Under Bonnet Mysteries

I'm a bit OCD when it comes to wiring, I like everything to be neat and tidy. That means all wires bundled together, all connections soldered and heatshrinked. So every time I get a new car, I go through all the wiring to make sure everything was done right. Unfortunately, this is never the case...

While cleaning up the engine bay wiring, I found something really strange. Check out the photos below:

Had to do some intense detective work to figure this out. Turns out it's a 1.5kOhm resitor that is in series with the coolant temp sensor for the ECU. This temp sensor's function is tell the ECU when to use the cold map (rich fuel mixture), this resistor in series tricks the ECU into thinking that the engine is actually cold.

In parralel with the resitor is a thermoswitch which closes when temperature rises to a specific point (not sure what temp though), this then shorts out the resistor and everything is back to normal so the ECU now thinks that the engine is warm.

I believe whoever did this wanted the engine to run on the cold map for longer because of Mazda's stupid coolant routing desing. This design flaw causes the rear of the engine to be hotter then the front. Meaning that the coolant temp sensor at the rear of the engine is reading warm when the engine is actually still a bit cold.

Although the mod works, there is no reason to have it. After removing all that junk, the car runs normal and no longer runs rich. So confused why they did this.... so silly!

The thermoswitch used -

Also spent the weekend replacing all the hoses and flushing out the coolant. Pretty happy to brand new hoses in the engine bay =)

Intake and hoses removed.

All back together and happy again =)

And just another sneak peak before the new wheels and tyres go on.
Taken while we were at MX-5 Plus.

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  1. Are you sure it’s not parallel?

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