Unusual coolant problem.

[105.4]

Driving home from work tonight, (10 miles or so), I noticed the temp gauge wavering about, from almost cold to nearly middle temp. At all times the air coming out of my vents was toasty hot.

When I got back home I expected to see the expansion tank almost empty, but it is 3/4 of the way between Min & Max.

Coolant is pretty fresh and nicely pink.
Radiator, water pump and thermostat were all changed a couple of years ago. There’s not been any signs or any smells of coolant leaking.

From what I understand, a temp gauge either works or it doesn’t. Correct?

From what I understand, thermostats either fail open or fail shut, not half, kinda almost working.

Any ideas fellas?
The engine is a 1.5dci Renault K9K.


Thanks in advance

[GreenV8S]

Sudden big changes like that would make me suspect an electrical fault rather than a cooling system fault.

[Demelitia]

I’d agree with the above.
I’d be looking to check the function of the coolant temp sensor.

[105.4]

@GreenV8S
@Demelitia

Thank you both

[aka_kerrly]

During the cold weather a temperature gauge bobbing around can be a sign that the thermostat is potentially sticky. You will be getting heat to the cabin (which shows your water pump is working fine) as that runs from a coolant supply not influenced by the engine thermostat.

[105.4]

aka_kerrly said:

During the cold weather a temperature gauge bobbing around can be a sign that the thermostat is potentially sticky. You will be getting heat to the cabin (which shows your water pump is working fine) as that runs from a coolant supply not influenced by the engine thermostat.

Yeah, I’d been pondering the same over breakfast.
For the sake of £20 and an hours work, I think I’ll fit a new thermostat this weekend.

Cheers.

[Peanut Gallery]

Air-lock and the bubble moves around? - Is there an odditiy with how that engine is bled?

My Fiesta did this for a while, warm air out the vents after going around a right hand bend, cold after a left. Bled the system and it behaved itself.

[steveo3002]

gauge will be erratic if the coolant is low as it will be air not coolant some of the time

[ARHarh]

Classic stuck open thermostat. Warms up when sat in traffic cools down when moving off, appears warm but rad is cooling and warming quickly where the temp sender is likely to be located. The engine block will stay warm where the heating water comes from.

[paintman]

I've had a few with identical symptoms on our own vehicles, last one being my Sprinter.
On each occasion it's been a failed thermostat & a new one has solved it.

IME when I've had a temp sensor fail the gauge either hasn't moved or shot off the hot end of the scale!

[105.4]

Thanks for the input chaps.

I’ll fit a new thermostat on Sunday and report back on how things are on Monday evening.

[Panamax]

My bet,

• Heater is al;ways warm because that's the way it's plumbed. Manufacturers don't plum it through the thermostat for obvious reaons and owners want their want their windscreen demisted asap.
• There's a good chance all you're seeing on the dial is the thermostat opening and closing so sometimes the sender is in hot coolant and sometimes it's in cold coolant. Some manufacturers fudge the gauge to a very wide range of temperatures that will display as "normal". Others don't.

If it ain't broke, don't fix it.

[105.4]

Panamax said:

My bet,

• Heater is al;ways warm because that's the way it's plumbed. Manufacturers don't plum it through the thermostat for obvious reaons and owners want their want their windscreen demisted asap.
• There's a good chance all you're seeing on the dial is the thermostat opening and closing so sometimes the sender is in hot coolant and sometimes it's in cold coolant. Some manufacturers fudge the gauge to a very wide range of temperatures that will display as "normal". Others don't.

If it ain't broke, don't fix it.

I’ve had this van for a couple of years now.

The usual routine is that once it’s up to temperature, the needle stays dead middle. For it to misbehave is quite unusual.

[Panamax]

If a thermostat is stuck closed you will obviously overheat. You're likely to hear the electric fan (where fitted) kicking on and staying on.

If a thermostat is stuck open you should be able to tell in winter by either,

• Slow warm-up, or
• Get the car warmed up and take it for a gentle run on a dual carriageway. If the thermostat has failed you'll see the coolant temperature drop as the high airflow cools the system more than is needed.

[catman]

I've had this before, a new thermostat cured it. Make sure to get a decent quality replacement, it will save you having to do it again.