Checking coolant temperature and thermostat operation

[SteveSarre]

Hi All,

I was driving along an A road, and had a cheap Bluetooth reader plugged in and was using Torque to monitor my engine coolant temperature. As one does.

I noticed a few things:
The gauge on the instrument panel reaches 3 o'clock at about 82C - well before full operating temperature.
It stays at 3 o'clock until the temperature gets up to about 97C.
So the instrument does not give a very precise indication of coolant temperature.
When I was coasting along the flat, the temperature would rise up to about 91C
When I was coasting downhill the temperature would drop to 83-85 C
Flooring it, up a hill, the temperature rose to 97-99C

So I would guess that the thermostat was opening at too low a temperature, and possibly not opening fully.

I took the thermostat out and put it (with 2 new thermostats and 2 other used ones) in a saucepan of hot water and turned the heat on.

I have an Extech EX330 multimeter which allows me to measure water temperature. I also have a cheap infra-red non contact thermometer gun. (The multimeter and the infra-red don't agree precisely but well enough).

Using a torch, you can pretty much work out when the thermostats open and close, and how far they open. (So I mark them with marker pen).

I believe that the OEM Volvo thermostats 271664 are set to open at 90C

I didn't test by boiling the water, just bring it up towards boiling
https://mechanics.stackexchange.com/...-my-thermostat

So I picked my best (which happened to be a new non-OEM thermostat) from polar parts.

On the same road the car now drives at between 92C and 97C.

I have just ordered an OEM 271664 and may fit it and report back.

Meanwhile, does anyone else know what is a good operating temperature to expect?
(1998 V70 T5 Auto)

Thanks
Steve

[cheshired5]

With a 90°C thermostat, once up to temperature, I would expect to see a virtually stable 87-89°C when driving around.

Anything drastically higher or lower (I see your figures as drastic) could be the thermostat but could equally be caused by the cooling system not being completely free flowing as designed or a combination of both.

If you're certain that the cooling system is free flowing then certainly fit a new thermostat.

The dash needle not really reflecting this fluctuation isn't that surprising and wouldn't cause me concern.

[SteveSarre]

Yes,

I wonder if this tells me anything about the water pump?

Steve

[cheshired5]

Originally Posted by SteveSarre

I wonder if this tells me anything about the water pump?

It may well but a coolant flush and thermostat swap are sensible maintenance precautions to take on a 20+ year old car regardless and they're cheaper.

If they don't work you'll need to consider all alternatives but given that genuine Volvo coolant pumps are normally bulletproof, I'd begin with the few things that are likely rather than all the things that are possible.

[Clogs]

An old car may have had Radweld or some such gunk put in the system after a leak as a get you home remedy. If that has happened and the coolant is anything but clear, flush it and make sure there are no leaks.

My water pump is original at 170,000 miles, and whatever the temperature outside the needle is always at 3 o'clock. Even going up in summer high mountain passes when the fan is going flat out (and will carry on for five minutes after ignition switched off) and the water pressure is very high (at 6000 feet boiling point is 94 C), and in -20 in the winter mountains... still toasty warm inside and needle at 3 o'clock.

The sensitivity of the gauge is doubtless "adequate" to alert a driver to overheating, which will mean temperature is way past boiling point for a 2 o'clock to show. Quite honestly does it really matter if the water is 85 or 95 so long as there are no leaks and all the parts that need cooling are being cooled - especially the turbo and engine block?

[Bob Meadows]

Central heating descaler is also useful if you decide to flush: ~ follow the pack instructions- not harmful to mixed metals etc. & far better than the product sold for cars.
Bob.

[Luxobarge]

Originally Posted by Bob Meadows

Central heating descaler is also useful if you decide to flush: ~ follow the pack instructions- not harmful to mixed metals etc. & far better than the product sold for cars.
Bob.

Another tip - if you've had a rad failure or other situation where oil has mixed with the water, fill it up with pure water and a little clothes washing liquid and run it for a day or two, then flush and repeat if necessary before finally draining and filling with the correct coolant. Don't do this if it's likely to freeze overnight, but otherwise this does a great job of cleaning out all the old oil and mayonnaise. I did this when my rad failed, and it worked well.

[Derek UK]

The Volvo temp gauge isn't very informative. They have a compensation circuit which I think started in the 240. It means that it gets up to a "normal" position and stays there until the coolant gets quite a bit hotter. Then it jumps up to a higher setting. Volvo did it to calm down those drivers who get paranoid about the needle rising and falling due to small changes in temperature. This compensation circuit can fail and some people have gone in and tampered with the wiring to take it out of circuit. With it working how Volvo, in their wisdom, intended, it does mean that you should at least check the gauge now and then. Bottom line is if your thermostat is good, your electric fan works, and the gauge doesn't get up close to the red you should be fine.

[DaveNP]

As clogs points out, and in my experience, the temp gauge is 'adequate' and there will always be some fluctuation of temperature due to the basic way the cooling system works.
The little gauges one gets inserted into the main dial are usually/often (but not always*) made to be readable at a quick glance with the needles pointing the same way, if you've driven the vehicle for a while it becomes instinctive where the needle should be and you would react quicker to any variations, especially if those variations are more marked eg the temp going from a steady 9o'clock to 10 rather than fluctuating constantly from 8 to 10 and 'oops it's gone to 11'

*Many years ago in one job I had there was one vehicle in the fleet where the manufacturer had used a gauge probably originally intended for one side of the dial and put it on the other side, effectively it was upside down which if you hadn't driven that vehicle for a while was disconcerting if you just glanced at it, I can't remember what the gauge was but as an example imagine if the fuel gauge was moved like that and full was the bottom of the dial, zipping up the motorway you glance at it and all of a sudden it's '#### I'm out of fuel! oh no wait a minute I just filled up, the gauge is upsidedown so I've got a full tank, phew' ... and you've just covered nearly 200 yards at 60mph in the time it took to read that.

[SteveSarre]

Hi all,

Just to follow up, I did fit a genuine Volvo stat 271664 and the temperature now sits fairly steadily at 93-95.

Occasionally it dips to 92 or 99 (and the fan kicks in at about 101).

MPG has jumped back up from 14mpg on one fill up, 21mpg on the next but now back to 28ish (but dependent on driving style and traffic conditions).

So definitely worth keeping an eye on coolant temperature (but the instrument gauge is not accurate enough).

Hope that helps.
Steve