Peculiar cooling system problem

[Finlandia]

On a Volvo T5, when at idle or driving slow it reaches normal temperature, but driving 50+ the gauge drops quickly, to rise again once slowing down. I have tried it with the torque app and normal temperature is around 84-92 on a 90 degree thermostat, on motorways it barely reaches 70.

The thermostat is new and tested to work in boiling water, no error codes, car runs fine but doesn't blow hot air inside like it used to, no leaks in the cooling system.

Could it be the temperature sensor, even if it's newly changed too? Or if anyone has any suggestions on something else?

[Finlandia]

TooMany2cvs said:

Finlandia said:

The thermostat is new and tested to work in boiling water

That's nice. It's still duff, though.

Lets hope you're right, but I don't really see how it can be faulty when it opens and closes as it should in a pan of water.

[Finlandia]

I haven't actually measured the temperature when it opens, but before the water is boiling it's open and closes as the water cools down again.

There is only the one thermostat in the old T5, at least to my knowledge.

[Finlandia]

littleredrooster said:

Finlandia said:

Lets hope you're right, but I don't really see how it can be faulty when it opens and closes as it should in a pan of water.

As you bring the pan up to heat from cold, at what temperature does it open; and just as important- at what temperature does it start to close?

My money says it may open and close in warm water, but nowhere near the correct temperature.

You have a point there, will check and see the temperatures. On a 90c thermostat, when should it start opening and when should it be close?

[Finlandia]

GroundEffect said:

It doesn't sound particularly odd. To me it sounds like a cluster issue.

Cruising along with a normally low ambient (i.e. ~15C) would see a 70C ECT. I have seen those temps before in testing.

Typical thermostat failure modes are:

1) (if stuck open) the system never warms up unless excessive load
2) (if stuck closed) fine operation at low loads/town work but overheating at higher speed/load

Your heater comment is too subjective to diagnose. What is the time to temperature? Get a thermocouple in there and do a test!

It doesn't really sound like it would be stuck open or close, maybe just not opening and closing at the right temperatures. Will have to check further.

At what temperature should a 90c thermostat start opening and closing?

[Finlandia]

DonnyMac said:

I had the exact same problem on an old 5 Series 15-20 years ago just after a major service.

The car eventually seized.

The dealer had put the brand new thermostat in the wrong way around.

BMW gave me a new engine, which was nice.

Mine is the right way in there, but the thought of the engine running cold is a bit worrying and I really don't fancy having it seize on me.

[Finlandia]

Jimmyarm said:

Put a genuine Volvo thermostat in it

It is a genuine Wahler/Volvo one, may try an aftermarket one tomorrow just to be sure.

[Finlandia]

ShayneJ said:

1. Your thermostat is buggered

Starting to sound like it.

ShayneJ said:

replace it also i don't know if
the T5 has a viscous fan or electric
but maybe check if viscous that the clutch hasn't seized
or leccy check the switch is not failed short making the fan run constantly

It's electric and seems to be working as it should.

[Finlandia]

feef said:

To clarify, it SHOULD be sitting at 84-92, but the dash display and the Torque app are showing it as <70?

On the dash, when the temperature is where it should be at 3 o'clock, the torque app shows the temperature range at between 84 and 92, this is idling or driving slow city traffic, when doing motorway speeds the dash gauge falls to 4 or 5 o'clock or around 70 on the app.

[Finlandia]

Thanks for all the replies and help.

I will try a new thermostat before doing anything else, then get one of those IR thingies too.
I thought the Volvo cooling systems are self bleeding, leave the cap off and run the car at idle for 10 minutes and let cool, top up coolant and done?

Will try a more thorough bleeding once I have the new thermostat in as well.

[Finlandia]

andy-xr said:

Just feel the hoses to the matrix, either they've been plumbed back in wrong or there's an airlock that's stopping heat getting around that part

That doesn't solve the main problem which I think is a stat that's either fighting the fan or sticking open

The hoses to the heater matrix are thin and well insulated so cant really feel anything, but I will try bleeding the system properly once I have a new thermostat in there too.

[Finlandia]

So I went to the local dealer and bought a new Wahler/Volvo thermostat, fitted it and bled the system with the heater on max, while keeping an eye on the dash gauge as well as the Torque app, and it works!
The car now gets to temperature and stays there no matter what.

Thanks all for your help and suggestions

[Finlandia]

I had the top off for some work and a new thermostat was fitted during that, a genuine Volvo made by Wahler, which was the one that gave these issues, today I bought another new Volvo/Wahler thermostat, which seems to work fine.

Will be going back with the old new one tomorrow.

[Finlandia]

morgrp said:

T5 engine has a self bleeding system - I've never had to bleed one - One of the reasons they can soldier on for years with a weeping head gasket as they are constantly relieving pressure in the system. One issue they do suffer with is cracked expansion tank caps - prob worth replacing it too - just make sure it is the correct kPa rating

That's what I thought too, but I let it run with the cap off for a while and the heater on max, just to be sure. I also replaced the cap earlier, the old one was worse for wear, with a 150kPa green one.