I have a Mk2b and just yesterday the needle on the temperature gauge has started to creep up above the halfway mark once I stop in traffic. Upon moving off, it quickly drops back to the halfway mark.

I did some testing by stopping the car and putting it in N. Once the needle has creeped up, revving the engine at about 2k steadily brings the needle back down to the halfway mark.

Opened up the bonnet with the engine running and the radiator fan seems to be turning.

What could be the problem? I did not do any maintenance to the engine or cooling system recently, so this problem has popped up out of the blue.

flashbang said:

the radiator fan seems to be turning.

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Seems to be turning? The fan should be very powerful and can be easily heard inside the car when the car is idling - bit like a vacuum cleaner noise .

If you are finding the problem is going away in neutral at 2000K but persists while idling it might be the fan gets more volts when you do that.

Does the fan sound powerful?

Once the temperature goes over halfway the fan comes on to assist in cooling. That sounds normal.
Over heating at standstill and dropping when you move off I would say your thermostat has failed and has stuck partly closed.
Revving at standstill increases the pump speed and might be enough to cool it with the fan already running.
On balance I would say the thermostat has failed. Before making any decision to replace it make sure the system is fully bled. Do you have a gurgling noise from the heater on start up ?

judderbar said:

Seems to be turning? The fan should be very powerful and can be easily heard inside the car when the car is idling - bit like a vacuum cleaner noise .

If you are finding the problem is going away in neutral at 2000K but persists while idling it might be the fan gets more volts when you do that.

Does the fan sound powerful?

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By "seems to be turning" I meant it was confirmed visually that it was turning. Also, I can hear the fan running and feel a slight breeze when placing my hand in front of the radiator. Am I right to say it's working like it should?

Brendan W said:

Once the temperature goes over halfway the fan comes on to assist in cooling. That sounds normal.
Over heating at standstill and dropping when you move off I would say your thermostat has failed and has stuck partly closed.
Revving at standstill increases the pump speed and might be enough to cool it with the fan already running.
On balance I would say the thermostat has failed. Before making any decision to replace it make sure the system is fully bled. Do you have a gurgling noise from the heater on start up ?

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I don't remember hearing gurgling sounds but I will check again the next time. I live in a tropical country (30C here) so the A/C is on cold all the time, does this make a difference? Where is this heater you are referring to?


I found this page that can help to diagnose a thermostat problem: http://hubpages.com/hub/How-to-Determine-if-a-Cars-Engine-Thermostat-is-Stuck

Thermostat Symptom Test & Explanation

Cold engine test. Open hood and remove radiator cap. Start engine. Observe coolant action. It should not be moving.
When an engine is cold the thermostat should be closed, thus bypassing the radiator.

Warm (Not Hot!) engine test. Open hood and remove radiator cap. Start engine. Observe coolant action. It shoud be swirling vigorously.
When an engine is warm the thermostat is in the open position. This allows the coolant to flow throught the radiator causing vigorous movement.

Cold engine test. Start the engine. Time how long it takes the engine to warm up enough to produce heat when the heater is turned on.
If it takes more than five minutes for the heater to produce heat it is a sign that the thermostat is stuck open allowing all the coolant to flow all the time. It takes a good deal longer for the engine to warm up.

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Is this accurate and can I use this to do a simple diagnosis of my problem? Gotta check everything you read on the internet, lol.

flashbang said:

By "seems to be turning" I meant it was confirmed visually that it was turning. Also, I can hear the fan running and feel a slight breeze when placing my hand in front of the radiator. Am I right to say it's working like it should?

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The fan is very powerful. Can you hear that powerful fan sound inside the car while idling with the windows closed? Slight breeze? is that from outside the car?

Those thermostat testing instructions do not apply to your car.

If your thermostat is partially closed then when the car overheats if you put your hand on the bottom hose it will not feel very warm.

judderbar said:

The fan is very powerful. Can you hear that powerful fan sound inside the car while idling with the windows closed? Slight breeze? is that from outside the car?

Those thermostat testing instructions do not apply to your car.

If your thermostat is partially closed then when the car overheats if you put your hand on the bottom hose it will not feel very warm.

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I will have to check again on that fan sound, I didn't really notice. The slight breeze I felt was from outside the car at the radiator side. With the window wound down the fan sound is quite obvious and sounds as though it's on high speed. Yet, the temperature needle still creeps up.

By the way, the temp needle creeps up exactly 3 boxes from the middle mark and stabilizes there when I'm idling. Here's an example picture to show what I mean about the 3 boxes. The picture below is exactly at the halfway mark, mine is 3 boxes higher.

If the guage goes above middle when idling and then goes back down when you start moving again
Thennit sure does sound like a cooling fan failure

If the temperature ever goes above half way
Stick the cabin heater on hot and speed 4 and points the fans outside the car with wibdow down

Dont turn fans off till either u stop or it sits middle again

Its to prevent a hg blowing

Ziggy

Ziggy122 said:

If the guage goes above middle when idling and then goes back down when you start moving again
Thennit sure does sound like a cooling fan failure

If the temperature ever goes above half way
Stick the cabin heater on hot and speed 4 and points the fans outside the car with wibdow down

Dont turn fans off till either u stop or it sits middle again

Its to prevent a hg blowing

Ziggy

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Actually I don't need to move to get it going down, in neutral gear and revving the engine for about 10 seconds also gets it back down to the middle. Is it normal for the radiator fan to fail even though I see and hear it running?

Thanks for all the replies guys! Really trying to figure out how cars work so I can at least do a basic diagnosis instead of letting the workshops try whatever they want.

flashbang said:

Is it normal for the radiator fan to fail even though I see and hear it running?.

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The issue is a simple one of knowing if your fan sounds powerful or it is something less than that. If you are outside the overheating car with the bonnet down and you can hear an impressive fan sound then it would sound normal to me.

Just change the thermostat. It is highly unlikely to be the fan.
The 'stat will be stuck partially open.

flashbang said:

Is it normal for the radiator fan to fail even though I see and hear it running?

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No, absolutely not.. It will not be the fan.

Starting to think it's likely to be a half-open thermostat. The temperature gauge took longer than usual to move up from C. Is it a complicated job to replace the thermostat?

flashbang said:

Starting to think it's likely to be a half-open thermostat. The temperature gauge took longer than usual to move up from C. Is it a complicated job to replace the thermostat?

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It is an easy job if you have 3 or even 4 hours to spare and are prepared to spend at least an hour cleaning up the place on the engine where the thermostat fits. Everybody is using a silicone sealant for the new thermostat but i used the supplied thin paper gasket along with the silicone - others are not using this gasket. I got no leaks. Some people do this job and get leaks. Cleaning the engine seems to be the biggest part of being successful.

You will need to get the bottom hose undone from the radiator which needs to be done with care to avoid breaking the plastic radiator. The hoses tend to get stuck on and have to be removed slowly rather than with brute force. The hose on the thermostat also has to come off but that is metal and you can be more forceful. Getting the hoses off requires time and persistance rather than brute force. Twist and hold one direction and twist and hold the other direction and that sort of thing

The job is more or less impossible unless you remove the coils above the thermostat so you can get your hands in there for cleaning the engine - that is an easy job however but I found it easiest to pull the plugs off the spark plugs rather than get the spark plug leads off the coil - you need to move the coils out of your way without stressing the spark plug leads which do not like being pulled upon.

You will need to refill with antifreeze even in the tropics to prevent corrosion and before you refill you should flush thru the system as best you can by putting the water hose inside the hoses from various directions till you have sufficiently clean water.

Most of the forum seem to be stressing getting air out of the system once you refill with coolant, but providing you open the radiator bleed screw the rear one is not important, but you must keep an eye on the reservoir level till the engine is hot and make sure the reservoir is near the max level before you go off for a test drive and keep an eye on it for a few days. If you break off the air bleed screws that is another job you will have to spend time fixing, particularly in a hot climate where the cooling system is stressed to the max - and they are easily broken.

So it is not a totally easy job which is why i was wanting you to be sure the fan was working properly. Too many people today just seem to change this and change that to fix things rather than being systematic about it. Maybe changing this and changing that is a faster way to get it fixed though. Diagnosing can be more of a lifestyle or hobby!

Once you have the old thermostat out there is no harm in putting old and new in the same saucepan and bringing them to the boil to check the new one opens later than the other. If they are the same then the old thermostat was ok or you have the wrong new thermostat

Well surprise, I decided to try topping up the radiator with about 500ml of distilled water and the overheating is gone. This seems to imply there's a leak somewhere in the system, but it's so slight that there hasn't been any problem for the last 2 months.

flashbang said:

Well surprise, I decided to try topping up the radiator with about 500ml of distilled water and the overheating is gone. This seems to imply there's a leak somewhere in the system, but it's so slight that there hasn't been any problem for the last 2 months.

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cardboard under car overnight and you shall see if it leaks

flashbang said:

Well surprise, I decided to try topping up the radiator with about 500ml of distilled water and the overheating is gone. This seems to imply there's a leak somewhere in the system, but it's so slight that there hasn't been any problem for the last 2 months.

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There is only about 500ml difference between Max and Low so that is a bit surprising and I just checked the difference by siphoning out 500ml. If it was a litre to bring it to the max you might have been drawing in air into the pump to make it less efficient whereas 2000RPM overcame that inefficiency??

judderbar said:

There is only about 500ml difference between Max and Low so that is a bit surprising and I just checked the difference by siphoning out 500ml. If it was a litre to bring it to the max you might have been drawing in air into the pump to make it less efficient whereas 2000RPM overcame that inefficiency??

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the higher rpm - it just allows better flow

Bearing in mind pump sucks water in via the water rail - which drags
Warm coolant from heater and cooled coolant from the bottom of the radiator

The min and max wont make any difference

As long as the coolant isn't below the lower coolant hose, it'd "technically" not suck air in, but probably would overheat due to lack of coolant actually been cooled

If the pump had lost a fin off it - been slow flow
So higher RPM actually flow the coolant around sufficiently

ziggy

Ziggy122 said:

the higher rpm - it just allows better flow

Bearing in mind pump sucks water in via the water rail - which drags
Warm coolant from heater and cooled coolant from the bottom of the radiator

The min and max wont make any difference

As long as the coolant isn't below the lower coolant hose, it'd "technically" not suck air in, but probably would overheat due to lack of coolant actually been cooled

If the pump had lost a fin off it - been slow flow
So higher RPM actually flow the coolant around sufficiently

ziggy

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I am not convinced yet

The 500ml or 750ml or even a liter missing amount from the reservoir would be effectively dead water not much involved in cooling surely? The difference between max and min seems negligible unless there is another factor involved.

The lost thin theory is ok but how does extra water make a difference? The car is in the tropics so we can assume most of the time the reservoir is fairly hot but can it really radiate so much extra heat when at max compared to when the level is low sufficient to make a difference to a car engine??

By the way do you know if the heater control valve for these cars is a simple one pipe valve or more complex? You and I seem to have a little battle going on about the water flow on these cars. I am assuming the heater control valve is a simple one pipe valve and when closed the engine temperature sensor works totally fine no problems at all and so the heater in the cabin can be totally blocked and you still get good sensor readings.

I will probably monitor the situation for a while to see if it still overheats. I was quite convinced the symptoms were for a stuck half open thermostat. No way an extra 500ml of water could solve that. Perhaps it magically unstuck itself?

Just a note re de-ionised water. It is just simply not worth bothering with. Use the tap - Adams Ale. What is important is corrosion inhibitor in the antifreeze.
I would suggest ref your apparant issue that you simply remove the thermostat and test it in a pan of water. It is not difficult.
How long have you had the car ?. You can have 'interesting' results if the PO was a dab hand with radiator / cooling system treatment for leaks. that can cause a whole pondfull of naughties.:yum: