Cooling System

The pattern itself is the diagnosis. Moving air through the radiator comes from the car's own speed above roughly thirty miles an hour, and from the fan below it, so a problem confined to standstill sits with the fan or whatever tells it to run. Open the bonnet with the engine hot and see whether the fan turns. If it does not, work through the fuse, the relay, the control output and the fan's own supply before condemning the motor. If it does turn, look instead at the radiator core for blockage.

The loss is real even if the floor is dry. On a modern circuit the most common culprits are a plastic pipe with a hairline split that only weeps hot and under pressure, a seam on the expansion tank doing the same, and a cap that releases early and vents to atmosphere every time the system heats up. None of those leaves a pool. Fit a pressure tester to the filler neck when the engine is cold, pump it to the stated figure and watch. If pressure holds and coolant still vanishes, look internally.

Watch the warm up rather than the running temperature. An engine that takes an unusually long time to reach normal, with a heater that blows lukewarm on a cold morning, is telling you the valve is not closing. The opposite fault shows as temperature that climbs steadily while the bottom radiator hose stays cool to the touch, because nothing is being allowed through. A cheap infrared thermometer makes both obvious in a couple of minutes. Rule out low coolant and trapped air first, since both produce readings that mimic a stuck thermostat.

Because a circuit with air in it cannot be judged. Until the system is properly full, the temperature sensor may be reading vapour, the pump may be cavitating and the gauge may be telling you something that has no relation to what the metal is doing. Workshops see plenty of thermostats and pumps replaced on the strength of readings taken from a system that was never bled. Many modern vehicles need a scan tool routine that drives the pumps and cycles the valves. Run it, let the car cool, then check the level and retest.

Whatever the handbook specifies, and treat the colour as a label rather than a specification. What separates one product from another is the inhibitor package protecting aluminium, cast iron, solder and rubber inside the same circuit, and those chemistries are not designed to be blended. Mix two and the additives can drop out, forming a sludge that blocks the narrow passages in a heater core or a radiator. If the history of the coolant in the car is unknown, drain and refill with the correct product rather than topping up and hoping.

Usually yes, because the seal and spring in the cap age along with everything else, and a tired cap can undo a new part quickly. A cap that opens too early lets coolant boil sooner and vents fluid every time the system gets hot. One that sticks shut as the system cools leaves a vacuum behind. That shows up as a top hose squashed flat by the morning, or an expansion tank sitting far above its mark. Each cap is made for one pressure rating, so read the OE number on the old cap and search for that.

An electric pump rarely leaks its way out of service the way a belt driven pump does. More often the brushless motor, its bearing or the control electronics give up, and the pump runs slowly, now and then, or not at all. Symptoms include a heater that suddenly blows cold, a thermal system warning on the dash and reduced power as the car protects the battery or drive unit. Listen for the pump running with the ignition on and read the fault codes before ordering anything. Then match the OE number printed on the pump body.

Electrified vehicles usually keep separate loops at different temperatures. The battery works best in a narrow, fairly cool band, the motor and power electronics can take more heat, and an engine, where one is fitted, runs hottest of all. Valves, heat exchangers and extra pumps move heat between those loops. A pump, tank or hose that looks the same as its neighbour may belong to a different circuit, with different connectors or flow. That is why fitting by appearance goes wrong. Note which circuit the old part sits in, then search for the OE number stamped or printed on it.

Squeeze it when the system is cold. A healthy hose feels firm and springs back. One that feels crunchy, rock hard or spongy, or bulges near the ends, has lost its structure and will fail under heat and pressure. Look closely at the ends, where cracks start, and at plastic quick connectors, whose locking tabs turn brittle and whose O rings flatten with age. White or crusty residue at a joint marks a slow weep. Moulded hoses are shaped for one route only, so take the OE number from the old hose before ordering.

It depends on where the blockage is. Leaves, insects and road dirt packed between the fins can often be rinsed out from the engine side with low pressure water, as long as you take care not to fold the soft aluminium. Internal blockage from mixed coolant or corrosion is harder to shift, and a flush rarely clears the narrow tubes fully. Replace the radiator if cold patches stay on the core while the rest is hot, if the fins crumble when touched, or if the plastic end tanks weep at the crimped seam. Match the OE number on the label or tank.

The sensor tells the control unit how hot the fluid is, and the fans, the pump speed and the warm up all follow that reading. When the sensor drifts or fails, the gauge may swing, stay on cold or jump to hot for no reason. The fans may also run all the time as a safety fallback. Compare the live reading on a scan tool with an infrared thermometer pointed at the same spot. If the two disagree, check the connector and wiring for corrosion before replacing the sensor, then order it by the OE number on the old one.

It usually makes sense. Both jobs mean draining the system and bleeding it again afterwards. On many modern units the thermostat also sits in a plastic housing that turns brittle with heat and can crack when disturbed. Putting a new pump next to a tired thermostat risks a second drain down a few months later. Always fit new seals or gaskets rather than reusing the old ones, and clean the mating faces. Some thermostats are electrically heated and wired into the control unit, so check the OE number on the old part and match it exactly.

Yes, within reason. After a hard drive, a hot day or a rapid charging session, the control unit may run the radiator fan and coolant pumps for several minutes to pull heat out of the engine, the battery or the power electronics. The car is designed to do this. It becomes a fault when the fan runs after every trip whatever the conditions, or never stops. That points to a sticking relay, a faulty temperature sensor or a control fault, and it can flatten the low voltage battery. Read the fault codes first, and ask a qualified mechanic if it carries on.

Go by the OE number on the old part, not by how it looks. Pumps, tanks, fans and hoses often change partway through production, gaining a different connector, mounting point or port size while looking almost the same in photos. The number is usually moulded into the plastic, printed on a label or stamped on a metal flange, and you may need to clean the part to read it. Where a part is split by build date, check the date on the identification plate as well. Type the number into the search box, or send it through the quote form. We reply within 24 hours, usually sooner.