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007 wrote: dude, replacing the thermostat will not change a thing.

The thermostat changing will change lot of things. First of all the car will working at proper temperature range thats mean more efficient and fuel saving. Second thing the wear of engine will reduce because of the optimal working temperature.
Then the interior heating will work good as it should be.

007 wrote: the interior heating system completely bypasses the main radiator and the heater core acts as a secondary radiator throwing heat inside the cabin. the thermostat valve controls flow to the main radiator but not the heater core. you can change whatever you want but when you turn the heater on, your temperature will drop. short of adding a second thermostat valve inline with the hose that goes to the heater core, there is nothing you can do.

Yes, but you forget that if thermostat is always stays opened then the coolant goes also to the main radiator too. The result is that coolant doesn't reach proper temperature.
The thermostat duty is to act as coolant traffic policeman when the optimal temperature of the coolant reaches then thermostat opens to prevent the engine from overheating.
In the situation of mynameisowen his car's thermostat does stay always opened and because of this fact the coolant temperature isn't on the working range and the heater doesn't not work properly.

007 I see agree with what you're saying but Betek is correct in saying that if coolant has to go through heater core "mini radiator" AND main radiator then engine will have an issue getting to temperature. If coolant only goes through "mini" radiator (thermostat closed) then it still has chance to warm up and operates like you would expect.

If you are experiencing what I am experiencing then maybe you should think about changing your thermostat aswell.

I live in India. don't need no heater.

that aside, coolant cannot flow through both radiators until it is at the point where the thermostat valve is rated to open. let's say that point is 80*C. because of you turning your heater on, let's say coolant is at 60*C. the thermostat won't open at all and only the mini radiator will be active. until you reach 80*C, all the engine cooling will be done by the mini radiator.

if your thermostat is stuck open like you suspect, then you should see similar results even with the heater off. if turning the heater off is restoring the situation back to normal, then your thermostat is fine.

also, there is a little "jiggle valve" in every thermostat that lets a tiny amount of coolant - and air if present - through even when the main valve is closed. I however doubt that the tiny amount of flow it creates will disturb things much.




Betek, read the underlined bit:

mynameisowen wrote:It's now approaching winter and every year I have the same problem, my car does not warm up properly (settle at about midway on the temp gauge) if I drive the car with interior heating on.

this leads me to assume that everything is fine when he does not use the interior heater. which will also mean that his thermostat valve is working like it is supposed to. Owen, correct me if I am wrong here.

007 wrote:that aside, coolant cannot flow through both radiators until it is at the point where the thermostat valve is rated to open. let's say that point is 80*C. because of you turning your heater on, let's say coolant is at 60*C. the thermostat won't open at all and only the mini radiator will be active. until you reach 80*C, all the engine cooling will be done by the mini radiator.

if your thermostat is stuck open like you suspect, then you should see similar results even with the heater off. if turning the heater off is restoring the situation back to normal, then your thermostat is fine.

If thermostat is stuck open then I am receiving more cooling than I need. Engine warming up properly depends on outside temperature and air flow rate through radiator. During winter and on motorways more cooling is experienced, thermostat should then stay closed. If it stays open then cooling system will not work as required. Honda will have designed this system to be able to warm cabin even though it is cold outside. Priority is warming the engine for me so I have heating off till warmed, if I turn heating on engine never warms up. Once warm and I turn heating on engine temp usually drops but that depends on air temp and vehicle speed. This should not happen. If engine is warm and I turn heating on thermostat should then close to keep engine at operating temp. Thermostat must be not closing: replace thermostat.

Do you not agree? I value your input.

It is normal not to turn the heater untill the engine reches working temperature. But when that happens and you put on the heater, the temp should not drop much if the termostat is ok. If it drops to zero point then you know its the termostat and you have to change it.

mynameisowen wrote:

007 wrote:that aside, coolant cannot flow through both radiators until it is at the point where the thermostat valve is rated to open. let's say that point is 80*C. because of you turning your heater on, let's say coolant is at 60*C. the thermostat won't open at all and only the mini radiator will be active. until you reach 80*C, all the engine cooling will be done by the mini radiator.

if your thermostat is stuck open like you suspect, then you should see similar results even with the heater off. if turning the heater off is restoring the situation back to normal, then your thermostat is fine.

If thermostat is stuck open then I am receiving more cooling than I need. Engine warming up properly depends on outside temperature and air flow rate through radiator. During winter and on motorways more cooling is experienced, thermostat should then stay closed. If it stays open then cooling system will not work as required. Honda will have designed this system to be able to warm cabin even though it is cold outside. Priority is warming the engine for me so I have heating off till warmed, if I turn heating on engine never warms up. Once warm and I turn heating on engine temp usually drops but that depends on air temp and vehicle speed. This should not happen. If engine is warm and I turn heating on thermostat should then close to keep engine at operating temp. Thermostat must be not closing: replace thermostat.

Do you not agree? I value your input.

I agree that *if* the thermostat is stuck open, the engine will run colder than it is supposed to. what I am trying to say is that at this point, you don't know for sure whether or not the thermostat is stuck open. you need to diagnose and confirm your suspicion before you throw new parts on the problem.

to do so, drive around with the heater off and see if engine temperature is still low. keep in mind that it will not rise as quickly as it does in summer because your engine will also lose heat through the block and head metal and colder ambient temperatures will cause more heat loss through the metal than usual.

here is what I would recommend:

1. idle the car with heater turned off. monitor the radiator top and bottom hose temperature by feeling them. when the thermostat is closed, the top hose will be hot and the bottom hose completely cold. when the temperature needle is near its usual steady state position, the thermostat will open and the bottom hose will warm up. if that happens, your thermostat is just fine. measure the amount of time it takes for the bottom hose to warm up for a completely cold engine.
2. repeat the above with the heater on and measure the amount of time. note the difference.

if the bottom hose starts to warm up way before the steady state mark, *then* you have a problem with it. take out the thermostat and check it as described in the service manual here:
http://blackworks.in/cars/resources/tstat/tstat.png

archyman wrote:It is normal not to turn the heater untill the engine reches working temperature. But when that happens and you put on the heater, the temp should not drop much if the termostat is ok. If it drops to zero point then you know its the termostat and you have to change it.

the heater has a very significant effect on engine temperature. I have personally faced many problems with thermostat valves that are jammed shut of radiator fans that don't turn on, causing the engine to overheat until the temperature needle shoots up all the way to the top. in this pinch, turning on the heater at full blast gets the temperature down almost immediately.
check Point #2 under "If you need to drive the car" here: http://www.wikihow.com/Stop-an-Engine-from-Overheating

Use the heater to disperse engine heat. Flip the climate controls to vent, turn the heater all the way up, and turn the fan all the way up. If the weather is hot, the inside of the car will heat up quite a lot. Point the vents out the windows as much as possible to help reduce the temperature.

conclusion: the interior heater takes away a *significant* amount of heat from the engine. keep that factor in mind.

Good logic, however, if I am having the symtoms as described by archyman, would you not agree that it is the thermostat?

I will inspect the thermostat before replacing anyway.

My suspicion is that the thermostat isn't sticking all the way open, maybe it is just clogging and getting stuck or having trouble opening fully. If so a clean should sort it out.

mynameisowen wrote:Good logic, however, if I am having the symtoms as described by archyman, would you not agree that it is the thermostat?

I will inspect the thermostat before replacing anyway.

at this point, all we are doing is guessing. my guess is that the thermostat is alright. I could be wrong. once you open and inspect the thermostat, things should be pretty clear.

in case the thermostat is alright, I have given you the reason why temperature might still be low. just keep it in mind.

The experience with civics from many owners have showed that the thermostats on these cars stay opened after many years of use.
From the description of mynameisowen I can say 99% sure that thermostat is the problem.
One another problem would be clogged interior heater radiator from dirt but on civics this problem is rare.
But clogged heater test is very easy, turning on the interior fun and testing it in scales. If the air flow is good that means the heater is OK.
Anyway the proper testing of the thermostat is this: