catch is that you need to do this with the engine running (to get an accurate reading) and the dipstick is located close to what will therefore be a hot exhaust system.

All of this means it’s really a job for a specialist with a hoist and is a great example of a carmaker saving a few cents per car in production at the expense of the greater running costs for the owner over the life of the vehicle.

A car like this shouldn’t frighten too many mechanics, largely because this is a fairly simple machine using well-understood technologies. In a situation like this one, most workshops will go back to first principles. That is: Air, fuel and spark.

On the surface, it sounds like a fuelling problem that is causing the engine to run out of fuel when it needs it most (when accelerating). So a check of the fuel filters, the fuel pump delivery pressure and volume and the injectors would be a good place to start. However, stumbling under load can also be caused by poor ignition, so the spark plugs, leads and coil(s) and charging system should also be checked out. Don’t forget things like a dirty air filter which also cause a case of the stumbles.

If all that checks out, then you need to start looking for a dud sensor somewhere in the engine bay. An electronic scan is a must for any electronically-injected car.

This behaviour is probably a function of the way Mazda has calibrated the cruise control. In order for the car to initiate a downshift to regain some lost speed on a hill, it needs to recognise that it has, in fact, slowed down. But if it shifted down after losing just a single kilometre per hour, you’d find it would be constantly shifting gears in even slightly hilly terrain. And that would probably be even more annoying. The fact that it then overruns the set speed is probably the tall gearing (and physics) letting the car momentarily push through your 111km/h setting before it settles back to that speed.

But it’s worth keeping an eye on the speedometer even when using cruise-control, as this behaviour won’t work as an excuse during any roadside chats with the Highway Patrol.

The smart money here suggests that there’s something your car’s electrical system and body computer don’t like about replacement LED lights. That’s assuming you’ve installed them correctly, of course. The first thing you should do is replace the standard globes and see if that restores the lights’ operation. If it does, you can be reasonably sure there’s a problem with the way the car reacts to the LED replacements.

Not all cars appreciate the conversion to LEDs. These newer lights draw a lot less current than conventional globes and, while that sounds like a good thing, the lack of current-draw detected by the body computer can cause problems. Another common problem with LEDs is that the low current-draw makes the car’s computer think that there’s a blown globe at which point it can trigger a warning light on the dashboard.

That seems odd, but there might be a really simple explanation. I’d love to know whether you’re calculating your fuel consumption by working out how far each tank is taking you, or whether you’re relying on the trip computer in the car’s dashboard. If it’s the latter, the reason for your higher figure after a service could be very simple.

It could easily be that the mechanic test drives the car after each service to make sure everything is working as it should be. To do this, the tester is very likely to give the engine a couple (or more) acceleration runs with lots of throttle. It’s also possible the car is left to idle for a time for the engine to come up to temperature while the mechanic gets on with other things. Both these activities will lead to higher fuel consumption than you might experience in normal driving with no harsh acceleration or extended idling periods. Because the car’s trip-computer works out fuel consumption based on recent use, this treatment at the workshop will lead it to deliver a higher reading on the dashboard. That it happens only after a trip to the workshop is the big clue here.

To test this theory, drive the car normally for a week and see if the fuel consumption number drops. If so, you’ve solved the mystery. If not, you need to return the car to the workshop because something is not right and is causing the car to use more fuel.

Proper maintenance is overlooked by many car owners, and it’s a real problem. Modern engines work very hard and have lots of intricate oilways and mechanical clearances that simply won’t tolerate dirty oil. On paper, the car you’re looking at hasn’t had clean oil or a filter change in 20,000km and that’s an awfully long time between drinks. In fact, it’s enough to perhaps give Mazda cause to void any new-car warranty remaining on it, should something mechanical go wrong.

Your best option would be to find another CX-5 with the options and in the colour you want, but with an up to date and complete service record. That’s the best way to prevent any major disasters down the track. The good news is the CX-5 was a popular model, so there should be lots to choose from.