Coolant header tanks
Lotus Excel Coolant header tanks
"Barn find" eclat engine bay, showing the single-cap header tank.
Twin-cap header tank, blanking cap at the front, pressure cap (110 kPa, 16psi) at the rear
The cooling system in the Excel is pretty conventional - a belt-driven water pump on the front of the engine circulates coolant through the block. A thermostat at the top of engine opens at a set temperature (72 or 82 degrees Celsius, depending on market) to allow it to circulate through the radiator, and a take off at the back circulates coolant through the heater matrix behind the dash when the valve is opened using the dashboard controls.
One or two electric fans operate when the electric otterswitch senses coolant in the top hose getting too warm (somewhere around 90 celsius). Thanks to some clever wiring, involving diodes, if a fan motor fails or the fan relay fails (where fitted), the "fan fail" light on the dash will illuminate. These fans also run if aircon is in use, to keep the a/c condenser cool.
The "fan fail" doubles up as a low coolant warning, via the sensor on the header tank, and it's common for new owners to be caught out by this. Rule 1 of fan fail is - check the coolant level.
Depending on age and specification - two different header tanks were fitted. The simpler is the single cap, single compartment, tank - this has a standard pressure cap and, if it needs to vent, will lose coolant through the overflow pipe onto the road.
The late models, particularly SEs, have a twin tank system, where the larger twin-cap tank is actually partitioned roughly in the middle. The front compartment is the usual coolant reservoir and is linked, by a small capillary pipe, to the normally empty rear compartment. As heat builds up, coolant can overflow into the rear compartment, being drawn back through the pipe when the system cools down again. This reduces instances of coolant loss - but only if you have the right sort of cap on each compartment. The front needs a standard BLANKING cap (not pressure relief, so no springs) so that pressure can build up. (If you do put a pressure relief cap on the front - there's nowhere for the coolant to go when the valve operates anyway - there's no overflow on the filler neck). The rear, which is a larger cap on the Lotus tank, is a pressure relief to allow coolant to escape via the overflow pipe as a last resort. (If you put a blanking cap on the rear tank, the system will never pressurise because it's open to atmosphere, at the overflow, all the time.)
Topping up is done through the FRONT tank and the level should sit just below the bottom of the filler neck. Don't worry about level in the rear compartment, it'll sort itself out as long as you keep the front at the right level.
Beware, though, the tanks are steel and do corrode. If you get a build up of rust inside the twin-cap tank it can block the capillary tube and the whole system will stop working properly, leading to leaks and overheating. Best bet then is to replace with an aluminium tank - though you may need to play around a little to get the wiring for the coolant sensor right, depending on supplier.
It's a simple job - the three nuts that hold the tank to the body can be accessed from the wheel arch, without taking the wheel off. The level sensor can be pulled out of the old tank and inserted into the new one, using something like windscreen washer pipe to make an electrical insulating grommet, and a small blob of silicone sealant to ensure that coolant won't leak out.
Aluminium twin-cap tank ready for fitting.
Underside of tank showing mounting posts and added flying lead for level sensor earth.
Tank installed. Old blanking cap on front, new 16psi pressure-relief cap on rear above vent pipe.
Old level sensor transferred to new tank, using an off-cut of washer pipe as a grommet to insulate it from tank body.
Level-sensor flying earth lead connected to loom using spade connectors.