Reverse The Heater Matrix Flow on a Lotus Exige

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Have you ever looked into your engine bay and said "What are these hoses for?" or "I wish it was easier to get to things"? Then you're in luck, several of those hoses can be shortened and/or discarded completely freeing up precious engine space.

A Lotus has four coolant bypass (hot coolant skips radiator before entering engine block) locations and only two of which are needed in my opinion. The remaining two I recommend blocking off making it easier to cool the engine, but with the disadvantage of longer warm up times.

Bypass locations:

  • Bypass Line (Not needed)
  • Throttle body (Not needed)
  • Heater Matrix (Required unless removing Heater)
  • Swirl pot (a.k.a, coolant tank, header tank) (Required)

Bypass line: There is a bypass line flowing hot coolant directly from the engine block back into the block bypassing the thermostat and radiator. The reason for this is to speed warm-up of the engine. Blocking adds minutes to engine initial warm-up, but makes cooling more efficient once warmed up (important when on Track).

Throttle Body heater: There are two lines circulating hot coolant through the throttle body. This is done to prevent the butterfly valve from freezing / sticking in cold temps. Unless your Lotus is driven in freezing temperatures there is no good reason to leave this in place. The coolant return also bypasses the radiator and thermostat, blocking this off improves cooling.

Heater Matrix: The heater matrix is a web of hoses running all around the engine compartment. Due to a few engineering decisions and the routing of the bypass hoses mentioned above, this results in about 4' (yes, four feet) of excess hose. Simply reversing the direction coolant runs through the heater matrix allows for the removal of the excess.

Before beginning any of the instructions below READ THE ENTIRE INSTRUCTION SET and ENSURE YOU HAVE PARTS BEFORE Starting..

I assume no responsibility for the quality of these instructions or your workmanship.


  1. Remove the under trays below the engine. This can be done with them on, but coolant will spill and it's easier to clean the floor than those panels.
  2. Remove / Open the rear clam.

All engines with a thermostat need some type of bypass to avoid localized hot spots, minimize development of steam pockets, enable quick warm-up, and prevent water-pump cavitation.

If you're running a thermostat then two 3/16" holes need drilled into the thermostat flange ( the outer diameter, looks like the frame of the thermostat, do not drill the inner piece that moves) to allow a constant circulation of coolant. This is much better than the bypass and the coolant runs through the radiator first and provides all the benefits listed above.

If you're NOT running a mechanical thermostat (e.g., you installed an Electric Water Pump and controller) then you're all set as long as that controller pulses coolant when below the "hot" temp.

Bypass Line - Block off


Obtain access to the coolant port.

  1. Remove the fuel rail, manifold, and supercharger. If running an aftermarket supercharger follow the instructions that came with it to remove.

You may be able to get to it from the bottom of the car, but it won't be easy and you're going take a bath in coolant.


Assemble the block off for engine port

  1. Cut a 1/2" ID heater hose to an approximate length of 57mm (2.25")
  2. Insert the Fleece Performance PFE-34071 plug into one of the hose ends and secure with a hose clamp. Constant tension clamps are used in these instructions, worm clamps, T Clamps, or the like will also work.
  3. Place a hose clamp on the other end of the hose as close as possible to the other clamp and if adjustable (e.g., worm clamp) snug to the hose. DO NOT TIGHTEN.

You should have something similar to what is pictured below. Ensure the hose open end clamp is all the way against the other clamp. This will allow slipping the hose on to the engine port quickly.

Image 8191 from Reverse the Heater matrix flow on a Lotus Exige

Block off 1/2" ID coolant exit at rear of engine (under intake manifold / supercharger)

  1. Place multiple paper towels, old T-Shirt, rags, etc.. under the coolant hose and ports. This will be used to capture any coolant spillage.
  2. Disconnect the battery for safety. Notice that electrical connector (A) in the picture on the starter.. Don't get coolant in there with the power on.
  3. Use Pliers to expand the hose clamp and disconnect the hose from the coolant return hard line. Be ready to capture a small amount of coolant draining from the hard line.
  4. Place a Dorman bypass cap (B) on the hard line port as a temporary plug
  5. Loosen the clamp on the hose securing it to the engine port, do not remove hose yet.
  6. Have the hose with the plug in one hand and with the other hand pull the hose off the engine port. Slide the plug hose on quickly to minimize coolant spillage.
  7. Place a hose clamp to secure the hose with the plug to the engine port.

Your engine should resemble the picture below.


Decide if you're going to reverse the Heater Matrix or not.

If not you'll need to create another plug and install it on the coolant return hard line to replace the Dorman cap. Do not trust that Dorman bypass cap, they're complete junk and will rot and burst in a year or two.

Throttle body heater


Determine the current state of the Throttle body / IAT coolant lines.

  • OEM configuration of a Lotus 2ZZ-GE motor has two small (1/4") coolant lines running from the engine block (A) to the throttle body (C) and returning into the silver coolant return hard line (B) directly below the heater matrix coolant output port on the block.
  • It is very common to replace the OEM configuration with a bypass to avoid heating the throttle body and the air within it to enhance performance. As a result, you may just see a (1/4") coolant line running from the same location on the block (A) to the coolant return hard line (B). The hose may have a copper looking sleeve midway through it, this is a thermostat and only there as carry over from the OEM configuration. It's worth noting heating the TB is for (1) cold start emissions and (2) prevent freezing / sticking in very cold weather. Provided you're not driving in Alaska during the winter you should be okay without it.
  • Alternatively, both ports (A, B) may already be blocked off completely. If this is the case skip this section as your work is already done.
  1. Use Pliers to release the hose clamp securing the coolant line marked as (A) in the picture above from the engine block.
  2. Use Pliers to release the hose clamp securing the opposite end of the line from the throttle body or coolant return hard line depending upon the current state of your setup.
  3. Place multiple paper towels, old T-Shirt, rags, etc.. under the coolant hose and ports. This will be used to capture any coolant spillage.
  4. If the throttle body has not been bypassed - Disconnect the line marked (B) in the image above from the throttle body and then the coolant return hard line.
  5. Drain residual coolant from the throttle body and place vacuum caps on the throttle body ports now exposed by removing the lines.

Vacuum caps are perfectly acceptable for the throttle body as no coolant or pressure exist. Those caps are purely to keep debris from entering the ports.


Block off the engine port by choosing one of the following methods.

Method 1: Hose with Block off plug

  1. Cut a 1/4" ID heater hose to an approximate length of 57mm (2.25")
  2. Insert the Chi Company barbed hose plug (04C03103) into an hose end and securely clamp. Constant tension clamps are used in these instructions, worm clamps, T Clamps, or the like will also work.
  3. Place a hose clamp on the other end of the hose as close as possible to the other clamp and if adjustable (e.g., worm clamp) snug to the hose. DO NOT TIGHTEN.
  4. Install on the engine port exposed when disconnecting line A and tighten the hose clamp.

Method 2: Convert to an AN fitting and cap

Reverse Heater lines


Return coolant line.

  1. Use Pliers to release the hose clamp and disconnect the Heater Return line (A) from the hard line at the drivers side of the engine. Move this hose out of the way up toward the firewall as it will be eventually routed to the thermostat location under the supercharger / intake manifold.

Remove heat soak pump

  1. Use a 13mm wrench and socket to remove the nut and bolt securing the emergency heat pump to the bracket connected to the transmission. remove the nut and bolt.
  2. Disconnect the wiring harness from the pump
  3. Place multiple paper towels, old T-Shirt, rags, etc.. under the coolant hose and ports. This will be used to capture any coolant spillage.
  4. Use Pliers to release the hose clamp and disconnect the coolant hose (A, B) from the engine block.
  5. Follow the hose (B) to the other end (C) and use Pliers to release the hose clamp securing it to the hard line below.
  6. Use a 18mm socket to remove the two bolts securing the pump mount to the transmission.
  7. Discard the mount and pump; put the bolts back in for future use.

Regarding the heat pump. In my opinion this is like a life jacket for your car. Happy you have it for the .0001% of the time when it's needed and totally pointless every other time. If you wish to retain it for peace of mind, leave it in place and ensure the new hoses route through it during the following instructions.


Disconnect passenger side

  1. Place a catch pan or something to capture the coolant that will spill when disconnecting the hose.
  2. Use Pliers to release the hose clamp on the return heater coolant line (a) where it connects to the hard line by the AC condenser on the belt side of the motor. The picture below is of an Exige without Air Conditioning, if you have A/C then there will be two large rubber lines blocking your view.
  3. Both ends of the hose are now disconnected, carefully remove the hose from the engine bay

Remove coolant return hard line

  1. Use Pliers to release the hose clamp (A) securing the swirl pot / coolant tank return line to the hard line above the thermostat.
  2. Use a 10mm socket to remove the three bolts (B, C) securing the hard line to the block.
  3. If you have a catch can setup with a bracket use a 10mm (E) & 13mm (D) sockets to remove the bolt/stud securing the hard line to the block above the starter
  4. If not use a 13mm deep socket to remove the bolt (D) securing the hard line to the block
  5. Remove the hard line, be sure not to loose the gasket between the line and the block by the thermostat

Reroute heater Matrix coolant


Connect the engine block output to the hard-line by the air filter

  1. Use a 3' (1m) length of 3/4" diameter coolant hose with a suitable hose clamp and secure to the hard-line. Depending on the route chosen a 90-degree elbow may be needed to avoid collapsing the hose at the bend.
  2. Route the hose to your liking and secure it to the engine block port with a suitable hose clamp. Beware of the shifter linkage on top of the transmission, those move, ensure your hose routing does not interfere/block the movement.

If using silicone hose ensure a silicone hose compatible clamp is used. You need one with out exposed holes if using a worm clamp.

Comp Cams G31220 (summit) or Pegasus Racing SHL29-BLE work well.

Image 8208 from Reverse the Heater matrix flow on a Lotus Exige

Shown with BOE Fabrication Swirl Pot installed

You'll need:

  • (2) 90 degree elbows
  • 2' straight hose cut into two lengths
  • 8 - G31220 hose clamps

Swirl Pot return line


Connect swirl pot return line to thermostat port

In order to connect the swirl pot return line you'll need to either modify the existing coolant return hard line or create a new flange from scratch, which is a lot easier than you think. Below are instructions for two options, which assuming you have access to a welder or a dude with a welder, anyone should be able to follow.

Reuse OEM Coolant return line

This method will support reversing the heater matrix while reusing a modified coolant return hard line. The advantages of this over the other method below is less fabrication and no modification to the coolant system is required.

Overview: This will explain how to modify the coolant hard line to support connecting the heater matrix return line and swirl pot return while minimizing complexity and weight.

  1. With the hard line removed, cut it about 4.5" in from the side (see picture). You need to ensure clearance with the sensor port on the block.
  2. Weld a 12AN Stainless Steel weld bung on the end of the pipe and use an 12AN Flare Cap to seal it. Alternatively you could just weld a cap on the line and cut it even shorter, but this method allows you to reverse the modification easily.
  3. Reinstall to the thermostat port. Use the original gasket if in good shape or replace with a new one.
  4. Reconnect the swirl pot return line to the OEM port on the hard line (the penis shaped port by my finger in the picture).
Image 8921 from Reverse the Heater matrix flow on a Lotus Exige

Option 2 - create a new flange

  1. Trace the outline of the OEM flange onto a 3/16" thick 6061 2" wide Aluminum flat plate. Cut the outline and deburr. The flange is 2.125" long x 1.375 height, any aluminum plate larger than those dimensions will work.
  2. Copy the hole locations to the new flange, drill them out with a 6mm drill bit
  3. Drill out the center hole with an appropriate drill bit. Not all weld bungs are created equal. Some have flat bottoms others have a lip that seats the mating object. Depending on yours the hole size needed may vary.
  4. Weld a 6AN Male Aluminum weld bung on over the center hole
  5. Reinstall on the thermostat port, ensure to replace or reuse the gasket
  6. Attach a Aluminum 3/8" Barbed Hose End - 6AN Female, 90 degree adapter to the new flange. Validate the return line from your swirl pot / cooolant tank is 3/8" before ordering/installing the part.
  7. Cut a new clean end to the existing swirl pot return line and install on the barbed end of the 90 degree adapter.

Optionally, paint, powder coat, or anodize the new plate.

Image 8922 from Reverse the Heater matrix flow on a Lotus Exige
Image 8923 from Reverse the Heater matrix flow on a Lotus Exige

Heater return line - tie into Coolant line

** This only works if not running a thermostat.*****

The heater return line needs to be tied back into the engine block

Overview of this procedure 1. Cut the main coolant return line below the alternator, insert a custom Y joiner, rejoin the coolant line and hook the heater return line to the Y adapter. The main coolant line and heater return will share the same 1.25" coolant hose to return to the thermostat housing.

Lacking a welder or access to a dude with a welder I outsourced my Y adapter to Universal Parts. They make similar parts for Jaguars and only required slight modification to their Z17555 part to work on the Lotus.

You need a single 1.25" diameter aluminum tube with a bead on both ends and a single 3/4" port with bead running out at ~30 degrees back flush with one end. See picture below for a visual. The 30 degree is important, do not make it more than 45 and make sure it runs WITH the coolant flow (must point to the left as pictured). You need to minimize the potential of coolant running backward through the return coolant line.


Find the correct location in the coolant return line to insert the Y adapter.

  • Place the Y adapter on top of the (A) coolant hose and find the location where the Y is pointing directly at the (B) heater return hard line.
  • Mark the coolant hose as close as possible to the Y joint.
  • If the coolant line is still secured to the firewall (C), cut the zip tie to free it.
  • Place a drain pan below the hose where you'll cut and have a few rags close by.
  • Cut the hose (A) in half at the mark and capture the coolant spilling out

Prepare the elbow for installation

  1. Cut one leg of the 6" 90 degree hose to 4". If a 4" was purchased skip this step.
  2. Cut the other leg down to 3"
  3. Slide the G31220 hose clamps on each end and tighten down just to prevent falling off.. do not tighten, not even snug.

Your mileage may vary from car to car.. ensure you eyeball the cuts before making them to ensure a good fit and that you don't cut too short.

Image 8309 from Reverse the Heater matrix flow on a Lotus Exige

Connect heater return line to Y adapter

  1. Install the 90 degree hose on the heater return hard line first. Push down far enough the other leg is level with the Y fitting.
  2. Connect the elbow to the Y fitting
  3. Tighten the hose clamps
Image 8310 from Reverse the Heater matrix flow on a Lotus Exige

Reinstall the 1.25" line from the Y adapter to the thermostat housing

  1. Mark the hose where to cut to mate to the Y adapter
  2. Cut hose and retest fit
Image 8312 from Reverse the Heater matrix flow on a Lotus Exige
  1. Slide a 3613 hose clamp (A) on the end of the hose connecting to the Y adapter.
  2. Install hose on the thermostat housing and Y adapter.
  3. Reuse the OEM constant tension clamp (B) on the thermostat housing
  4. Tighten the 3613 hose clamp (A) to secure to the Y adapter.

Refill and bleed the coolant system of air and check for leaks.

DISCLAIMER Last updated August 28, 2021

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