WHAT IS THE PART CALLED THAT CIRCULATES HEAT FROM THE RADIATOR INTO THE PASSENGER COMPARTMENT OF A 1998 MB S420. IT HAS THREE ALUMINUN CYLINDERS ABOUT 2"TALL AND 1/4" IN DIAMETER

The most common cause is low coolant level, so check the radiator and overflow tank and fill as needed. If coolant is over 2 years old, it is a good idea to change the coolant and thermostat-start afresh.
Check the radiator cap. If it doesn't hold pressure of about 13-15 psi in the coolant system, the seal is bad and a new radiator cap is called for. The coolant system is a sealed system, so you should never have to add coolant, save for a small loss through evaporation in the overflow tank. If you are losing coolant, there must be a leak that needs found and repaired-a pressure test of the system should show up a leak.
When the engine is warmed up, the thermostat opens and coolant is circulated through the engine (pushed by the water pump on the front of the engine), into the upper radiator hose to the radiator, and down through the radiator where it is cooled, then back to engine and water pump through the lower radiator hose-a continuous circulation. If anything blocks this circulation (like a partially plugged radiator, or leaves, trash or debris on the outside of the radiator preventing good radiator cooling) then you could get engine overheating. You may want to have the coolant circulation checked, and/or have the radiator and engine flushed. Also possible that the water pump may not be promoting good flow-the impellers on the pump may be worn down to stubs-this can happen if regular maintenance is not observed.
Your radiator fans should also be checked that they work when called for.
The worst case would be a blown cylinder head gasket-another common cause of overheating, and the most expensive fix. A compression test could check for this condition- a cylinder or two with low compression indicates head gasket failure. Coolant can get mixed in with the oil, or it may wind up in the cylinders and get burned, putting out white smoke out the exhaust.
Overheating is bad on engines, get it checked right away.

There should be a plug sticking out of or near the front grill. Block heaters are generally connected to the oil pan.
If you meant the heat exchange that brings heat from the engine into the passenger area, that is on the psssenger side betwern the firewall and the dashboard, usually near the glove compartment. This heater core will leak sweet-smelling radiator fluid (engine coolant) onto thr floorboards if it is defective. If you just aren't getting heat, be sure your coolant level is full, and if it is (but there is no leaking into the passenger area,) then look at the valve inside the engine compartment, and at what is called the Blend-door.

I hate to say it, but I think that you installed the thermostat upside-down. If so, the coolant is constantly circulating through the radiator, and it takes a very long time to come up to normal operating temperature, which means no heat to the passenger compartment. Try removing the "gooseneck" again and make sure that the "long" end of the thermostat is facing down into the engine. Good luck.

You may have

- a coolant leak somewhere, comes out when the engine heats up and pressurises the radiator. Camrys do develop a crack in the plastic radiator top tank, which can be hard to see if on a seam. Another place which leaks is under the intake manifold, which is not visible. There is a test, called a "leak-down" test

- thermostat stuck closed, preventing coolant circulation

- a leak through a head gasket from a combustion chamber to the coolant jacket. This will quickly cause overheating, and sometimes causes white bubbles in the radiator neck, like dry ice. There is a test kit available for gases in the coolant.

#5 spark plug is the one closest to the back of the engine on passenger side. The location of the #5 spark plug wire on the coil pack is on yhr passenger side closest to the bulkhead between the engine compartment and passenger compartment. Thank you

Your car's heater is a life saver in cold weather. Before attempting to repair your car?s heater, you should understand how your car?s heater system works. First, on this page is an explanation of how your car?s heater system functions, further down the page are some pointers on heater system trouble-shooting. To warm the passenger cabin of your car, your car?s heating system makes use of excess heat from the engine?s internal combustion process. A car?s engine, in fact, when in operation produces so much excess heat that if the excess heat is not removed, it would destroy the engine. It is your car?s cooling system that removes that dangerous excess heat. Some of an engine?s excess heat is released through the exhaust. Most of an engine?s excess heat is absorbed by a circulating liquid coolant, which is a mix of water and antifreeze. The heated liquid coolant is carried from the engine through hoses to the radiator, which transfers the heat from the coolant to the outside air. That heat transfer lowers the temperature of the liquid coolant, which is then circulated back to the engine to absorb excess heat again. Whereas the radiator is located at your car?s front grill, the unit that transfers heat to the passenger cabin is located inside the dashboard. This unit is something of a mini-radiator and is referred to as the heater core. Heated liquid coolant circulates through tubes in the heater core and a heater fan blowing across those tubes, as well as through little fins encasing the tubes, directs warm air through heating vents into your car?s passenger cabin. Because your car?s heating system works off of its cooling system, heating system malfunctions are often caused by problems in the cooling system.

There are many many reasons for car heater not coming ON or not working properly:-- i will explain you the procedure to check this possibilities Your car's heater is a life saver in cold weather. Before attempting to repair your car’s heater, you should understand how your car’s heater system works. First, on this page is an explanation of how your car’s heater system functions, further down the page are some pointers on heater system trouble-shooting. To warm the passenger cabin of your car, your car’s heating system makes use of excess heat from the engine’s internal combustion process. A car’s engine, in fact, when in operation produces so much excess heat that if the excess heat is not removed, it would destroy the engine. It is your car’s cooling system that removes that dangerous excess heat. Some of an engine’s excess heat is released through the exhaust. Most of an engine’s excess heat is absorbed by a circulating liquid coolant, which is a mix of water and antifreeze.

The heated liquid coolant is carried from the engine through hoses to the radiator, which transfers the heat from the coolant to the outside air. That heat transfer lowers the temperature of the liquid coolant, which is then circulated back to the engine to absorb excess heat again. Whereas the radiator is located at your car’s front grill, the unit that transfers heat to the passenger cabin is located inside the dashboard. This unit is something of a mini-radiator and is referred to as the heater core. Heated liquid coolant circulates through tubes in the heater core and a heater fan blowing across those tubes, as well as through little fins encasing the tubes, directs warm air through heating vents into your car’s passenger cabin. Because your car’s heating system works off of its cooling system, heating system malfunctions are often caused by problems in the cooling system.
The heater core in your car is similar to the radiator in the front of your car; in fact it looks like a small radiator. The difference is the heater is mounted inside the car and air is blown through the fins of the core. The heater hosestransfer engine coolant from the engine to the heater core, this allows the heat from the engine coolant to be utilized and warm the passenger compartment. When a heater stops functioning determine what type of failure has occurred to execute a repair. We have listed the most common problems below.

please click on this link directly for more detailed help
http://www.2carpros.com/first_things/heater_failure.htm
check the diagrams and parts in the link to troubleshoot your care heater
This will help. Thanks please keep updated. please do rate the solution positively .thank you for using fixya

It sounds like water pump may be your problem. It circulates a 50/50 mixture of glycol (or some form of) and water through the engine block and cylinder head and back through the radiator. In the radiator the mixture is cooled when air passes over the fins of the radiator. This is accomplished by driving or a mechanical/electrical fan when stationary. The water pump also circulates the mixture through the heater core giving the car heat. If the water pump is not circulating the water back to the radiator to cool, it will overheat. More heat is caused by the engine when driving (much less heat when idling), hence the overheating while driving.
I hope this helps and good luck!

When you say the heat doesn't work are you meaning the fan doesn't blow out hot air or just doesn't blow out any air? if it blows cold air sometimes when you have the heat on then it could be poor circulation through your radiator or a weak water pump. make sure your radiator is clean on the inside, not too corroded or nasty inside. You can buy a radiator flush thing from wal-mart or any auto parts store. See if it helps.

providing you cooling system is full....your thermostat may need to be replaced....please rate my response