Coolant rerouting

More power = More heat

Posted on

When the MX5 was born, Mazda took an exiting engine, the BP 94 and rotated it 90 degrees to make it fit into the engine bay and to power a rear wheel drive car. Unfortunately when they did this, the original engine coolant routing (which took hot liquid from the back of the engine to the radiator, cooled it and returned it to the front of the engine colder) was no longer viable.

Mazda made an adjustment to the engine, adding a new pipe to take take heat out of the front at the top of the engine into the radiator, then returning it at the bottom of the engine at the side and into the water pump.

This means that all of the cooling, inlet and outlet is now at the front of the engine except for a small branch which runs some of the engine coolant out and through the heater matrix to heat the cabin. As a result, Cylinder 4, the one nearest the cabin, is starved of coolant and is prone to running hotter than all of the others.


The evidence cited comes from a change that Mazda made in the head gasket when they introduced the Mk 2.5 which constricted the flow of water on the front two cylinders and forced more coolant to the back cylinders. On friend, whose engine have had a lot more heavy driving miles on did show an increased wear on the 4th cylinder when they came to strip it down, but this engine also had no oil in it when it was purchased so, maybe that didn’t help?

Cylinder 4

The main website I came across, with lots of lovely diagrams of the issue ( is run by SkidNation, who sell a reroute kit to fix the issue. Conversely, Super Miata ( in the US state: “The Qmax coolant reroute is a must have for any Miata experiencing cooling issues.” But, also this is trying to sell a reroute kit.

So… under normal driving conditions, it seems likely that the existing coolant system on the MX5 is absolutely fine. It took Mazda until nearly 15 years of the MX5 to worry about making a change, and it’s possible this was only needed when they introduced the higher powered VVT engine.

But we’re about to supercharge Ermy, she already has 100k of mileage on her and if we put more power through the engine, well bad things might happen. She is also often driven like she is stolen, so coolant rerouting cant hurt.

One alternative some people have proposed to the reroute is to fit a bigger, thicker, aluminium radiator with higher flow and a second fan, which also dissipates heat. We did that too.


Our Reroute kit came from Skidnation, though of course other kits are available. Some of these are even in anodised orange too so we clearly missed a trick. But why do we need a kit? Surely its just a bit of hose and some blanking plates?

The main reason that you cannot just bolt a different outlet pipe on the back and a blanking plate on the front is the thermostat, which is located in the front outlet.

This is a mechanical thermostat which opens once the engine gets up to temperature. It helps the engine retain heat when it first starts, and once it is at running temperature gets the coolant loop opened up. The Thermostat needs to be relocated so that this function still works, with the thermostat needing to be located in the outlet pipe. We thus need a thermostat spacer/housing to hold this.

Ermy doesn’t have a heater, which reduces complexity a little, but if you did, you would also need to have a point for this hose to be attached and to return the coolant to the cooling loop. Skination have a different kit, for those who still have their heaters.

This is the kit Skidnation sent us.


The main specialist part – Is this piece of machined metal, a spacer which sits at the back of the engine and creates space for the thermostat (Shown)


A Kia branded part which has the same bolt spacing sits on the back of this, providing a connector for the hose which runs down the side of the engine to the front and connects to the top of the radiator.


You remove the outlet pipe and thermostat at the front of the engine and this is then blanked off using one of the branded blanking plates provided.

Front Plate

The problem is, you cannot fit the kit if you have an EGR valve as this involves a steel hose which runs from the exhaust to the EGR Valve. The large block at the rear of the engine which routes coolant to the front sits right where the EGR pipe is. You can see it here in its original form, tucked tightly under the connector where the water temperature sensor is. In fact the OEM part is specially shaped to accommodate the EGR. The revised part is not.


So… We ordered an EGR delete kit.

EGR Delete

Sort of. An EGR “delete” is typically the two plates above, but you still need to block up the hole in the exhaust.

EGR exhaust

To do this, we removed the existing EGR screw thread from the pipe and and welded a plate over the end, grinding it to shape.

EGR Exhaust closed

The problem we have here is that we’ve changed the radiator, fitted a second fan and done the coolant reroute. If the engine is colder, we won’t have a clue why, or what is making the most difference. We also don’t know if cylinder 4 was that hot or not.

But, colder is colder so on with the supercharging!


Welcome to Twits with Spanner. A blog following two chaps as they attempt to build an Exocet kit car with bits from a donor MX5, spanners and lots of hope. Read more about us