Mitigating GR Yaris Gen 2 Heat Soak: Why Your Boost Disappears Between Heats

The Gen 2 GR Yaris has a heat soak problem. The MAP sensor on the intake manifold gets heat soaked, and the ECU responds by pulling boost.

What took us a while to work out was when it happens. It isn't while you're driving. It's when the car is standing still. A typical TimeAttack day: 15 to 20 minute heat, car runs perfectly. Then 45 minutes parked in the pits. Go out for the next heat and the boost bar sits under +1 where it's normally close to +2. No warning light, no fault code, nothing. The car just feels flat. Sometimes it comes back after a few laps and sometimes it doesn't.

Why the ECU does it

The MAP sensor reads intake air temperature as well as pressure, and the ECU uses that reading to protect the engine. If the incoming air really is hot, pulling boost and timing is the correct response.

The trouble is that a heat-soaked sensor isn't measuring the air going into the engine. It's measuring itself. Sit in the pits for 45 minutes with everything around the sensor cooking, and it reports a number far above reality. The ECU sees dangerously hot intake air and protects against a problem that isn't there.

The threshold is commonly reported as around 55 °C. We haven't verified that ourselves, but it fits what we see.

What didn't work

We went through the obvious things first. We run an AIRTEC intercooler and the AIRTEC auxiliary cooler kit, plus intercooler spray. All of it does exactly what it should for charge air temperatures, and none of it helped, because none of it does anything for a sensor being cooked by its surroundings while the car is parked. Bonnet up between sessions helps the engine bay in general but it's slow and unfocused.

The one thing that made a real difference was a leaf blower aimed at the area before each session. That was the clue. Cooling the charge air changed nothing; forcing air directly at the sensor changed something. But standing in the pits with a leaf blower isn't a fix, and you can't do it on the grid.

The pipe

Looking at the area properly, we found a coolant pipe running about 15 to 20 mm behind the MAP sensor, carrying coolant at close to 100 °C.

It sits there radiating heat straight at the back of the sensor, for as long as the car is parked. That reframed the whole thing for us. It doesn't matter how much cold air you eventually get to the front of the sensor when there's a 100 °C heat source 15 mm behind it, pointed right at it.

This is why we think a cooling duct on its own only gets you halfway. A duct is genuinely useful, but it spends the whole time fighting that radiation. And when the car is stationary there's no ram air and barely any flow through the bumper, so the duct has almost nothing to work with while the pipe carries on radiating.

Block the radiation and the duct isn't fighting anything.

Building it

We scanned the area for accurate geometry and designed two parts: a duct feeding air from the bumper opening onto the sensor, and a heat shield sitting between the coolant pipe and the sensor.

Simple enough on screen. Less so in the car. The space is tight, nothing can touch the coolant pipe, nothing can foul under engine movement, and we didn't want the customer cutting or drilling anything. We also wanted a smooth internal path through the duct rather than sharp direction changes that kill the flow before it arrives.

It took about 20 printed prototypes and three redesigns to get there. Almost all of that was fitment. The gap between "fits on the bench" and "fits in the car, with the bumper on, and stays put" is where the iterations went.

Material

This mattered more than we expected. The part lives next to a 100 °C pipe, on a car that gets tracked and then parked outside in Swedish weather. PLA and PETG were never in the conversation, and even ordinary nylons soften at temperatures this part will actually see.

We landed on PAHT-CF, a carbon fibre reinforced high-temperature polyamide. Heat deflection is 194 °C at 0.45 MPa and 170 °C at 1.8 MPa, so there's a wide margin over anything it will meet. It absorbs far less moisture than standard PA-CF, which matters for something permanently exposed to road spray, since nylons that take on water go soft and move dimensionally. The carbon fibre keeps it stiff enough to hold its shape under sustained heat.

What you get

The duct feeds ambient air onto the sensor from the front. The shield blocks the radiant heat from the pipe behind it, which is the part that still works when the car is sitting still with no airflow at all.

Every other solution we've found deals with the airflow side only. As far as we know this is the only kit that also handles the pipe, which based on everything we saw is the part that actually matters in the pits.

It's a bolt-on fit. No cutting, no drilling, nothing permanent, and you can put the car back to standard whenever you want.

Being straight about it

This doesn't eliminate heat soak. Nothing does. The sensor still lives in a hot engine bay next to a hot engine, and no plastic part changes that.

What it does is take away the constant heat source behind the sensor and feed it ambient air from the front, so it runs cooler and steadier than it does with a duct alone. The reading stays closer to the truth, and the ECU has less reason to pull boost it doesn't need to pull.

We built this because we had the problem, at real events, and nothing we bought fixed it. If your car makes full power in the first heat and goes flat in the second, this is what we came up with.

GR Yaris Gen 2 MAP Sensor Cooling Duct + Heat Shield Kit →

Fits Toyota GR Yaris Gen 2 (2024+, G16E-GTS). Designed, tested and printed in Sweden.