Ask The Guide

You could be looking at something internally wrong with the transmission, but the clue that this might be something simpler is in the way the engine will start in Reverse which, of course, it shouldn’t (for safety reasons). The suspicion would be that the relationship between the selector (inside the cabin) and the actual gearbox is somehow out of synch. So when the selector is telling you the car is in Reverse, it’s actually in Park or Neutral, which is why the engine will start.

That might also explain why the car won’t select fourth gear; the position that actually coincides with Drive (or fourth gear) looks – from inside the cabin – to be the Neutral position. So, to check this, put the car in what looks like Neutral and, in a safe place, see if it moves when you give it some throttle.

Failing that, you’ve got two separate problems. The first is the inability to select fourth gear (which could be a hundred things) and a failure of the safety switch which is allowing the engine to start in Reverse.

Mazda (and other car-makers) have been having lots of problems with the oil level rising in the engines of some of their diesel models. The problem is caused by the Diesel Particulate Filter attempting to regenerate itself. In an ideal world, these diesel vehicles would be driven for at least an bour or so at freeway speeds at least once a month. That would get the exhaust system hot enough for the DPF to regenerate on its own (burn the contaminants from the exhaust to ash and expel them, basically).

But in cars that don’t see this sort of use and are more commonly driven around the suburbs at moderate speeds, the exhaust never gets hot enough for this to occur. So, the solution has been to inject extra diesel into the engine to make the exhaust hotter and, therefore, get it all to a temperature where DPF regeneration can occur. The problem is that some of that diesel can get into the sump of the engine where it dilutes the engine oil. And that’s why you’re seeing the level rising on the dipstick over a period of weeks and months.

Diluted engine oil is bad news because it may not lubricate and protect the engine as well as non-polluted oil. The warning light you’re seeing is telling you that you might be at that point and that an engine oil change is probably a good idea, even if the service interval hasn’t been reached yet. Just draining some of the contents of the sump away is not the answer as the oil is already diluted by the diesel.

It’s not a problem and is actually completely normal. What’s happening is that the car’s computer is detecting that the engine or transmission (or both) are hot enough that they need a little extra cooling. That’s why the electric fan comes on. It stays on for a minute or so after the engine is shut down to try to bring down temperatures a little more in anticipation that you may return to the car and restart it in the next few minutes.

It’s not that the vehicle is overheating, but modern cars are all about efficiency and thermal management is a big part of that. The fan is more likely to come on if you’ve just spent a few minutes idling at a red light or driving slowly through a car-park to your eventual parking spot. Low (or no) speeds mean there’s little to no air passing through the car’s radiator, so the fan switches on to create some airflow of its own.

There are literally hundreds of things that can cause a check-engine light in a modern, computer-controlled car. A problem with the ignition, fuel system, emissions-control system, air-intake, lubrication, cooling and much, much more can cause this light to illuminate.

The best way to proceed is to have the vehicle electronically scanned. That way, the computer can tell the mechanic where the problem is and, hopefully, precisely what’s causing the check-engine light to flicker on.

For the record, however, a common fault with the V6 engine in this series of Commodores is a stretched timing chain that the computer interprets as incorrect valve timing (which it is). Plenty of owners of this model have discovered this as the cause of their check-engine light.

This sounds like a case of a split fuel pick-up pipe in the fuel tank. When the tank is full, the split in the pick-up is covered by fuel, so the pump works normally at supplying the injectors with fuel. But when the level in the tank falls, the split is exposed and the fuel pump suddenly finds it’s much easier to suck air rather than fuel. And engines don’t run too well on just air.

This could be interpreted by a scan-tool as a fuel leak, so a check of the tank’s pick-up tube seems like a good place to start looking. Any actual fuel leak large enough to cause this problem would, presumably, be visible, but it’s worth a check of the fuel rail and return lines as well. Failing that, you could have a dud sensor that is telling the computer there’s a fuel shortage and sending the car into limp-home mode to protect the engine (which would also explain the loss of power). It would also be worth checking to see that the new Suction Control Valve (SCV) is working properly, even though it's brand new.

Even though your caravan is not at the towing limit of your vehicle, more than 2000kg still puts a pretty big load on the Isuzu’s engine. And that, simply, put, is why it’s using its gearbox to keep the engine happy and revving at the appropriate speed to safely make the power and torque you require to move that caravan at highway speeds.

The short answer is that you won’t be hurting the engine by allowing the gearbox to determine what gear it needs to be in. Quite the contrary, and over-riding the transmission to force the vehicle into a too-high gear would, indeed, damage things by overloading various components.

Short of fitting a more powerful engine, there’s not a lot you can do about this. After all, you’ve taken a two-and-a-bit tonne vehicle and more than doubled its weight (with the caravan) meaning you’re asking it to do twice as much work with the same engine. Fundamentally, you can’t beat physics.

The first conclusion many mechanics will arrive at is that the engine’s head gasket has blown, allowing the oil and coolant to mix. Have a look at the dipstick. If the engine oil is milky, then this is a very real possibility. Perhaps when the cylinder head was replaced, the gasket has not sealed properly.

But if the oil cooler was replaced because it was leaking into the radiator (which is pretty common) then you might find that the oil you’re still seeing is just residual oil from the previous failure, and not a problem at all. Even so, it would be wise to replace the coolant and flush the system to try to eradicate all traces of the old oil.

A workshop can perform a chemical-based test to see if the head gasket is leaking. It’s worth the relatively small cost to rule this out and begin to look elsewhere.

The service intervals for this model were every 15,000km or 12 months, so the fact that so many for sale have the same mileage showing is more likely to be a coincidence or the fact that at 10 years of age, most owners have been covering the national average of about 13,000km per year.

Some cars that need timing belt replacements often hit the second-hand market all together as the owners try to avoid the cost of this, but the engine in your car has a timing chain which should be good for the life of the vehicle.

Other factors contributing to the pack-trade-in phenomenon include psychological barriers like a mileage of 100,000km recorded and the age of the car. In this case, the car’s 10^th birthday might be the catalyst for the mass selling.

Coolant leaks can be very tricky things, often starting at one point and then running down a hose, bracket or wire to drop on the ground where you’re not looking for them. If you definitely can’t find a leak, however, you need to start looking for another problem which could even be a leaking head gasket which is allowing the coolant to enter the engine’s cylinders.

The first thing I’d do, however, is change the radiator hoses., If they’re flattening out when the engine is running, that suggests they’ve collapsed internally and are unable to cope with the suction created by the engine’s water pump (which circulates the coolant around the engine). Even if the hoses look fine, they can be damaged inside and, left this way, can cause a blockage in the cooling system which can lead to the engine overheating. Perhaps this is already happening, and allowing the boiling coolant to escape the system via the pressurised radiator cap or the overflow system.

This goes against everything we were told a generation ago about the transmission fluid in automatic gearboxes. In the old days, bits of clutch and metallic band would wear off the internal parts of the transmission, requiring the fluid to be changed to avoid this dross doing damage to the actual transmission.

The theory is that with today’s better transmission designs, materials and precision engineering, this is not going to happen and the vehicle can use the same transmission fluid for its entire operational life. Maybe it can, maybe it can’t, but I’m with you in being a bit confused about it all. My preferences (and that of many transmission specialists) is that even in a sealed-for-life transmissions, you won’t be doing any harm by replacing the fluid at sensible intervals. And if you do happen to flush out some rubbish in the process, the expense might be well worth it.

Bear in mind there are two types of fluid change. The cheapest is simply a case of emptying the transmission’s pan and adding enough new fluid to take the level back to where it should be. The second, more expensive, option is what’s called a flush. In this case, the fluid in the gearbox is changed, as well as all the fluid in the lines, coolers and other plumbing around the car.

The only catch with this would be if Nissan regards a fluid change as some kind of tampering and takes the view that this would void any factory warranty. So check with Nissan first. The correct type of fluid for your car’s CVT is also crucial.