Repairing a Coolant Hose Problem, Coolant Temperature Sensors, Cooling Fan, Bleeding


Compiled by Dot

A label on the expansion tank should tell you what coolant was used. There are basically 3 main types of coolant (different types should not be mixed):

  • "IAT" (Inorganic Acid Technology) is the traditional (older style) "green" formula antifreeze that contains phosphate and silicates. This provides good protection for cast iron and aluminum engine parts, as well as copper/brass radiators in older vehicles and aluminum radiators in newer vehicles. The silicates in IAT coolants even lube rubber hoses ;) The corrosion-fighting chemicals are fast-acting but wear out after two to three years or 36,000 miles of average use.
  • "OAT" (Organic Acid Technology) is usually dyed orange or pink to distinguish it from other types of antifreeze. The OAT corrosion inhibitors are slower acting and provide protection over a longer period of time. OAT coolants typically have a service life of up to five years or 150,000 miles. Though OAT provides good protection for aluminum, it may not be the best choice for older vehicles with copper/brass radiators because of the lead-based solder used in the radiator. Some say OAT-based coolants may also provide little protection against cavitation erosion in water pumps with aluminum housings (unless the pump impeller is carefully designed to minimize cavitation).
  • "HOAT" (Hybrid Organic Acid Technology) antifreeze is usually dyed yellow but may also be dyed orange or green. The additive package in a HOAT formula coolant contains organic acid corrosion inhibitors (OAT) plus silicates for added aluminum protection. Most of the antifreezes in this category also meet the European "G-O5" specification for hybrid extended life coolant. The service life for HOAT is also five years or 150,000 miles.

From mid. 2002, Rover/Lotus changed from the older IAT to OAT Coolant. Most older non-OAT cars will be now have been fully bled and replaced with a HOAT coolant. Colour is also not an infallible guide to coolant type as some manufacturers mimicked colours to increase the prices of their products (*allegedly); and indeed the older S1 yellow coolant (non-OAT) is Unipart super plus 2 (or if not available, use an ethylene glycol based antifreeze that meets specification BS 6580 and BS 5117); and the newer S2 green coolant (OAT) is Unipart super plus 4.

The different coolants should not be mixed. Arguments abound on just how bad mixing coolants really is (some anecdotal evidence of a precipitation (sludge), although it's not conclusive), but the advice is that it reduces the benefits at least - so just don't mix them. To swap from ony type to another, fully flush the old coolant out first. For small topping up you can use plain water, but if you live in a hard water area, distilled, de-ionised or filtered rain water should be used.

The MIN level on the expansion tank is actually the correct coolant level when the engine is cold (when the engine is warm the level will rise). Be very careful if you try to remove the cap when the engine is warm - it may splash you with hot coolant.

Opinion on the use of "Water Wetter" (eg from Redline Oils) is divided. It is designed to helps to ensure Head Gasket Failure does not happen because it evens out the temperature within the engine block by creating a slick surface and stops isolated heat spots building up and damaging the engine and components. However, some people have found that it reacts badly with the rubber in some after market replacement head gaskets (causing HGF).

Bob van Melzen


There was a faulty batch of pressure caps (you can tell by the hole not being in the middle of the cap). A faulty cap means that the temperature is allowed to get too high and is therefore a possible cause of head gasket failure.

It is also possible for air to get trapped in the pipe that runs on top of the exhaust manifold on older cars. Older cars have a pipe running above the exhaust manifold (this point being higher that the bleed nipple), so some people have added a bleed nipple at that point. Later cars have this pipe running below the exhaust manifold so it is not a problem.

Mike Knowles


The standard Elise radiator is apparently (according to Lotus) good for 200bhp without modification. It contains a fan on the underside which is switched by means of the temperature sensor in the pipework next to the engine in the back. It turns on at roughly 99-102C, and turns back off at roughly 90-92C. The coolant reading on the Stack dash flashes at anything over 100C.

The standard radiator has plastic side panels, which as a result of the heat/cool cycle have a tendency to crack and leak. This is a clam off job to replace the rad, and there are 3rd party variants available with steel side-tanks.

A frequently occurring fault with the radiator system is the left hand side radiator return hose coming off the radiator. This is normally held in place with a jubilee clip, and some either leave the factory with the clip not especially tight, or they become loose over time. If your rad hose clip has never been checked, never failed, or you were unaware of this problem, it's a good idea to have it checked by a garage when next in for service.

Sometimes, the gauge on the Stack dash can start to fluctuate wildly. This can be caused by several things. The causes are mainly due to the way the Stack dash gets it's information about the coolant temperature. This is via a sender that is located at one of the highest points in the cooling system, hence the following can indicate what's wrong:

- Broken sensor or wire
- airlock in coolant system at the sensor
- loss of coolant

However the main point here is to stop driving immediately as the causes range from trivial to fundamental, and the latter can be very expensive and lead to warping of the alloy cylinder head if the car isn't stopped immediately and the engine turned off.

In terms of the broken sensor or wire, the sensor is a known failure item, and is available at any Lotus or Rover (or trusted 3rd party Rover) dealer.

In terms of the airlock in the coolant system, this requires the coolant system to be bled properly.

Loss of coolant can occur from various places, namely the front rad hose, from the rad pipes that go through the chassis sidemembers, and from an Inlet Manifold Gasket or Head Gasket Failure (this is the worst case scenario).

In all cases, the temperature reading is liable to jump about in the region of roughly 20C in a second, thereby pointing to one of the above scenarios.

Some people have warned against the use of Redline Water Wetter and other such coolant additives, and also against the use of 3rd party coolant as they are a supposed cause or contributor to Head Gasket Failure.

Repairing a coolant hose problem - Bleeding The System
Bob van Melzen


I was waiting behind a (German) driver at a petrol station (who decided to go for a P* and left his car next to the pump). The temperature rose and suddenly there was steam and green / yellowish stuff everywhere. The left hand hose just popped off the radiator.

At that moment I realised that someone had told me that the securing clips at the hoses are not of the best quality (probably cheapest or lightest available).

Fortunately we were at a petrol station, just outside Hamburg, so the easiest thing to do was to call the nearby Hamburg dealer. Normally I do a lot of repairs myself, but with specific things, I need a workshop manual, and that I left at home! I did not know how to fill the system and where the (many) air-traps are in the system, and you do not want any air in you cooling system for obvious reasons.

We put the hose back on the radiator, secured the clamp, called the dealer, and waited. The fastest service vehicle on earth to appeared less than 10 minutes later! - a . yellow 220 TURBO-ELISE, with can anti-freeze, tools and pressure-test pump.

The fixing procedure was easy, and showed to us by Lutz Weinschenk, the technical brain behind the Elise-tuning at Thielert.

Fill the system with the anti-freeze (and water), and let the air out at 2 points :

  1. In the water rail below the distributor is a bleed-plug. It's best to use an 8 mm socket (long) and open it until coolant comes out (take care not to misplace the sealing washer).
  2. In the radiator outlet hose (on the left hand side) is a bleed plug that you can reach by taking the tyre-repair-bottle out and reaching forward.You can undo this one by hand. Open the plug, and close again when more no air comes out

Next, put the pressure test pump on the header tank and pressurise the system ( 35 kPa = 5 lbf/in ) to circulate the coolant. Open the 2 bleed nipples again until a steady stream of coolant flows from the bleed ports, then close and tighten. When necessary refill the header tank, and repeat.

Take the pressure pump off; fill the system till the max level and still with the cap off, run the engine at idle until the water temperature reads 60-65C. Increase the engine speed to 2000 rpm and observe the running temperature gauge which goes up to 93-98C, before dropping to 90 due to the opening of the thermostat and circulation of air pockets.

After a further rise and fall, re-bleed the radiator hose with the engine running (still at 2000 rpm) This is when you burn your fingers, so take a cloth!

If all air is out of the system (and the coolant temp not yet hot), then let the engine run at 2000 rpm until the return pipe feels hot, then put the pressure cap back on the header tank.

Last thing to check is that the radiator fan cuts in at 103-105C and brings down the temperature to 90 - 95C, before the fan cuts off and the cycle repeats.

Stop the engine now to cool down before filling the header tank to the "low" mark.

The only problem with this by the roadside is that you do need a pressure pump to circulate the coolant and get the air out.

Thanks to Lutz (and Dealer Thielert) we were back on the road in half an hour.

One last conclusion.change those bl**dy hose clamps! It is also recommended that you change the coolant once every 2 years with the proper antifreeze-mixture (from Lotus or Rover/MG) to protect the aluminium Engine against corrosion.

Coolant Temperature Sensors

Compiled by Fd


The Elise has two temperature sensors, one for the ECU and one for the Stack temperature display. They are located on the coolant outflow elbow on the left hand (gearbox) end of the engine.

The Stack temperature sensor is coloured Blue and is the lower sensor of the pair.

New Rover Part Number : YCB100420 - Old Rover Part Number : GTR270

Resistance is 100C - 75ohms, 0C - 2250ohms, measured between either of the sensor wires (they are common) and the sensor body (earthed to the engine).

The ECU temperature sensor is coloured Brown and is the upper sensor of the pair.

No Rover part number at this time, but it is a standard Rover MGF part.

Resistance is 100C - 280ohms, 0C - 5000ohms measured between the two sensor wires (this IS different from the blue sensor !).

Cooling Fan

Compiled by Fd


The cooling fan is located under the radiator in the front crash structure, it is a front clam off job to remove the radiator or fan.

The cooling fan is controlled by the ECU, which reads the coolant temperature from it's own private temperature sensor. Manufacturing tolerances between the Stack sensor and the ECU sensor can cause the fan to cut in and out at different actual and displayed temperatures.

If the cooling fan runs all the time it is likely an ECU temperature sensor or related wiring fault. Under these circumstances the ECU will run the cooling fan permanently.


Compiled by FDA


Air in the cooling system of the Elise can cause all sorts of temperature problems, specifically airlocks and subsequent overheating at low speeds, it is essential to make sure the cooling system is in perfect working order as coolant flow problems and temperature instability are thought to cause the dreaded Head Gasket Failure.

Due to the nature of the cooling system in the Elise it can be difficult to bleed. Earlier cars are more difficult to bleed than later cars as the coolant pipework in the engine bay was redesigned. You can tell whether you have an older or newer design by looking for an alloy pipe running across the bulkhead just in front of the engine, if the pipe is high on the bulkhead then you have an older design, if the pipe is low on the bulkhead (near the bottom of the bulkhead) then you have a later design.

The cooling system has 2 bleed screws designed to aid with bleeding the system should air get into it. One is on the metal coolant rail running just under the left hand end of the cylinder head and is an 8mm bolt. The other is a thumbscrew located on the coolant outflow pipe on the radiator, this is the left hand pipe, the pipe leaves the radiator at the very front of the clam, and can be reached simply by pulling back the left hand end of the plastic radiator shroud and reaching into the clam, there is no need to remove the wheel arch liner as suggested by the Lotus service manual.

Disclaimer : All information is supplied as a guide only.
No Guarantee as to its reliability can be issued.
You use this information entirely at your own risk.

No Reproduction or Reuse without prior written consent.

© Elise FAQ Team 2002