Lotus Elan Central
Thermostat Problems (Diagnosis and Replacement)


The Thermostat sits on the end of the engine and regulates the coolant flow from the engine to the radiator. It its normal state it is CLOSED and it keeps the coolant within the engine. This allows the engine to get up to running temp and helps with emissions and mileage. Once the coolant within the engine rises to running temp/thermostat temp, the thermostat will open and allow the hot coolant out of the engine and start the circulation loop to and from the front radiator.

During hot and cool driving the thermostat will open and close as you drive to regulate the engine coolant temperature. Thermostats are made to open at "SET temperatures." The O.E. thermostat of the 4XE1-T engine is 180F/82C !!

Thermostats usually last about 60-70,000 miles. They usually fail in the closed position which traps the coolant within the engine with no way to circulate to the radiator. This leads to overheating and can result in problems such as blown gaskets or worse! If the thermostat fails in an open position (very rare), it will cause the coolant loop to be permanently open which will cause the engine to take forever to warm up and run at a very low operating temperatures.

How will you know when the thermostat goes out ??

The car will warm up as normal, but the temperature gauge needle will continue to rise. It will become hotter and hotter until something is done about it. Since nothing is wrong except the thermostat, you will NOT see the engine light come on, nor will you notice the engine running abnormally!! I was lucky enough to see the needle rising (my pilot skills paid off with my regular gauge scans!) when mine went south! The radiator cooling fans did NOT turn on at the appropriate time (3/4 of the gauge) and the needle continued to rise. I realized later the reason the cooling fans did not turn on is that the radiator temp probe "thermal switch" is located on the far side of the thermostat housing (see diagram below) and since the hot coolant was not circulating it was not there to make the probe hot!!

The best thing to do when this situation happens is to stop the car before the high temperature causes a serious problem!! I do not recommend continuing driving with the thermostat out. You should find a good parking spot and turn the engine off and let it cool! Do not open the radiator cap or you will be rewarded with scalding hot water in your face!

I was close enough to home AND I realized what was happening so I risked it and continued home with my eye glued to the needle! In this situation there are only a few things you can do to continue driving:

(1) Minimize engine RPMs as you want to keep the heat production down to as little as possible. Do not try to drive fast thinking the air flow over the engine will cool it!!--this will just cause the heat output of the engine to go up and the temperature will continue to rise.

(2) Turn on the heat full blast and point the vents away from you. You will notice the heat coming out of the vents is extremely HOT. The engine is linked to the heater core (a small radiator!) from within the engine circulation, so using the heater will actually draw heat off the engine coolant at a small rate! This may be enough to keep the temp needle from rising more or bring it down a needle width.

If these tricks do not stop the advance of the temperature needle then you MUST stop (or you will be stopping soon anyway!).

Condition: Overheating

Coolant Leak Fill/pressure test system
A/C Condenser fins clogged Remove/clean condenser
Radiation fins clogged Remove/clean radiator
Thermostat stuck closed Replace thermostat
Clogged cooling system passages Clean/flush cooling system
Water pump malfunction Replace fan clutch
Retarded ignition timing Reset ignition timing
Cooling fan malfuncion Test cooling fan/circuit
Cooling fan motor malfunction Test fan motor
Cooling fan relay malfunction Test fan relay
Faulty radiator cap Replace radiator cap
Broken/slipping fan belt Replace fan belt
Restricted exhaust Repair exhaust system

To replace the Thermostat you will need to acquire a new Thermostat and a thermostat gasket. You can contact your local Isuzu (or Lotus) dealer and see if they have the parts in stock--they may not. Luckily thermostats are a common enough part to be readily available in automotive parts stores! And they are cheap too! Call or go to your favorite auto store and see if they have the parts you need. They will likely have thermostats of varying set temperatures, all of which would fit the car! The common temperatures are 180F/82C, 185F/85C, and 195F/90C . Any of these will work properly in the Elan, but if you cannot find the 180F/82C then get the 185F/85C as it is very similar.

If you cannot find a thermostat you like, you can put the car in a "temporary mode" by simply removing the thermostat and re-assembling the engine without one! This will allow you to drive the car home or to somewhere that has the proper part.


Difficulty level = 2 out of 5      —       Cost = about $10

What's Needed

Essential Parts:

  • 1 to 2 hours' time
  • a small flat screwdriver
  • a 12mm socket wrench
  • a new Thermostat (test it -- see text below)
  • a new Thermostat housing gasket

Possible Additional things:

  • Loctite for replacing the bolts (optional)
  • Thermostat gasket sealant (optional)
  • "fine" and "super-fine" grit sandpaper
  • a small adjustable wrench (optional)
  • new hose clamps and/or hoses
  • small flat scraper tool

Removing The Old Thermostat

  1. Let the engine cool !!
  2. Turn the heater dial to the hot end (for the entire procedure).
  3. Locate the area below the Cam Angle Sensor where the rounded aluminum Water Outlet Pipe sits -- This is what houses the Thermostat. (see picture)
  4. INSPECT the condition of your existing hoses and hose clamps in the vicinity of the thermostat as now is the PREFECT time to replace any of these!
  5. Remove the Air Filter canister and intake pipe to the turbo to make working room.
  6. Plug the turbo inlet with a rag so nothing gets in there while you are working!
  7. Remove the 2 small hoses coming off the intake if needed. These have small hose clamps and be sure not to let them fall off into the abyss of the engine!
  8. HINT: When removing hose clamps you may want to remember or write down how many turns of the screwdriver you used so you can replace them to the same or similar tightness!!
  9. If you have a bypass valve you will likely need to loosen the end clamps to rotate it up and out of your work way--removing the pipe is not necessary.
  10. Clean all the gunk and dirt out of the area BEFORE you go further and open anything vital up.
  11. Remove the radiator hose from the end of the Water Outlet Pipe by loosening the clamp -- Coolant will spill out and all over the tranny and floor so be prepared and be sure it is cool !!
  12. Remove the small hose off the front of the Water Outlet Pipe by loosening the clamp.
  13. You may need to remove the 'Radiator Temp Sensor (thermal switch)' (see picture above) from the Water Outlet Pipe which can be done now -- you can leave it makes the job easier if you remove it.
  14. Remove the 2 housing screws (12mm) from the Water Outlet Pipe -- clean them with a wire brush if needed to remove corrosion or grit.
  15. Carefully pull the Water Outlet Pipe off the engine to expose the Thermostat.
  16. The gasket is between the Water Outlet Pipe and the Thermostat and must be removed COMPLETELY before installing the new one !!
  17. Carefully remove the entire gasket from whichever side is has stuck to. You may need a scraper tool to assist you but be gentle and DO NOT scratch up the aluminum parts !!
  18. If the Gasket is stubborn (as was mine--very!) you may need to use some "fine" to "super fine" grit sandpaper to help remove it and leave a smooth surface.
  19. If the Radiator Temp Sensor is removed you can easily handle and clean the Water Outlet Pipe as it will be free of the engine. If not be careful not to stretch/damage the Radiator Temp Sensor's single wire!!
  20. Rinse the area clean of sandings and gasket pieces as needed.
  21. It is better to wait until the gasket is removed to remove the Thermostat so as to keep grit from getting in the outlet pipe area.
  22. Carefully remove the old Thermostat -- Be sure to notice the PROPER direction orientation of the Thermostat -- coolant will again spill out of the engine.
  23. Have a beer; the hard part is over with !!

Installing The New Thermostat

  1. Check to be sure you have the right Thermostat (see above discussion) and Gasket before proceeding !!
  2. Place a super-thin layer of gasket sealant on the rims of each side of the joint.
  3. Place the gasket to the Water Outlet Pipe and the Thermostat in the proper orientation to the engine outlet.
  4. Replace the Radiator Temp Sensor now -- you can use Loctite if you want.
  5. Replace the Water Outlet Pipe to the engine and secure it with the 2 bolts -- you can use Loctite if you want.
  6. Replace ALL hoses and pieces you removed to gain access to the area.
  7. Tighten the hose clamps to the same as before.
  8. Be sure all tools and things are clear of the engine.
  9. Clean area as needed.
  10. Refill coolant to a running level as needed to replace lost coolant from the procedure.
  11. If you used gasket sealant, let it dry a bit before running engine!
  12. Now start the car and let it warm up.
  13. Check all hose connections to be sure they do not leak -- Tighten them as needed!
  14. If your engine runs properly then you are done and give yourself a pat on the back !

Now is the perfect time to replace your engine coolant as well !!

To check your new and old thermostats for proper functioning you can do this simple procedure:

Place the thermostat(s) in a sauce pan full of water on the stove. Heat the water until near boiling. When the water temp reaches the opening temperature of the Thermostat (80 to 90 degrees, depending on its specifications), the thermostat should open fully in the center!! This will help verify the new Thermostat works properly and the old one does not. The picture here is of my old thermostat in 100 degree boiling water and it is fully closed !!

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