How to Install a Top Feed TGV Housing on Subaru WRX STI

Quick answer

Installing a top feed TGV housing on a Subaru WRX STI means removing the intake manifold and factory side-feed TGV assemblies, then bolting on top feed housings with new gaskets and torquing the manifold in the factory sequence. The job is mostly labor: plan a full day, label every hose and connector, and use the correct torque specs to avoid vacuum leaks. If you are deleting the TGV butterflies, you will also need a tune to clear the related check engine light.

Swapping to top feed TGV housings on a Subaru WRX STI is one of those jobs that looks intimidating until you have the intake manifold off. Once you are there, the actual housing change is straightforward: unbolt the old side-feed TGV assemblies, clean the mating surfaces, and bolt the top feed housings in place with fresh gaskets. The time goes into the removal and reinstallation around them.

This guide walks through the process in order, with the checkpoints that keep most people out of trouble: labeling vacuum lines, protecting the injector seals, and torquing the intake manifold in the correct sequence. It also covers what changes when you delete the TGV butterflies and why a tune is part of the job, not an optional extra.

What you need before you start

Gather parts and tools first so the car is not sitting open overnight. You will need the top feed TGV housings, new intake manifold gaskets, TGV-to-head gaskets, and top feed injectors with the correct seals and adapters for your setup. Have new intake manifold bolts on hand if yours are corroded, plus a torque wrench that reads in the 5-25 lb-ft range. Basic hand tools, a 10mm and 12mm socket set, pliers for hose clamps, and a pick for connectors cover most of the work.

If you are doing a TGV delete at the same time, confirm whether your housings are already machined or need the butterfly shaft removed. Some top feed housings come ready to install with no butterflies; others require you to swap or modify the shaft. Check the housing before the car is apart so you are not improvising mid-job. Also confirm your engine management can be tuned for the delete, because the factory ECU will throw a code once the TGV motors are unplugged.

• Label every vacuum line and electrical connector with tape and a marker before removing it.
• Photograph the engine bay from a few angles before disassembly; the photos are faster than a service manual for hose routing.

Step 1: Depressurize and disconnect the fuel system

Relieve fuel pressure before opening any fuel line. Pull the fuel pump fuse or relay and run the engine until it stalls, then crank it once more to confirm pressure is gone. Disconnect the negative battery terminal and wait a few minutes before touching fuel components. Place a rag under the fuel line fittings when you crack them, because a small amount of fuel will still be in the line.

On the STI, the fuel lines and injector connectors sit under the intake manifold, so most people remove the manifold first and then disconnect the fuel system. Either order works, but do not leave open fuel lines sitting while you work on other steps. Cap or tape the ends to keep debris out.

• Common mistake: skipping the pressure relief and getting a face full of fuel when the first fitting opens.
• Checkpoint: the engine should not start after the fuse is pulled; if it does, you pulled the wrong fuse.

Step 2: Remove the intake manifold and accessories

Disconnect the throttle body connector, idle air control hoses, and any vacuum lines attached to the manifold. Unbolt the manifold in a cross pattern, working from the outside bolts inward, and lift it straight up. On turbo models, the manifold often hangs up on the turbo inlet and fuel lines, so lift slowly and check for anything still attached before forcing it.

Once the manifold is off, set it on a clean bench. This is the point to inspect the factory TGV housings and the intake ports. Look for oil residue in the intake tract, which is common on older turbo Subarus and worth addressing while the manifold is off. If you are replacing the turbo inlet or adding an air-oil separator, this is the easiest access you will get.

• Common mistake: prying the manifold before all bolts are out; a missed bolt can crack the manifold flange.
• Checkpoint: the manifold should lift with light hand pressure. If it resists, something is still connected.

Step 3: Remove the factory TGV housings

Unbolt the TGV assemblies from the cylinder heads. There are typically two bolts per side plus the TGV motor connectors and any coolant lines on some models. Remove the TGV motors and set them aside if you plan to keep them for a future build. Pull the housings straight off the head studs; if they are stuck, a soft mallet tap on the side is safer than prying on the mating surface.

Scrape the old gasket material off the cylinder head and the TGV flange with a plastic scraper. Do not use a metal scraper or abrasive disc on the aluminum head surface. Wipe the surfaces with brake cleaner and let them dry. Any leftover gasket material becomes a vacuum leak once the new housing is bolted down.

• Common mistake: reusing old TGV gaskets. They are cheap and a leak here is hard to diagnose later.
• Checkpoint: the head surface should be clean, dry, and free of deep scratches before the new housing goes on.

Step 4: Install the top feed TGV housings

Set the new gaskets on the head studs, then slide the top feed housings into place. Start the bolts by hand to avoid cross-threading, then snug them in a cross pattern. Torque the TGV housing bolts to the factory specification for your model year; if you do not have the spec, use a conservative 18-20 lb-ft for the M8 hardware and verify against a service manual before final tightening. Over-torquing can distort the housing and cause a boost leak.

Install the top feed injectors with new seals and the correct adapters for your fuel rails. Lubricate the injector O-rings with a small amount of clean engine oil so they seat without tearing. Route the fuel lines so they do not rub on the TGV housing or the manifold, and check that the injector connectors reach without stretching the harness.

• Common mistake: pinching an injector O-ring during installation. If the engine idles rough or smells like fuel, this is the first thing to check.
• Checkpoint: with the fuel system pressurized, look for weeping at the injector bases before you reinstall the manifold.

Step 5: Reinstall the intake manifold and torque in sequence

Lower the manifold straight down onto the TGV studs, guiding the fuel lines and connectors as it goes. Start all manifold bolts by hand, then torque in the factory sequence, usually a cross pattern from the center outward. The intake manifold bolts on most Subaru EJ engines are in the 18-22 lb-ft range, but confirm the spec for your year. Tightening one bolt fully before the others can distort the manifold flange and create a leak.

Reconnect the throttle body, idle air control, vacuum lines, and electrical connectors. Double-check that every line you labeled during removal has a home. A single open vacuum port will cause a rough idle and a lean code, which is easy to misdiagnose as a bad tune.

• Common mistake: torquing manifold bolts in a circle instead of a cross pattern; this is a common source of intake leaks.
• Checkpoint: after the first start, spray brake cleaner around the manifold flanges. An idle change indicates a leak.

Step 6: Reflash and first start

If you deleted the TGV butterflies, the ECU will set a check engine light for the missing TGV motors. You need a tune that accounts for the delete before the car runs correctly. Do not drive the car hard until the tune is loaded and you have confirmed there are no vacuum or boost leaks. On a top feed conversion, the injector scaling also changes, so the tune is not optional.

On first start, let the car idle and watch for fuel or coolant leaks. Check the intake manifold area with a flashlight. Once it is warm, blip the throttle and listen for a whistle that would indicate a boost leak. If everything is clean, take a short low-load drive and recheck for codes before a full pull.

• Common mistake: driving on the factory tune after a TGV delete. The car may run, but it will not run right.
• Checkpoint: no fuel smell, no coolant drips, and no check engine light after the first heat cycle.

Install order at a glance

1. Relieve fuel pressure: Pull the fuel pump fuse, run the engine until it stalls, then disconnect the battery. Checkpoint: no fuel spray when you crack the first line.
2. Remove the intake manifold: Label every hose and connector, unbolt in a cross pattern, and lift straight up. Checkpoint: manifold lifts with light pressure; if not, find the missed bolt.
3. Remove factory TGV housings: Unbolt the TGV assemblies, disconnect motors, and clean the head surface with a plastic scraper. Checkpoint: no old gasket material left on the head.
4. Install top feed TGV housings: New gaskets, hand-start bolts, torque in a cross pattern, and install injectors with new seals. Checkpoint: no weeping at injector bases after pressurizing.
5. Reinstall the intake manifold: Lower straight down, hand-start all bolts, torque in the factory cross sequence. Checkpoint: spray brake cleaner at flanges; idle change means a leak.
6. Reflash and first start: Load a tune that accounts for the TGV delete and injector scaling before driving hard. Checkpoint: no codes, no fuel smell, no coolant drips after a heat cycle.

Parts that support this job

• PLM High Flow Performance Metallic Catalytic Converter & Flange Extension Pipe: PLM High-Flow 200-Cell Metallic Catalytic Converter is designed for enhanced exhaust flow, performance, and emissions compliance, and the bundle includes the flange and extension pipe to save time and labor on fabrication. If you are already doing top feed TGV work and planning exhaust changes, bundling the converter with the flange and extension pipe reduces fabrication time and keeps the exhaust side matched to your setup.
• Precision Works Aluminum Boost Control Solenoid Cover for 08-21 Subaru STi: Precision Works Aluminum Boost Control Solenoid Cover replaces the steel boost solenoid cover on 2008-2021 STi with a powder-coated aluminum plate, includes required hardware, and is a bolt-on installation. The boost control solenoid sits near the intake manifold area you are working in. If your factory steel cover is corroded, this is a simple bolt-on replacement to do while access is easy.

A top feed TGV housing install on a Subaru WRX STI is a full-day job that rewards patience with clean labeling and correct torque. Take your time on the gasket surfaces and injector seals, confirm your tune before you drive hard, and the conversion will behave like a factory setup with better flow.