car temperature gauge reading 'ok' but is it overheating?

[A nice cupra T]

Hi Everyone,

I hope everyone had a great festive period and wish everyone a prosperous new year!


A couple of months ago my engine was running COLD; the temperature gauge barely made it up to the quarter mark (Never mind the middle), only for it to go back to zero again. I went to a local garage who've done my tyres, oil change, cam belt etc and mentioned it to a chap there who owns a Golf MK4 1.8T GTi and they said it could be the Coolant Temperature Sensor or the Thermostat.

Anyway, I replaced the Coolant Temperature Sensor myself and nothing changed; my car temperature never budged from cold. The Thermostat seemed a bit of a bugger to change (At least for someone non-mechanically minded like myself) so I got the garage to do it for me. They sorted me out a quote for the price of OEM part and fitting.

On the 16th of December 2019 I had my thermostat fitted whilst having my MOT done. My car temperature gauge now appeared normal and the needle would nestle the middle when warm. They tested the old thermostat and said it was "OK". (Unsure why the new one affected the temperature gauge and the old one didn't if they are both OK).

The morning after (17/12/19) I checked the coolant level to find coolant marks over my engine bay. Also, the coolant level wasn't as high as I'd like it; it was JUST above the minimum mark. I returned to the garage and to ask them to top up my coolant a wee bit. Though on the short journey to the garage (Approx 3 miles) when they lifted the bonnet I noticed the coolant had risen and was no longer just above the minimum mark ; it was just under the maximum mark. They proceeded to add extra coolant. I was kinda concerned as to why it had risen and ask them if coolant should rise and they said "it shouldn't do"(?). It was then topped over the maximum mark and I asked if would it damage the car if the coolant level is a bit over maximum and they said it (The coolant) would evaporate?

Couple of days passed, noticed new coolant marks on engine bay. I thought it might need a new coolant reservoir cap (I know these things can potentially go bad). I installed a new one but the situation still the same.

Which leads me to today, opened the bonnet after a short run to see the coolant literally boiling inside my reservoir and had risen way above the maximum mark (Car Temp Gauge reading normal). I've phoned up the garage and they are going to look at it on Monday 30/12/19.

Does anyone have any suggestions as to what it could be? I'm kinda worried about my engine now with all that heat and wonder now if it's goosed :-(

 

[Alexis27]

Just let it run at stationary and see how it goes. With no air moving through the radiator you'll be able to see if it really is ok.

Coolant top up after thermostat change is normal. Syringe the excess out if it's over the max mark now.

My thermostat worked fine when I took it out of the car and tested it in on the stove. But it wasn't working in the car and a new thermostat fixed my symptoms 100%. So whether a thermostat works outside the car is irrelevant I've found.

 

[martin j.]

Often the test for the thermostat by putting it hot water proves nothing without measuring the temp it opens at, they are designed to operate at a certain temp, opening too soon means the engine is not reaching the correct water temp but, the thermostat is working!, after a change it I should possible to get an air lock in the system that can cause problems short term until it works around the system, is the rad fan coming on?

 

[mty12345]

Most of the time the thermostat fails in the open position causing the car to underheat.

The coolant level only means anything when the system is cold. It will move a fair bit when the system is warm/hot, this is normal.

The bubbling you saw could just have been steam expanding into the tank or an airlock. If the temp guage is at 90C then i wouldn't worry too much, chances are that it's fine, but no harm in getting the garage to check.