How to check accuracy of temperature gauge at the roadside

[Stefvan]

Hi
I am buying a van tomorrow with a 1.6TD, when I test drove it the temp gauge on the dash seemed high, the owner says "it has always been like that for 10 years". He has suggested that he will get a digital thermometer and show me that at normal running temperature the coolant is between 87 and 91 degrees - what is a safe procedure for doing this at the roadside and can you measure the expansion tank coolant while the engine is running ? I realise the system is under pressure so theres risk of burns etc...
tia

Stefvan

[Aidan]

just measure the temp of the expansion bottle, will be pretty damm close to the temp of the contents

[Stefvan]

Thanks Aidan, but I am trying to establish the accuracy of the dash gauge, is this an accurate way of doing it ? Its like a metal probe sensor, that came with my digital multimeter, are you proposing I just touch it to the side of the expansion tank ?
cheers

Stefvan

[tigsbirman]

Hi folks - Ermie here with Tigs.....when the van had its recent service, I questioned my temp guage...it is reading high.....they had a laser temperature sensor....while the engine was running they aimed it the block...the fan came on and off at the right times too! They could test the temp at the block, and at the rad, and anywhre in between!! I was satisfied that the thing was not overheating!!

Neat bit of kit!!

Em
x

[Louey]

One thing that will help is : check that the temp gauge takes about 5 mins to get to just before the middle of the gauge - unless the engine has been run recently (within last hour or two).

[Red Westie]

One of the reasons for having a pressurised system is to increase the coolants boiling point which gives it greater heat absorbing capacity.
If the coolant is below 100 degrees C it won't boil when the pressure is released (presuming you don't live on top of a mountain or in a deep canyon lol)
When up to temp...slowly release the cap (with a rag over the top) does it boil? that's not to say that this temp means overheating.
Martin

[Fritz]

I hope you are not serious about that piece of advice, Martin

When up to temp...slowly release the cap (with a rag over the top) does it boil? that's not to say that this temp means overheating.
Martin

People have been badly scalded doing such daft things.


Regards


Fritz,,,,,,,,,,

[Red Westie]

Absolutely serious ...with caution and with several layers of rag over the cap so it doesn't spray upwards.
I agree this isn't exactly text book and under most circumstances the advice would be the opposite (to allow the engine to cool first) but if you want to prove wether the coolant is over 100C then the instantanious boiling once depressurised would be one way of finding out. Remember this is a screw cap on all but the early vans so can be quickly tightened back up again.
Call this a method...not strickly advice.

Although it doesn't really prove anything because coolant sometimes does go over 100C under extreme conditions, it doesn't necessarily mean that there is a fault. (so my answer was simply a means to an end)
Martin

[kevtherev]

the possibilities of a disaster are endless..

I would never take the cap off a hot engine... rag or no rag

[Jutstar]

Under no circumstances take it off!!!!!

I can talk from experience, 3rd degree burns to my hand and arm which resulted in 4 trips to hospital, 3 weeks off work and not to mention a stack of pain.

I only attempt it now when the engine has been turned off for at least an hour and i would still pop a old jumper over it just in case.

[lloyd]

Absolutely serious Call this a method...not strickly advice.

Although it doesn't really prove anything because coolant sometimes does go over 100C under extreme conditions, it doesn't necessarily mean that there is a fault. (so my answer was simply a means to an end)
Martin

If engine is at or over 100C it will spew out like a volcano!!!

Not good in big open engine compartments...

Definately not good in confined space of T25.

[Stefvan]

Ok so, back to my question.. and thanks to everyone for the advice.. if I want to accurately measure the coolant temp with a probe meter, allow the engine to cool a bit, get plenty of protection while we very slowly open the dalek cap to release the pressure, dip the sensor in the coolant, then could I run the engine and get the heat back up to working temp, obviously if it starts to boil in the expansion tank then stop ?? Because Im not sure sticking the probe on the plastic outside of the tank would give me an accurate reading...
tia

Stefvan

[tonytech]

When up to temp...slowly release the cap (with a rag over the top) does it boil? that's not to say that this temp means overheating.
Martin

A pal of mine slackened the pressure cap one half turn.... the pressure in the system along wit a worn thread on the bottle shot the cap completely off...... boiling water straight up in the air.... nasty scalds to his arm.....

T

[lloyd]

Do it as Em suggested... and even then it's not truly accurate as it's reading engine block surface of radiator, etc, not actual coolant temp.

To do it really accurate, you need to put a good senser / gauge system into pressurized coolant and monitor. Don't see how you can do this and use stock gauge / sensor at same time.

Best I can think of is to put probe against tank, hose running from engine to radiator, hose into radiator... and take a reading. The tighter and more insulated the probe is the better as it will allow less heat loss and more accurate reading of what coolant temp is inside.

[Mocki]

in any case, VW say normal is anywhere from the white square on the lower end of the scale to the while block at the upper end...... anywhere from 1/8 to 7/8 is normal.

a temp probe on the main pipe from the pump to the header tank will be a ccurate enough reading.

there are much bigger worries to look for methinks

never ever take the cap off a running engine.