GR Yaris Coolant Temperatures Too High Despite Upgraded Cooling System

[JohnCassidy]

Hi everyone,

I'm trying to diagnose an unusual cooling issue with my 2021 GR Yaris.

On track, my oil temperatures are excellent, but my engine coolant temperatures are consistently higher than I would expect, especially considering the cooling upgrades.

Relevant modifications​

• Airtec Stage 3 oil cooler
• Koyorad upgraded engine coolant radiator
• Airtec turbo and auxiliary radiators (replacing the front fog lights)
• Vented hood
• Airtec Stage 1 intercooler
• Modified intake, hybrid turbo, upgraded valve springs, EcuTek tune (those modifications are probably less relevant, but I included them for completeness)

All data is gathered using EcuTek logging software.

Observation 1​

Ambient temperature: 25°C

• 20-minute track sessions
• Fast circuit, average speed above 135 km/h
• Maximum oil temperature: 106°C
• Maximum coolant temperature: 101°C

For comparison, a friend's GR Yaris with similar power but a completely stock cooling system reached only 95°C coolant temperature during the same event.

Observation 2​

Ambient temperature: 42°C (!)

Same track and also 20-minute sessions.

• Maximum oil temperature: 116°C
• Maximum coolant temperature: 115°C

Quite bizarre correlation here.

Observation 3​

While waiting in the pit lane, idling before entering the track, the coolant temperature naturally rises due to the lack of airflow.

However, something interesting happens if I rev up the engine for some time (for example idle -> 3,000 RPM). The coolant temperature drops very quickly. Just to illustrate the drop can be something like from 90°C to 85°C.

Observation 4​

What is also important to mention, the car cools down very quickly. Depending on conditions, one slower lap (not even proper cool down lap) can result in over 10 degrees of coolant temperature drop.

Current condition​

• No power loss
• No drivability issues
• The car remains competitive on track

I'm mainly concerned because I would expect the coolant temperature to be at least 5–10°C lower, given the upgraded cooling system.

What I've already checked​

• Replaced the thermostat
• Performed a CO₂ test (negative, no combustion gases in the coolant)

My current theory​

Based on these observations, I suspect that coolant flow may be restricted.
Possible causes I'm considering are:

• Failing water pump
• Partially blocked coolant passages
• Some issue with my cooling system setup

My next step is to replace the water pump and pulley.

Has anyone experienced something similar or have any ideas about what else I should investigate?

I can also provide EcuTek logs from the track sessions if that would help with diagnosing the problem.

Thanks a lot in advance!