It’s that awful feeling; watching the water temperature needle rise while you frantically look for a place to pull over, mind racing at a million miles an hour. Have you cooked your engine? Cracked a head? Maybe it’s just a faulty gauge – oh god please let it be something minor!
Sound familiar? Well, we’ve got bad news, it’s not getting any cooler and those little issues that you didn’t notice or attend to over winter are about to rise up and bite you square on the arse. That is, unless you grab your pen and paper for another five minute masterclass, as we explain five of the most common causes of overheating 4WDs.
CAUSE 1: OVER- OR UNDER-FUELLING
Petrol engines run hot if they lean out; that is, if there’s not enough fuel getting into the engine. Diesel engines are the exact opposite – too much fuel and they get hot. This is where air-fuel ratios come into the question, and why it’s so vital – especially in a diesel – to have your vehicle maintained and correctly tuned.
Dirty fuel injectors in a diesel have a tendency to not seal properly, and can dribble excess fuel into the combustion chamber. Too much fuel means too much heat, and all of a sudden your EGTs are sky-rocketing, closely followed by your coolant temperatures. Petrol motors running lean from undersupply of fuel isn’t as common an occurrence, but can stem from a blocked fuel filter, stuck injectors or a failing fuel pump. Most of the time however, the vehicle will just run poorly rather than run hot.
Cross poor tuning off your list by having a mechanic check your air-fuel ratios ideally on a dyno, then address the cause of the over- or under-fuelling.
CAUSE 2: FAULTY WATER PUMP
As the name suggests, a waterpump’s job is to circulate the coolant throughout your engine and your cooling system. It needs to move the coolant a decent distance too – up through the engine and cylinder head’s water galleries, and in some cases through external components like water-cooled turbos, carbies and LPG mixers.
Often, the problem is the water-pump’s vanes (fan blades) can and do wear down over the years – rusting away due to poor servicing of the cooling system or age. It then gets to a point where the water pump vanes simply aren’t big enough to move coolant through the system, so the hottest bits never circulate through the radiator to cool down. This is a tricky one because your waterpump can look and act fine from the outside, spinning without any play or resistance in the bearing. The fan hub itself too can fail – the viscous coupling will just spin freely even when the engine is hot if it’s passed its used by date.
If you suspect your water pump isn’t flowing enough coolant, simply replace it. They’re cheap enough that if nothing else, you’ll know it’s good for another 100,000km. Test the fan hub by warming your 4WD up fully. Shut off the motor and once the fan has stopped spinning, give it a spin manually. It should move no more than a quarter of a turn at the absolute maximum – any more and there’s a good chance it’s slipping too much and needs replacing.
CAUSE 3: BLOCKED RADIATOR
So you’ve been playing in the mud again, and now your 4WD’s running hot. You’ve given it a good wash and gotten rid of all mud you can see, and your radiator’s looking clean as far as you can tell, but it’s still getting hot running up the hills. What’s the go? The problem here is that when you hit a mud hole, the mud punches up through the fins in your radiator, and more often than not the majority of it dries on the back (fan) side of the radiator. Unless you have a habit of driving everywhere in reverse, that’s the side the air should exit, carrying away the heat from the radiator. When you give your radiator a wash-out from the front with the garden hose, you’re not getting all of the mud – only the stuff at the front. Have a look at your radiator from behind – look through the fan and chances are you’ll be able to quickly see just how blocked it is.
This is a simple one, but take it easy so you don’t stuff anything up. Remove your fan, shroud and finally your radiator, and then with the garden tap on a medium flow, clean the radiator from both sides. Use a soft-bristled dustpan broom to help if you like, but be gently. Don’t use a pressure washer as it will distort the cooling fins. If you need to, let it soak for a while. Wash every little bit of mud out of the fins until the water from the hose runs clear.
Be careful when you are refilling your cooling system, that you don’t get an air pocket in the top of the motor. The best way to guard against this is to first fill your radiator until it’s full, then undo one of the hoses leading into the heater tap on your firewall and top the coolant up again. Finally, with the radiator cap off, start the engine and let it sit at a fast idle (1500rpm or so) until the thermostat opens. ‘Burp’ the motor by leaving the cap off and massaging each of the coolant hoses until you’re sure no air bubbles are coming up out of the radiator neck. Do this any time you drain the cooling system.
CAUSE 4: LOSS OF PRESSURE
Without getting way too deep into the science behind it all (mainly because we never paid attention in science class), vehicles run pressurised cooling systems to raise the coolant’s boiling point. That’s why if you’ve got an aftermarket or numerical temperature gauge, you’ll sometimes see the coolant temp climb over 100°C – enough to boil unpressurised water in your kettle, but not a drama for a modern cooling system.
Problem is, if the system loses pressure in any way, the engine will start to cook. It could be as simple as the spring in your radiator cap losing tension, or it could be as painful as a blown cylinder head gasket. Don’t overlook other simple things like loose hose clamps and leaking hoses, too.
Any good mechanic will tell you to start with the basics first. Don’t book it in for a new cylinder head without first trying a new radiator cap. Loss of pressure will also mean loss of coolant so keep an eye on your radiator and overflow levels, and if need be speak to a mechanic to get the cooling system pressure-tested to identify the problem.
CAUSE 5: FAN SHROUD
Here’s one we’ve definitely been guilty of at some stage. It’s late, you’ve just about got your 4WD back together but the fan and the shroud are being pains in the arse. Then the shroud, brittle from 20+ years in a hot engine bay, cracks in three big pieces. “Stuff it”, you think, as you put it all back together without the shroud – and from there on in, the vehicle starts running just that bit hotter, especially when you’re working it hard.
Fan shrouds don’t just protect fingers from spinning fans – they actually create a funnelling effect, forcing air through the radiator so that even without any natural airflow, the parts of the radiator that aren’t directly in line with the fan will still have evenly distributed airflow.
This is an easy one. Get a hold of a fan shroud, and fit it!
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