07 xc70 lukewarm heat am stumped

benbug · Post by **benbug** » 09 Feb 2026, 21:01

Hey all,
I recently purchased an 07 xc70 with 168k miles on the clock to replace my last xc70 that was totaled in the fall.

When I purchased it I noticed the heat was very weak and has remained that way since putting on around 3k miles since then.

In an attempt to fix this I first flushed the core with a garden hose and refilled with coolant. No changes.

After taking it to a specialist, he also couldn't figure it out but gave me some ideas.
Here's what we've mostly ruled out/ may suspect
-Thermostat seems to be fine, car is getting to operating temp
-Blend doors may be stuck by we weren't able to find any codes and I can hear them opening and closing when I adjust temp control
-Inlet and outlet hoses on the core are different temps but have about a 30 degree difference. The specialist said this is too big a disparity in temperature as they should be closer.
-I see no signs of a bad head gasket and am not having to refill the car with coolant
-Adjustment knob for fan is finicky and will sometimes turn down when I'm turning it up and vice versa
- The airflow sounds louder than my last xc70, especially when the fan is cranked but I don't suspect or hear anything from the blower motor itself

With all that noted, if it is a blend door, is there one in particular that is known for going before the others?
Additionally, could old low quality coolant clog the core to the point where it's effecting my interior temp?

Any help is appreciated.

Thanks

Post by **cn90** » 10 Feb 2026, 06:53

I' d replace both Tstat (Facet is cheaper than Wahler) + heater core.

Re heater core, Behr is NLA, search forum for advices.

Re DIY, I wrote it up in XC90 forum.

Post by **chrism** » 10 Feb 2026, 11:11

A 30 deg drop between connections does sound like a lot but I don’t know what would be considered normal. It kinda sounds like the coolant is moving through the core very slowly.
When you saw the 30 deg drop was the blower fan running and was it blowing strongly, albeit not real hot, out of the floor vents?
How much does the temp drop if the blower fan ISN’T blowing.
What were the actual temps you witnessed going in and out of the core?
Did you flush the core in both directions?

Post by **cn90** » 10 Feb 2026, 11:58

At this mileage, I'd replace both: Tstat + heater core.
You WILL see the difference.

Post by **scot850** » 10 Feb 2026, 15:15

I wonder if the previous owner may have used head gasket in a can on this car. That stuff (depending on what version you use), is bad for clogging heater cores and other small diameter pipe work. I think I would suggest following cn90's advice and swap the tstat and heater core.

It is possible you have an air-lock in the heater core but if you have been driving the car around it should eventually clear.

Neil.

benbug · Post by **benbug** » 11 Feb 2026, 16:52

cn90 wrote: ↑10 Feb 2026, 06:53 I' d replace both Tstat (Facet is cheaper than Wahler) + heater core.

Re heater core, Behr is NLA, search forum for advices.

Re DIY, I wrote it up in XC90 forum.

Thanks cn90. I was looking into replacing the tstat but am getting some mixed advice on whether or not I can replace the tstat itself or need to replace the entire housing?

Additionally, it comes up to temperature fine but not sure if that in fact rules out the tstat being bad.

Any advice is appreciated!

benbug · Post by **benbug** » 11 Feb 2026, 16:59

chrism wrote: ↑10 Feb 2026, 11:11 A 30 deg drop between connections does sound like a lot but I don’t know what would be considered normal. It kinda sounds like the coolant is moving through the core very slowly.
When you saw the 30 deg drop was the blower fan running and was it blowing strongly, albeit not real hot, out of the floor vents?
How much does the temp drop if the blower fan ISN’T blowing.
What were the actual temps you witnessed going in and out of the core?
Did you flush the core in both directions?

Hi chrism, the specialist said 30 degrees was greater than normal. The temps he registered were out:105 in:140. It was only blowing about 95 degrees out of the vents inside. I'm not sure about the drop/ difference if the fan isn't blowing.
Yes, I did flush in both directions and saw some crud come out. At the end had good, clean flow

benbug · Post by **benbug** » 11 Feb 2026, 17:06

Does anyone have thoughts on SKP, 4 seasons or nissen heater cores?

Georgeandkira · Post by **Georgeandkira** » 11 Feb 2026, 19:08

A heater core was packed with "radiator leak-stop" putty.
Increasing the severity of cleaners, I finally got to gasoline. This dissolved the goo.
Use sacrificial lengths of parts store hose and a funnel and catch to do this...outdoors.
It was a fast process.

benbug: "whether or not I can replace the tstat itself or need to replace the entire housing?"

I've seen posts featuring the simple opening of that housing and replacing the 'stat.
Today I wouldn't trust the piecemeal parts acquisition routes to deliver the correct 'stat to fit the housing.
I think I'd buy the housing complete to avoid the risk. I don't know what the price is today. It was ~$156 pre-pandemic.
Also, I don't recall if there was a gasket used.

Post by **MoVolvos** » 11 Feb 2026, 19:45

Georgeandkira wrote: ↑11 Feb 2026, 19:08 A heater core was packed with "radiator leak-stop" putty.
Increasing the severity of cleaners, I finally got to gasoline. This dissolved the goo.
Use sacrificial lengths of parts store hose and a funnel and catch to do this...outdoors.
It was a fast process.

benbug: "whether or not I can replace the tstat itself or need to replace the entire housing?"

I've seen posts featuring the simple opening of that housing and replacing the 'stat.
Today I wouldn't trust the piecemeal parts acquisition routes to deliver the correct 'stat to fit the housing.
I think I'd buy the housing complete to avoid the risk. I don't know what the price is today. It was ~$156 pre-pandemic.
Also, I don't recall if there was a gasket used.

.
In the mid 2000's I tried a father and son shop that had a good reputation and was in a segment of a radio show I use to listen to. Immediately after picking up the 95 Plymouth Grand Voyager with the 3.0 V6 Mitsubishi motor for a headgasket replacement I had the following issues.

1) Ran terrible due to timing being off. Surprised they didn't road test with all that work done. Redo timing.
2) Fuel smell coming into cabin. Wanted to charge me an arm and leg for seals on the injectors as it was not included in the headgasket repair. Turn out to be the fuel line not fully tightened.
3) No heat. I remove the hose to the core and saw gunk. When confronted they said Copper Seal is added in all headgasket repairs. They never had a clog heater core. Never fully flowed even after cleaning.

As for the thermostat assembly Rock Auto is very reasonably prices with name brands like Mahle, SKP, VEMO and Four Seasons from $36 to $66.
.