Happy NY all!

Recently noticed that my 1.6tdi is taking longer to heat up, esp in this cold snap and when it does, temp will drop down from 90'C when say going down hill.

No coolant leaks and water pump was done around 30k ago & car has done 142k miles.

I'm think thermostat stuck open? Is this likely? and is it a diy job?

When did you last change your coolant? If it's over 5 years old, the coolant may be weak and not transferring heat properly to/from the engine. With that said, the thermostat is also a candidate. It's located around where the water pump is....

51 minutes ago, varaderoguy said:

When did you last change your coolant? If it's over 5 years old, the coolant may be weak and not transferring heat properly to/from the engine. With that said, the thermostat is also a candidate. It's located around where the water pump is....

Coolant was changed 3 years ago, i noticed a while back that the car seemed to be taking longer to warm up but put it down to weather, but yesterday, temp didn't get near 90'C and on a down hill section, went cooler, then Amber DPF light came on.... i believe the thermostat is on battery side of engine (its a Diesel, not a Petrol)... & looks like a mission to get at....

This morning i checked coolant tank and level is low.... when the pump was changed after it leaked, along with cam belt, the car still reached temp correctly...

Top up with some G12EVO and try again.

12 hours ago, varaderoguy said:

Top up with some G12EVO and try again.

Why would a low coolant level when cold cause cool running?

Always have the correct coolant level - basic checks. Then look further at the issue; you are probably right that it is the thermostat though,

1 minute ago, varaderoguy said:

Always have the correct coolant level - basic checks. Then look further at the issue; you are probably right that it is the thermostat though,

Ok thanks, i was surprised to see it so low, def just below min mark, which is making me think there could be another issue, water pump.

Yes, if you are running with the original water pump; they will just die (impellers just grenade and destroy themselves). New VAG waterpumps are metal;

You get two warnings with waterpumps; temperature rockets....goes down.....rockets up again then the pump dies.

9 hours ago, varaderoguy said:

Yes, if you are running with the original water pump; they will just die (impellers just grenade and destroy themselves). New VAG waterpumps are metal;

You get two warnings with waterpumps; temperature rockets....goes down.....rockets up again then the pump dies.

No, the pump was replaced around 35k ago, though i can't be sure if it was a genuine part.....

This morning took car for a 30 mile drive (to get Thermostat) temp stayed on 50' for 4 miles, then very slowly went to 70, then held at 80 for about 10mins, eventually stuck at 90 and then stayed there..

Normally would be on 90 within 5 miles, whatever the weather.

Will change it out and see what occurs.

Sounds like the thermostat in that case....but it is worth replacing both and ideally with genuine VAG parts.

It does sound like thermostat but question would be where the coolant went to. Check its not leaking anywhere especially around thermostat etc. My old Octavia did the same. Fine in summer but took ages to warm up in winter. Thermostat was past it.

Alasdair

9 minutes ago, Alasdair1 said:

It does sound like thermostat but question would be where the coolant went to. Check its not leaking anywhere especially around thermostat etc. My old Octavia did the same. Fine in summer but took ages to warm up in winter. Thermostat was past it.

Alasdair

Is it fairly easy to do? looks fiddly from the YT vids.....

Yes the loss of coolant is a worry, as this is what happened when the water pump failed, just a gradual loss of coolant until it was having to be topped up daily but this was a very small amount to top up, maybe 250ml and i haven't checked it for a few months

Hopefully a small leak at thermostat housing. Check for tell tale crystal deposits.

Alasdair

As to where the coolant went to....the other niggle with the coolant header tank is the cap. These have a built-in pressure relief. All TDI's when they regen seem to pressurise the cooling system and they all loose a little coolant when the regen process occuring. The therory goes that the spring plastic slider inside the cap (that should be relieving the pressure) - works hard and the pressure relief stops happening (because the spring cannot open and relieve the pressure). Therefore the cooling system over pressures and the system will then dump coolant out of the overflow down the side of the engine bay.

The Gen 3.5 cars had a revised coolant bottle and cap. If you order: Genuine VAG MQB Coolant Expansion Tank Cap – 2Q0121321A - it may resolve the issue with the coolant loss.

I had these exact symptoms last year (2024! I haven't adjusted yet). Turned out to be the dreaded cracked water pump housing. Ended up replacing whole water pump assembly and belt for good measure as they are prone to fail on this platform. Though I have the 2.0 TSI petrol, but I think the diesels suffer the same problem? (somebody correct me if I am wrong, please)

Edited 6 January6 Jan by Ripmax12345

2 hours ago, Ripmax12345 said:

I had these exact symptoms last year (2024! I haven't adjusted yet). Turned out to be the dreaded cracked water pump housing. Ended up replacing whole water pump assembly and belt for good measure as they are prone to fail on this platform. Though I have the 2.0 TSI petrol, but I think the diesels suffer the same problem? (somebody correct me if I am wrong, please)

I think the Pump is in a different place on the diesels... mine has been replaced already.

On my 1.9tdi its inside the timing belt cover area. I replaceed it when I do the belt.

If you are still losing coolant after thermostat replacement try a plastic bottle with the expansion vessel overflow pipe in to it. If there is any coolant loss through expansion cap when driving you can see it collecting in bottle.

Check passengers side carpet area for dampness or signs of crytals in case its the heater matrix leaking.

Alasdair