Temperature gauge and coolant

[Porque223]

Since you are looking for a substantial and rapid change in temperature, almost any small, fast responding (thermal time constant measured in seconds) thermistor mounted on the radiator will register this.

In lieu of a thermistor you can also use a tiny silicon diode in series with an 8.2K resistor in series with a 9v battery. One degree C increase means a 2.2 mV decrease in the diode voltage drop.

 

[Belle427]

worth considering the water pump too, a loose impellor will not pump the coolant round efficiently and a very common issue on the vag cars. Also small water leaks in this area are very hard to detect.

 

[madder]

But would this not cause the car/radiator to overheat? What symptoms or problems ca I expect to have considering I've been driving this car for 4 hour roundtrips twice a week!!!!

 

[Mursal]

I wouldn't over stress, the increase in temp on start up could have been bad circulation after topping up. You know the warning circuit is OK, and you have 5 liters of water in the boot, so its all good.

Just keep a wee eye on the coolant level at least once a day, to see how things pan out.

 

[madder]

OK. Well, I may not have found out why this is happening, but I accidentally found out what it seems to be related to.
Today, I started the car, heard a noise in the air conditioning, and then, as if by magic, the temperature gauge climbed up to the 90 mark, and stayed there for several minutes. It dropped down again after a while, and so I started randomly pressing the air con buttons and controls, on, off, more intensity, less intensity, and then, at some point, the temperature gauge moved up again. It has since dropped down again, but at least this seems to be related to the air con.

 

[Porque223]

The A/C is a heat pump of sorts but how much temperature change can you expect in a properly functioning car?

BTW, the A/C compressor may take 7 hp.

 

[Burnerman]

Using the air con may just have the effect of turning the cooling fan(s) on, which naturally will reduce the engine coolant temperature at the same time.
I would suspect a lazy thermostat here - even if changing it just rules the problem out.
The coolant loss is a much more important issue, and as suggested the water pump is a frequent culprit. For sure, if there is coolant leaking there's always that characteristic smell!
Do keep the coolant level on max in the mean time.
Regards
John

 

[madder]

I think you're right about the 'lazy thermostat' John. Basically I have got it down to a fine art. Start the car, if the thermostat doesn't react, I blast the air con on full power(cold air since the weather is good right now ) and after a minute or two the thermostat moves up.

 

[madder]

bump! Changed the thermostat today but the problem persists! Stumped here. I do have a small oil leak in the engine now also.

 

[Mursal]

Hopefully the oil is leaking to the outside not into the coolant?

Why were you changing the stat, was it the warning light flashing or the temp gauge, I forget, sorry.

 

[madder]

Hi Mursal. The temperature gauge on the dash rarely moves, even though the engine must run at the correct temperature. After going though all the possibilities at the time, I put it down to a lazy thermostat. I felt it must be opening slower than normal, and one symptom of that, apparently, is that the temperature gauge doesn't always move up to the correct point. So, eventually, I decided I best get around to changing the thermostat, which I did yesterday, but it seems to have made no difference at all. Nothing has changed.

I don't think the oil is leaking into the coolant? Is there a good way to diagnose that? Oil seems to be leaking close to the front of the car, at least that is where the evidence is landing underneath, and on the drivers side.

 

[madder]

Just went out for a spin to the shops. Got the 3 beeps and red flashing light on the dash in the coolant/temp gauge position. It still didn't rise. SO I checked the coolant level in the reservoir. It is slightly below minimum. So perhaps I didn't top it up quite enough with the litre I put in yesterday, after changing over the thermostat. But would that impact on the temperature gauge? Obviously, I'll get some more red coolant tomorrow and top it up until it's correct, but what are the chances of it solving the other issue?

 

[Mursal]

Sorry I got side tracked, no the oil is not in the coolant, you would know.

Does the gauge move at all? If it does then, it could be a lazy gauge in the dash, or a faulty temperature sensor. We had one (lazy gauge) that moved when you tapped the glass on the dash?

I think (from memory) your sensor has four wires coming from it, and is usually on a water/coolant pipe between engine and gearbox (clutch area). Might have bad/corroded terminals.

But, have you plenty of interior heat?

PS low coolant might solve the problem, but doubt it ...........

 

[madder]

The gauge used to move occasionally, but it became less and less likely to move and now, I don't remember the last time it moved. I suspect a temperature sensor now too. I changed the thermostat for nothing!! Well, at least there's a new one it it! I suppose the next thing is to change the temperature sensor. Tapping the glass hasn't worked so, I doubt that it's the gauge, although who knows??!!!!

 

[Belle427]

I think you need to go back to basics and first establish are you loosing coolant?
You mention topping it up frequently but why?
If you fill the system and run it round for a few days the coolant should settle, it may take a few trips out to shift air locks as some vag stuff is problematic.