September 1, 2026 Daniel Ho

Lexus IS300 3UZ Swap Part 6: Flex Fuel & First Start

Our 3UZ-swapped Lexus IS300 finally runs.

After getting the 4.3-liter 3UZ-FE V8 and TL70 6-speed transmission into the car in Part 5, we're moving on to the supporting systems needed to turn this swap into an actual running and driving drift car.

For Part 6, we're installing an eFlexFuel flex fuel system on the 3UZ, wiring all eight injectors, installing the ethanol content sensor, finishing the fuel lines, bleeding the clutch, checking our TL70 transmission clearance, deleting the AC system, and attempting the first real startup.

The flex fuel installation is surprisingly straightforward. The injector connections use plug-in jumper harnesses, so we don't need to cut into the injector wiring.

By the end of the episode, we hit one of the biggest milestones of the entire build:

The 3UZ starts and idles inside the IS300.

If you're building your own IS300, check out our Lexus IS300 parts and Lexus IS300 ECU and tuning upgrades at Lunatics Co.

Watch the Lexus IS300 3UZ Swap Build Series

This is Part 6 of our Lexus IS300 3UZ-FE swap series. If you're new to the build, catch up on the previous episodes first:

Part 1: Starting Our Lexus IS300 3UZ Swap
Watch Part 1 on YouTube

Part 2: Lexus IS300 3UZ Swap Build
Watch Part 2 on YouTube

Part 3: Preparing the 3UZ-FE for the IS300
Watch Part 3 on YouTube

Part 4: Preparing the IS300 for the 3UZ
Watch Part 4 on YouTube

Part 5: Installing the 3UZ and TL70 Transmission
Watch Part 5 on YouTube

In Part 5, we paired the 3UZ-FE with a TL70 6-speed manual transmission, installed the clutch and flywheel, used the Xcessive Manufacturing adapter setup, dropped the entire drivetrain into the IS300, and installed an oil pan baffle for drifting.

Now it's time to get it running.

Can You Install Flex Fuel on a 3UZ-FE?

Yes. A 3UZ-FE can be set up to run flex fuel when the injectors, fuel pump, fuel lines, engine management, and other fuel-system components are compatible with ethanol and the increased fuel requirements.

For our 3UZ IS300, we're installing an eFlexFuel system.

The system uses an ethanol content sensor and its own controller to determine the ethanol content of the fuel and make the necessary fueling adjustments.

One of the biggest advantages of this system for our installation is the wiring.

Instead of cutting and splicing all eight injector wires, the system uses plug-in injector jumper harnesses.

Lunatics Co carries both the eFlexFuel E85 Flex Fuel Kit for the Lexus IS300 and the more advanced eFlexFuel E85 Flex Fuel PRO Kit for the Lexus IS300.

Our vehicle obviously isn't a factory 2JZ-GE IS300 anymore, so we're adapting the system around the eight-cylinder 3UZ-FE configuration.

What Does a Flex Fuel System Do?

A flex fuel system allows an engine to operate with varying concentrations of gasoline and ethanol instead of requiring one fixed fuel blend.

The ethanol content sensor measures the ethanol concentration in the fuel.

That information allows the system to compensate for the different amount of fuel required when ethanol content changes.

For a car that may see different ethanol concentrations from one fill-up to another, that flexibility can be useful compared with configuring the car around only one specific blend.

Lunatics Co carries a complete selection of eFlexFuel systems for several Lexus platforms.

Why We're Using eFlexPro on Our Modified IS300

Our IS300 isn't exactly stock anymore.

We have a 3UZ-FE V8, manual transmission swap, custom fuel-system components, and a car that's being built specifically around drifting and track use.

The eFlexFuel PRO system for the IS300 adds additional control and tuning functionality intended for modified applications.

The eFlexPro system offers features including:

Automatic ethanol-content compensation
Wideband AFR integration
Additional analog sensor inputs
Smartphone monitoring
Injector duty-cycle monitoring
Fuel-temperature monitoring
Advanced fuel tuning through eFlexTuner
Data logging

For a heavily modified project, having additional monitoring and adjustment capability gives us more flexibility as the build continues.

Installing the eFlexFuel Harness on a 3UZ V8

The first thing we did was assemble the injector harness.

Because the 3UZ is a V8, we need connections for eight injectors.

The flex fuel harness includes additional connections that allow us to expand the injector harness for the extra cylinders.

We started by plugging those additional leads into the main harness.

There's no need to permanently install everything on the engine immediately.

We wanted to preassemble as much as possible before putting it into the car.

Identifying the 3UZ Injector Power Wires

The next step is important.

Before finishing the injector jumper harnesses, we need to identify the power side of each injector connector.

We temporarily plugged the connectors into the 3UZ injectors and marked the corresponding side of each connector.

Once all eight were identified, we removed the connectors again.

Now we could finish assembling the flex fuel harness.

For our setup, the red wire from the flex fuel harness needed to correspond with the injector power side we had just marked.

Once we knew the correct orientation, each connector could be snapped together.

There's no cutting into the original injector wiring during this process.

Connecting All Eight 3UZ Injectors

With the harness assembled, we brought everything back to the engine.

Each jumper harness connects inline between the engine harness and its corresponding injector.

The biggest thing to pay attention to is keeping each individual injector circuit together.

You don't want to route one side of a jumper to one injector and accidentally connect the other side to another cylinder.

We worked our way around the engine until all eight injectors were connected.

Having the 3UZ accessible made the job considerably easier, although the same installation could still be performed with everything completely assembled in the car.

Cleaning Up the Flex Fuel Wiring

Once all eight injectors were connected, we started routing the harness alongside the existing engine wiring.

We ran the wiring underneath and around the engine wherever possible rather than simply laying the harness across the top.

Both sides eventually merge near the flex fuel controller.

From there, we still have wiring for the ethanol content sensor and temperature sensing.

The ethanol sensor wiring needs to travel toward the side of the engine bay where we're routing our fuel line.

The temperature sensor can remain within the engine bay so the system can monitor ambient engine temperature and make adjustments for conditions such as cold starts.

We're intentionally not making the wiring completely permanent yet.

Before spending time perfectly wrapping, mounting and zip tying everything, we want to know that the engine starts and that every system works.

Once everything is confirmed, we'll come back and make the installation look finished.

Grounding the Flex Fuel System

The flex fuel controller also requires a good electrical ground.

For our initial installation, we're grounding the system directly to the engine.

A reliable ground is important for any electronic fuel-control system.

The final connection needs to be secure and made against a suitable grounding point with good electrical contact.

With the injector harness and ground handled, we could move on to the fuel side.

Installing the Flex Fuel Ethanol Sensor

The ethanol content sensor measures the amount of ethanol present in the fuel.

For our installation, we're positioning the sensor near the fuel line in the engine bay.

We used a 90-degree fitting on one side and a straight fitting on the other because that combination follows the direction we need the fuel line to travel.

There's no reason to make the fuel line take unnecessary turns if the fittings can be configured around the existing layout.

Our final sensor mounting location will also keep it accessible if we need to inspect or service the system later.

Connecting the Flex Fuel Sensor to the 3UZ Fuel System

We already simplified the 3UZ fuel system earlier in the build, so integrating the ethanol sensor is relatively straightforward.

The sensor gets installed inline with the fuel supply.

We connected our fuel hose to the sensor and used clamps appropriate for fuel-system use.

The hose then continues toward the 3UZ fuel rail.

Whenever you're working on a fuel system, take the connections seriously.

Make sure the hose, fittings, seals, and clamps you're using are appropriate for the fuel and pressure involved. Components also need to be compatible with ethanol if the vehicle will be running E85 or other high-ethanol blends.

Before starting the engine, every connection should be inspected carefully for leaks.

With the plumbing finished, we connected the sensor to the flex fuel controller.

Can You Run E85 in a 3UZ?

A 3UZ-FE can be configured to run ethanol-based fuels, but converting to E85 involves more than simply filling the tank with E85.

The fuel pump, injectors, fuel lines, seals, engine management, and other components need to support the required fuel volume and ethanol content.

E85 requires a different amount of fuel than gasoline, which is why proper fuel control is important.

Our flex fuel system allows us to measure ethanol concentration rather than assuming every tank contains exactly the same blend.

That becomes particularly useful with a flex fuel setup because ethanol concentration can vary.

Finishing the 3UZ Drive-By-Wire Setup

Before we could really test the engine, we also needed to continue working on the throttle system.

The 3UZ uses an electronic throttle configuration, so the original IS300 throttle setup doesn't simply connect to it like the factory 2JZ-GE.

We're converting the car around the 3UZ drive-by-wire setup.

This is another example of why an engine swap is rarely just an engine swap.

Getting the engine physically into the car is only one part of the project.

You still have to make the fuel system, throttle, cooling, wiring, transmission, clutch, power steering, exhaust, and other systems work together.

First Start of the 3UZ-Swapped Lexus IS300

Once the flex fuel system was connected and we added oil, it was time to see if the engine would actually start.

It did.

Hearing the 3UZ fire inside the IS300 was a huge milestone.

The first startup wasn't the end of the work. The engine still needed some attention before it would consistently stay running and idle correctly.

But the important part was that it ran.

After everything involved with preparing the chassis, adapting the transmission, installing the drivetrain, and wiring the supporting systems, we finally had a running 3UZ inside the car.

Bleeding the TL70 Clutch

While continuing to button up the car, we also needed to bleed the clutch.

We're using a TL70 6-speed manual transmission behind the 3UZ.

If you missed that installation, check out Part 5 of the build:

Watch Part 5: Installing the 3UZ and TL70 Transmission

Our clutch hydraulics use AN lines and banjo fittings to adapt the IS300 clutch system to the new transmission.

The bleeder remains accessible with the drivetrain installed, although there isn't a huge amount of room.

It can get a little messy because getting a hose onto the bleeder isn't particularly easy, but we still have enough room to service it.

For anyone interested in a similar UZ/TL70 combination, browse our Lexus IS300 swap parts for swap-related engine, transmission, and drivetrain components.

Checking TL70 Shifter Clearance

With the drivetrain positioned, we could also see where the TL70 shifter actually landed inside the IS300.

This was one of the major reasons we wanted to experiment with the TL70.

It's relatively compact compared with some other transmission options, and packaging was a major concern with this car.

We moved the transmission through the gears and checked clearance around the shifter opening.

Even in first gear, we still have room.

That means we shouldn't have to completely redesign the center tunnel just to make the shifter work.

It's a small victory, but engine swaps are basically a long series of small victories.

Why We're Deleting AC From the 3UZ IS300

We've also decided to remove the air conditioning from this car.

That doesn't mean every 3UZ-swapped IS300 needs to delete AC.

It makes sense for what we're doing with this particular build.

The car has evolved into a dedicated drift-focused project with solid mounts throughout the chassis. Comfort isn't exactly the priority anymore.

Removing the AC system gives us additional room, particularly on the driver's side of the engine bay.

That makes it easier to access the exhaust manifold and other components when we're servicing the car.

We also plan on taking the IS300 to drift events farther from home rather than using it as a normal street car.

Simplifying the car is becoming more valuable to us than retaining every factory comfort feature.

3UZ Belt Routing With Power Steering and AC Removed

We're also simplifying the accessory drive.

The factory hydraulic power steering system is being removed because we're planning to use an electric power steering setup.

We're also deleting AC.

That means the accessory belt configuration needs to work without those original components.

We installed the belt configuration we're using with the power steering pump deleted and continued checking alignment before running the engine.

Later in the build, we'll address the electric power steering system separately.

Getting the 3UZ to Idle

After working through the remaining connections and intake setup, we tried starting the car again.

This time it settled into a steady idle.

That's the moment that made this episode worth it.

The engine wasn't sitting on a stand anymore.

It wasn't hanging from an engine hoist.

It wasn't just bolted into the chassis waiting for another dozen systems to be connected.

The 3UZ was sitting inside the Lexus IS300 and idling.

After everything we've done to get here, that's a huge milestone.

Does eFlexFuel Require Cutting the Injector Wiring?

Our installation did not require cutting the injector wiring.

The system uses jumper-style injector connections that install inline with the existing injector connectors.

For our V8 application, we needed to assemble the additional injector connections and correctly identify the power-wire orientation for all eight injectors.

That's considerably cleaner than permanently cutting into the injector harness.

Lexus IS300 3UZ Flex Fuel FAQ

Can you install flex fuel on a 3UZ-swapped Lexus IS300?

Yes, but a swapped vehicle is a custom application. The system needs to be configured around the 3UZ's eight injectors and the specific fuel system being used.

Can a 3UZ run on E85?

A 3UZ can be configured for ethanol fuel when the injectors, fuel pump, lines, engine management, and other fuel-system components are suitable for ethanol and the required fuel volume.

Does the eFlexFuel system require cutting injector wires?

Our installation did not require cutting the injector wires. The injector connections use inline jumper harnesses.

What does the ethanol sensor do?

The sensor measures ethanol content in the fuel. That information allows a flex fuel system to account for changing gasoline and ethanol concentrations.

Where did you install the flex fuel sensor?

We positioned our sensor in the engine bay along the fuel-line route leading toward the 3UZ fuel rail.

What transmission are you using behind the 3UZ?

We're using a TL70 6-speed manual transmission from the FR-S/BRZ/86 platform.

You can see the complete transmission and drivetrain installation in Part 5 of our 3UZ IS300 build.

Is the 3UZ IS300 keeping air conditioning?

Not in our current configuration. We're removing AC to simplify our drift-focused build and create additional room for servicing the engine and exhaust.

Is the 3UZ IS300 using hydraulic power steering?

No. We're removing the factory hydraulic power steering system and plan to convert the car to electric power steering later in the build.

The 3UZ IS300 Finally Runs

This was another major step forward.

We started the episode installing flex fuel and ended it with a running 3UZ-swapped Lexus IS300.

During Part 6, we completed:

eFlexFuel harness assembly
Eight-cylinder injector harness installation
Injector power-wire identification
Flex fuel harness routing
Ethanol content sensor installation
Fuel-line connections
Flex fuel controller installation
Drive-by-wire work
First 3UZ startup
TL70 clutch bleeding
TL70 shifter clearance checks
AC delete preparation
Accessory belt changes
Final startup and idle testing

The car still isn't finished.

We have more work ahead with cooling, electric power steering, final wiring, fuel-system cleanup, and everything else required before the car can actually go drifting.

But we're getting closer.

The 3UZ is in.

The TL70 is in.

The engine runs.

And for the first time, this project is starting to feel like a car again.