GR Yaris Coolant Temperatures Too High Despite Upgraded Cooling System

JohnCassidy

JohnCassidy

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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!
:)
 
I think the same, doesn’t seem unreasonable.

The other car has stock cooling and is better? It wouldn't be the first time an “upgrade” was actually not, either because the were developed in isolation (i.e. additional cooler(s) are blocking/heating airflow for others), not installed correctly (e.g. a constricted pipe as you mention), or just plain not actually any better than the factory system.
 
I agree that the engine coolant temperatures are within an acceptable range, but I believe they should be lower.

I personally know several people who use the GR Yaris as a track car. The modifications on my car are quite common, and I have never heard of them causing this kind of issue. The tuner who tuned my car has worked on around 40 GR Yaris builds, including full-blown motorsport cars, and he also believes the coolant temperatures seem higher than expected for this setup.

The cooling system was upgraded step by step rather than all at once. Because of that, I would expect to have noticed if any individual modification had introduced a negative effect.

The entire system has been checked multiple times for installation issues, leaks, and any other common problems I could think of. I have never observed any air in the cooling system, although I cannot completely rule that out.
 
Addition to Observation 1:

I also got data from another GR Yaris that was present that day. It has a very similar cooling setup, the only difference is the brand of the aftermarket main radiator, but it also has forged internals, cams and an even larger Garrett turbo.
  • Maximum oil temperature: 107°C
  • Maximum engine coolant temperature: 92°C
Despite running a larger turbo, its maximum engine coolant temperature was nearly 10°C lower. This is much closer to the temperature I would expect to see on my car.
 
Yeah, the fact that adding some revs when stationary brings the sensed temperature down suggests that there may be some air in there.

Maybe worth adding another temp sensor somewhere else to check it not just that and a bit of heat soak though.
 
I‘m nearly 100% sure you have air in your system…

When hot, have someone rev the car to 4000rpm and try to compress the big coolant hose which goes from the engine to the radiator over the gearbox with your hand.
Wear gloves!!!
You can instantly feel if it‘s full of coolant or nearly empty.
I‘d bet a fair amount, it‘s not full…

Bleeding it is a hilarious stupid process and can take ages.
 
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