Corolla GT Coupe - Royalty Black

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It's not really an issue.

Everything bolts on as soon as you place the parts near the holes.... It's really a lego....

If you had a levin, you have some smaller spacers that will not allow the Trueno lights to bolt you need to remove those from the front plates of the radiator.

That's it, that's the only change

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Back in 2015, I installed a 20V Blacktop while keeping the original 16V coolant routing.

At the time, I had no issues whatsoever with engine temperatures. I was only relying on the factory digital cluster, as I didn't have a Defi Link gauge to monitor the actual coolant temperature.

My setup was:

Cluster temperature sender at the back of the cylinder head (the hottest part of the engine).
Fan switch installed in the upper radiator hose (coolant returning from the radiator).

Despite the sender being located at the hottest point, I never experienced any temperature issues on the cluster.

The new setup...

When I installed the Defi gauge, I also made a few changes:

Low-temperature thermostat.
Cluster sender moved to the upper radiator hose (coolant returning from the radiator).
Defi sender installed at the back of the cylinder head.
Fan switch also installed at the back of the engine.

In hindsight, this wasn't a great setup.

Although the engine itself was running very cool thanks to the low-temp thermostat, the fan switch was reading coolant at the hottest point in the engine. As a result, the fans were running almost constantly, even when they weren't really needed.

Time to do things properly

I decided to redesign the cooling system.

The changes included:

Installing a genuine OEM thermostat (bench-tested before installation).
Moving the fan switch to the engine outlet, just after the thermostat.
Replacing the radiator with a brand-new one.
Flushing and checking the entire cooling system.

That's when the real headaches started...

The digital cluster temperature would suddenly climb much higher than before.

While investigating everything, I also discovered that I had been using a Daihatsu Charmant water pump with a 6-blade impeller instead of the correct AE86 7-blade version. ?

I honestly thought that had to be the culprit...

But after replacing everything, the behaviour was exactly the same.

The breakthrough

After a lot of troubleshooting (and with a little help from AI), I started looking at the ignition timing.

Sure enough, the timing wasn't quite right.

Correcting it made a significant improvement.

Before that, the digital cluster would climb almost to the top of the gauge, even with the fans running. Clearly something wasn't right.

Current status

At the moment:

The car drives perfectly, with the cluster sitting right in the middle.
When stopped at traffic lights, the temperature rises slightly, but the fans eventually bring it back to normal.
After a long drive, if I let the car idle, I still notice some heat soak before the temperature gradually returns to normal.

Overall, I think my fan switch is simply switching on a little too late, allowing the cluster temperature to climb more than I'd like before the fans start pulling it back down.

The reason I never really panicked is because my Defi Link gauge, which measures coolant temperature at the back of the cylinder head, has always shown perfectly reasonable temperatures of around 90–100°C, which is exactly what I'd expect from a fully warmed-up 4AGE.

So the engine itself doesn't appear to be overheating—it's just the cluster reading that likes to scare me!

What a saga... ?

I'm still not completely satisfied, though. I'd really like to understand why the cluster temperature rises so dramatically during heat soak and whether there's still something I can improve. Seeing the gauge climb like that isn't exactly reassuring, even if the actual coolant temperature remains within a normal range.

Cheers!