Hot under the Bonnet

The following article is the elaboration of a post that I made in the forums a while ago. My good friend KO insisted I turn it into a tech article as a reference for those who are not regular visitors around here.

Proper cooling for an engine is like oxygen for us – you don’t care about it until you’re not getting enough of it. That’s why cooling systems tend to go unnoticed until something bad happens, usually accompanied by a lot of huffing, hissing and steam.

A lot of people tend to overlook the cooling system. They think as long as it holds water and hoses aren’t flying apart it’s all cool (no pun intended). Local mechanic logic dictates a massive 4-core radiator accompanied by the biggest clutch fan that will fit in the engine bay should equal one invincible, unbeatable cooling system that will weather any temperature extreme Mother Nature can throw at it. While the formula does sometimes work, it is the equivalent of using a shotgun to kill a fly – completely unnecessary. If huge radiators and fans are necessary for effective engine cooling, how do the Japanese make wimpy 2-core radiators and puny electric fans perform under the harshest temperature extremes?

Ask anyone a question about the cooling system and the answer will undoubtedly lead to the radiator. While bigger is generally better, there are also other finer points to consider about radiators. We here in Pakistan have mainly two choices; either get a new locally manufactured ‘jaali’ (radiator core for the uninitiated), or a used imported jaali from the junkyard. The better choice would obviously seem to be the new jaali, but that is not the case. All jaalis are not created equal. Although locally made jaalis may do an OK job of keeping the engine cool under most conditions, I would not recommend them. If you are planning to venture out towards the desert in the summer, an imported jaali is a must. A close look at an imported jaali will show that the fins zigzag between the pipes and they are also split and fanned out to expose as much metal to oncoming air as possible. More numerous, thinner fins do a much better job of dissipating heat than fewer, thicker fins as found in local jaalis. The metal used in imported jaalis is also better suited to resist the corrosive effects of water, and hence they last longer. Imported radiators are also better because the construction is far sturdier compared to anything manufactured locally. The tanks are securely sealed and attached to the jaali to minimize the likelihood of leakage. Provided the pipes are clean, an imported radiator, complete with upper and lower tanks and at least a 3-core jaali, is the best bet for your jeep.

The next most obvious part of the cooling system is the fan. Locked clutch fans are all the rage in Pakistan. I say locked because the gel inside the clutch is usually removed by mechanics and replaced with epoxy to lock it solid. With a properly functioning clutch, the fan accelerates only up to a certain engine RPM beyond which the gel in the clutch allows it to slip. The fan does not need to spin any faster than that speed because the rest of the cooling system is designed with this fan speed in mind. This makes for less wind noise from the fan and also less horsepower robbing parasitic drag on the engine, especially at high engine speeds. However, since all clutch fans available here come from the junkyard, nobody wants to take a chance with them and simply locks up the clutch, defeating the entire purpose of the clutch. The only fans designed to run all the way to redline with the engine are flex fans, which are not available in Pakistan, hence I won’t discuss them here.

While factory fitted clutch fans are adequate, they are antiquated and can be unreliable. Besides, if adequate was good enough, we would all be running around in Mehrans and Cultuses (Cultii??) wouldn’t we? That’s why there are electric fans. However, local mechanics avoid anything even remotely associated with electronics like the plague, so you can imagine their aversion to ripping out a ‘perfectly good’ clutch fan in favour of an electric fan. Far superior to clutch or flex fans, electric fans don’t have belts to break, slip adjust, etc. They feature longer bearing life, and turn on automatically when used in conjunction with a temperature switch. A manual override fan switch also allows the driver to switch the fan on or off as deemed necessary. For instance, you don’t want the fan running while crossing deep water – the fan will act as a propeller and get pulled into the radiator. Electric fans also free up precious horsepower by placing zero parasitic drag on the engine and make far less noise. Electric fans also almost always outflow similarly sized clutch fans, mainly because of the higher speed at which they spin and the presence of an integral fan shroud.

The next most important and usually missing part of the cooling system is the fan shroud. Many ask, “What is this magical shroud you speak of?” My answer is it is what prevents the fan from recycling hot air in the engine bay instead of pulling fresh cool air through the radiator from outside. For the benefit of those who don’t know, the shroud fits between the radiator and fan. It acts as a tunnel through which the fan pulls air only through the radiator instead of around it. The fan shroud is critical on air-conditioned vehicles since the condenser causes a substantial reduction in airflow to the radiator. For the same reason many CJs feature an electric pusher fan mounted on the condenser. In the absence of the shroud, the fan will pull hardly any air through the radiator, and lead to an overheating condition, especially when the A/C is switched on.

Moving further back, we get to the thermostat. While hardly ever mentioned, removal of the thermostat by local mechanics is a huge problem here. Apparently, we as a nation seem to have accepted the notion that the average sidewalk mechanic who has no formal training in the finer points of automobile engineering knows better than the automobile engineer who designed the cooling system after studying thermodynamics for several years in tech school. Of course, automobile engineers, blinded by their Western depravity and decadence, made thermostats simply to help out the auto repair industry by increasing incidences of overheating. Therefore, the thermostat, having been designed and installed by Satan himself, has been reduced to a non-entity over here.

All sarcasm aside, the thermostat is a critical component of the cooling system. Its function is to regulate engine temperature and the flow of coolant through the system. No thermostat means the engine takes a long time to get up to operating temperature because the coolant is circulating freely and dissipating heat from the engine when the engine actually needs it. The optimal operating temperature for diesel engines is about 80 degrees Celsius. Since diesels don’t have an ignition system they need heat for complete combustion. That’s why diesels run so much smoother once warmed up. Heat is also necessary for burning off condensation that occurs inside the crankcase and causes acid formation and sludge deposits. In fact, EFI vehicles require a certain threshold temperature for the engine to enter normal operation, or closed loop operation. Removal of the thermostat ensures the engine never warms up enough to enter closed loop operation, especially on short trips.

Thermostats are also notorious for getting stuck either open or closed, and this is why lazy mechanics will often remove them completely. While getting stuck open won’t cause much damage, getting stuck closed causes severe overheating. Therefore, it is a good idea to always test junkyard thermostats by immersing them in boiling water – the valve should quickly be fully open. A neat trick taught to me by Mr. Taimur Mirza is to remove the thermostat from the boiling water and dunk it in a cup of automatic transmission fluid. This process repeated a few times will ensure the thermostat never gets stuck again! Also, carry a spare thermostat in the toolbox. And don’t forget a spare gasket too!

The radiator cap is also an often ignored part of the cooling system. It ensures that the system is pressure sealed, hence reducing the chances of a boil over. In extreme conditions the coolant temperature will often exceed 100 degrees Celsius. The only thing keeping the coolant from boiling is the pressure inside the cooling system. Once the coolant starts boiling and turns to steam it loses the ability to absorb heat from the engine. The cooling system will never remain properly pressurized in the presence of air pockets. Air is compressible, causing loss of pressure in the cooling system and increasing the chances of a boil-over.

Air pockets in the cylinder head cooling passages can cause localized stress, which leads to cracks in the cylinder head. This also happens to be the most common complaint with B-series engines. For this reason, the radiator cap should always be located at the highest point in the cooling system, and a proper coolant fill procedure should be followed. Coolant should be added to the radiator until it is full. Then the engine should be started and idled fast to bring it up to operating temperature while adding coolant as the level goes down. This ensures that the thermostat opens and the water pump cycles all the air out of the system before the cap is replaced. Failure to do so will result in less than optimal performance, regardless of the size of the fan and radiator.

Many people choose to retain the stock location of the radiator cap, which often happens to be the thermostat housing on Japanese engines, especially Toyota B-series engines. The reason why the filler cap is located here is because a great majority of these engines came out of trucks and buses where the only way to reach the engine is through the access panel under the seat inside the cab and the radiator is mounted fairly low. On the other hand, the design of the Jeep engine bay ensures than the radiator is the highest part of the cooling system. Therefore, it only makes sense to relocate the cap to the radiator. The case may be different with other modified vehicles, but whether one chooses to retain the stock location or not should not merely be a matter of preference – it should always be dictated by function.

While we’re on the topic of coolant, how many people actually know what coolant is? Most will say it’s water, and a few will say it’s green water, but hardly anyone will mention why it’s green and what it does. Coolant doesn’t merely cool the engine. It also protects the system from corrosion, calcification, sludge build-up, overheating, and freezing. The sweet smelling, slippery green liquid is ethylene glycol, and now many late model vehicles use another type of long life orange coolant. The green and orange stuff are not interchangeable and should never be mixed!

While not a good conductor of heat on its own, coolant is used in a 50:50 mixture with water. This ratio provides good boil-over and freeze protection besides preventing other nasty things from happening to the cooling system. It is a good idea to use distilled water for the coolant mixture whenever possible. Distilled water, being free of oxygen and minerals, poses no risk of damage to the cooling system like ordinary tap water. In fact, even the tap water we get here is hard water loaded with calcium that deposits itself everywhere. One look at neglected bathroom and kitchen fittings is enough to prove this point.

Finally, the most commonly overlooked part of the cooling system, the overflow reservoir. If I had a nickel for every time I have seen the overflow tube from the radiator dangling below the vehicle, I would be a rich man. It’s simple physics – coolant expands to a certain extent under heat, gets pushed out of the radiator into the overflow reservoir. Coolant cools down, ‘shrinks’ and sucks displaced coolant back into the radiator. No overflow reservoir means every time the vehicle is driven and parked, air gets sucked back into the system. Simply strapping an empty bottle of mineral water with the overflow tube stuck inside saved us gallons of water in Baluchistan.

7 thoughts on “Hot under the Bonnet”

  1. Hi,

    Imad; what a wonderful efort. You have addressed a very common problem faced by almost all the diesel converted vehicles, specially 4×4s.

    The article was very very informative.

    Looking forward to more technical tips and articles from you.

    Keep it up.


  2. Imad,

    Good article Ims. One correction – You need to get rid of third paragraph as its a repitition of the first paragraph.

  3. The first paragraph was actually supposed to be a leader for the main article with a “more” link appended to it. However, the website layout was revamped so it appears to be a repetition.

  4. Thank you for your insight into the cooling system.It was very informative! I have a question maybe you can awnser.with a carbeurated engine no concerns about pollution, just performance and long life what is The optimum temperature?I know some people will run 210 for a few horsepower!What would be the absolute coldest I could run an engine without concerns about excessive engine wear?

    My particular engine is an aluminum Yamaha V4 1200cc water cooled with electric fan. I would give up horsepower for engine life and less oil breakdown!

  5. hi

    great article. Can u suggest a good electric fan for radiator that is easily available in pakistan

    keep it up

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