Overheating & Cooling

Cylinder Heads and Pistons

For many years cylinder heads were made from cast iron and still is used for most diesel engines. However, in the 60’s aluminium took over, initially for it’s superior heat conductive properties, but these days also to reduce engine mass.

When you are looking to buy a cylinder head there are also important criteria to consider. Firstly you have the option of buying a cylinder head on an exchange basis with a guarantee. There are many exchange shops around that offer cylinder heads on an exchange basis. The cylinder head is a part on a motor vehicle that is vulnerable to drastic heat exchanges within an engine.

Cylinder heads are the first components that will let you down if you abuse them and this I mean they don’t like drastic changes in temperature all round. An engine is designed to run at between 80-89 degrees C and any more for a long period usually results in a blown cylinder head gasket if you are lucky and only get away with the head being skimmed and refitted with a new cylinder head gasket.

The not so lucky will endure the pains of replacing the cracked head with another one. Extreme overheating can also cause the block to crack or a combination of the two. Head and block sealer is an additive that can be used to temporarily seal a fine crack and maybe get you home. I have driven with head and block sealer from Harrismith to JHB without loosing too much water and only just made it home. Read the instructions carefully. The contents of the tin must be administered in a certain way for best results.

For the not so fortunate a cracked head is the result once it is determined by a compulsory pressure test. To have a pressure test the cylinder head must be removed from the sub assembly. I have witnessed desperate clients pleading to have the head stitch welded and machined. These guys usually take drastic measures only to sell the vehicle to some poor individual that buys it thinking he got a good deal on the car or bakkie, only to walk in weeks later with the same piece of junk and place it on the counter. When you see this go down you can only pitty the man and advise him accordingly.

Thermostats

Most modern day cylinder heads are all made from alluminium alloys, although most diesel engines still come with cast iron castings for the cylinder heads. Heads don’t like the heat to soar over the desired operating temp limit when driving so make sure your thermostat is replaced at regular intervals. Years ago I discovered and pointed out a faulty batch of thermostats supplied by Landrover.

We fitted one to a 4 cylinder turbo diesel that we had remanufactured the (head, block and sump), for a Landrover dealership. The vehicle overheated and had to be towed back from Central Africa somewhere. I couldn’t believe that a new engine we had remanufactured could overheat and seize. My gut feeling told me to remove the thermostat on inspection. If you place a cold thermostat in a kettle you will see it start opening before the kettle reaches boiling point, this one remained closed. The result was, we were off the hook and Landrover SA discontinued the batch of thermostats.

When overheating an engine not only do the cylinder heads become stressed, but the pistons also made from aluminium alloy collapse, distort and shrink slightly. The change cannot be detected with the naked eye but when measured with a micrometer these changes are magnified. An example would be a new piston measuring 91.88mm to fit a std bore of 92mm should not collapse more than anything under 3 hundredths of a millimeter. With a standard piston to bore clearance of 2 hundredths of a millimeter this would mean the piston to bore clearance is now borderline at 5 hundredths of a millimeter, providing the bore is still standard and ‘on size’, (give or take another 100 th of a millimeter for bore wear), and its over the top.

This is the limit and experience has show that any more than this, results in an engine that uses oil and will show signs of loss in power. In most cases severe overheating will cause the rings to glaze the bore and collapse, thus loosing power due to the tension loss in the spring qualities left in the piston rings. Rings also tend to seize within their ring grooves of the piston due to the lack of spring tension forcing themselves outwards against the bore of the sleeve and sometimes break the ring land. You will immediately notice a nice constant stream of smoke emitting from your exhaust as you drive and guess what, your sub assembly will now have to be removed to replace the pistons, which ultimately means it’s time for an engine change or complete engine overhaul.

My advise is not only replace your thermostat on a regular interval but change all your hoses when they have reached the 100,000 km mark and always fill the radiator with at least 50%, good anti freeze-anti boil additive.

Viscous Fan Coupling

Another cooling component that is most often overlooked is the viscous fan coupling or clutch fan coupling. It is important to understand how it works. This component is probably responsible for more engine failures than any other cooling part on a vehicle simply because it’s failure is so gradual and unnoticeable. I have seen guys build up a new engine and not bother to replace the viscous coupling. Manufacturers fit them to most model cars and trucks as they are fuel savers and they allow the engine to produce more power. A fixed fan will also drain horse power from an engine powerplant.

The viscous fan is free to turn on it’s axis as the motor revs. In other words the engine RPM and the viscous RPM differ in their speed of rotation. It is designed to start locking-up at round about the time the engine reaches it’s desired temperature range which is controlled by the thermostat. The thermostat actually controls the viscous fan in principal because it regulates the engines expected temperature range. As the radiator cools the hot water from the engine, the thermostat allows only enough water in at a time, keeping the water at a constant temp within the head and block. The viscous coupling will then only engage again once the temperature rises again.

The problem arises however when the viscous comes to the end of it’s life span which is difficult for most people to understand because it always appears to be spinning in time with the engine revolutions. There is only one way to check it and that is when it is cold, on start up in the morning. You will hear the distinct rush of air as you rev up the engine. This noise will then fade away and quieten down completely after a 30 seconds or so. Watch the temperature climb on the temp guage, listening all the time. Once the temp reaches the half way mark after a few more minutes, the viscous fan coupling will reengage itself and you will again hear the wooosh of air being propelled through the fan blades as you rev the motor.

This indicates that the viscous coupling is working. If the temp guage reaches the half way mark and I mean, not a hair over the half way mark and the guage keeps climbing over the half way mark then you know it’s time to replace the coupling. They don’t come cheap so what a lot of guys do is remove them and fit solid fans adaptors. The problem with these are that when the vehicle attains a speed of lets say 110 km/h the fan reaches a static condition whereby the fan actually slows down the air flow through the radiator. Fixed fans are fine for round town vehicles that don’t exceed this kind speed. They are however very reliable in that they never give up sucking air and are great for traffic congested driving as they are constantly working whereas a viscous coupling only starts to really work when it gets to the desired operating temperature.

OIL PRESSURE

The Idiot Light

I have seen many a motorist drive his or her vehicle regardless of any warning signals. Even when the red light on the dashboard comes on they still keep on driving as if the light were a disco light. This light is often referred to as an ‘idiot light’, the red one with the outline of an oil can on it, it’s unmistakable, you can’t miss it, it’s bright red and red means stop. You should pull over and immediately switch off the car. Get a tow truck, no matter where you are; the costs incurred will leave you flat broke and carless for weeks unless you have a Motorite Insurance policy.

Oil pressure failure is usually attributed to a faulty oil pressure switch, a loose or severed wire and ultimately oil pump failure or a lack of oil channeled through to a particular part of an engine. Sometimes one of the oil channels gets a particle usually like a dislodged piece of matter large enough to partially block the oil channel causing the flow of oil to drastically slow down to a particular area of the engine. This may stop oil flow to a single crankshaft journal for instance and cause the crank bearing to seize thus damaging a big end or main bearing journal on the crankshaft.

Why a piece of debris can become lodged or dislodged is besides all logic if the engine was assembled by the original manufacturing engine builder at the assembly plant, but these things can happen, no one is perfect. Usually one would expect the oil filter to filter everything that passes through it. Change your oil and oil filter every 10,000 km’s without fail. Oil is the life of an engine so use good multi-grade engine oil. Filters can be purchased at your local motor spares shop. Here you can purchase excellent quality filters and engine oils.