Upgrading your Celica GT-Four with an HKS Super Sequential Blow Off Valve (BOV) is one of the most rewarding modifications you can make to your turbocharged 3S-GTE engine. Not only does it provide that iconic pshhh sound that turns heads, but it also protects your turbocharger from compressor surge, improves throttle response between shifts, and can even extend the life of your turbo bearings. The HKS Super Sequential is widely regarded as one of the best aftermarket BOVs for high-boost applications because of its dual-stage design that vents at low and high boost seamlessly, preventing stalling and maintaining driveability. This comprehensive guide will walk you through every step of the installation, from gathering the right tools to fine-tuning your new setup for maximum performance.

What is a Blow Off Valve and Why Upgrade?

A blow off valve is a pressure relief device installed between the turbocharger compressor outlet and the throttle body. When you lift off the throttle during boost, the throttle plate closes, and compressed air has nowhere to go. This pressure spike slams back into the turbo compressor wheel, causing compressor surge—a harsh fluttering sound that can quickly damage bearings and reduce turbo life. The BOV opens to vent that excess pressure either back into the intake system (recirculating) or to the atmosphere (vent-to-atmosphere).

The Celica GT-Four (ST185, ST205) came from the factory with a plastic recirculating BOV that is prone to leaking under higher boost levels. The HKS Super Sequential is a vent-to-atmosphere (VTA) BOV that uses a unique dual-port design: a small primary valve opens at low boost for crisp throttle response, and a larger secondary valve opens at higher boost for massive flow capacity. This sequential action prevents the rough idle and stalling issues often associated with single-stage VTA BOVs on MAF-based systems (note: the GT-Four uses a MAP sensor, so VTA is perfectly safe without fueling issues). Upgrading to the HKS unit not only sounds better but also holds boost pressure more reliably, reduces turbo lag between shifts, and can free up a small amount of horsepower by reducing backpressure in the intake tract.

Tools and Parts Required

Before you begin, ensure you have everything on hand. The HKS Super Sequential BOV kit (part number 71199-AT008 for flange type or 71199-AT007 for weld-on) typically includes the valve, a mounting flange or adapter, gaskets, vacuum fittings, and sometimes a spring selection. Confirm your kit matches your Celica's intake pipe diameter.

Essential tools:
  • Socket wrench set with 8mm, 10mm, 12mm, and 14mm sockets
  • Phillips and flathead screwdrivers
  • Torque wrench (recommended range 8–12 Nm for flange bolts)
  • Vacuum hose (4mm ID silicone, about 1 meter)
  • Silicone coupler and T-bolt clamps (if your kit doesn't include hose adapters)
  • Safety glasses and mechanic gloves
  • Shop towels and brake cleaner
  • Thread locker (blue Loctite) for flange bolts
  • A boost leak tester or smoke machine (optional but highly recommended for post-install check)

Safety Precautions and Preparation

Working on a turbocharged engine involves heat, electricity, and flammable fluids. Follow these steps to stay safe and avoid damaging your car:

  • Park the vehicle on a level surface and chock the wheels.
  • Disconnect the negative battery terminal and wait 10 minutes for the ECU capacitors to discharge.
  • Allow the engine to cool completely—touch the intake manifold and verify it is cool to the touch to prevent burns.
  • Remove any loose clothing or jewelry that could catch in belts or fans.
  • Have a fire extinguisher nearby when working near fuel or electrical systems.

Also, take a photo of the stock BOV and its vacuum line routing before disassembly. This reference will help if you need to revert or if you decide to run a recirculating setup later.

Step-by-Step Installation Process

Removing the Factory Blow Off Valve

The stock BOV on the Celica GT-Four is located on the passenger side of the engine bay, mounted directly to the intake pipe just before the throttle body. It is a two-piece plastic assembly with a small vacuum nipple. Here is how to remove it cleanly:

  1. Locate the stock BOV. Trace the intake pipe from the turbo outlet to the throttle body. The BOV is the round plastic unit bolted onto the metal pipe with two 10mm bolts.
  2. Disconnect the vacuum line. Pull the rubber vacuum hose from the nipple on top of the BOV. It may be held by a small clip; gently pry it off.
  3. Remove the two mounting bolts. Use a 10mm socket and extension. Be careful not to drop the bolts into the engine bay. If they are stubborn, spray penetrating oil and let it soak.
  4. Twist and pull the BOV free. The stock unit may be stuck to a rubber o-ring. Gently rock it side to side until it slides off. Do not use excessive force—if it is stuck, use a plastic pry tool.
  5. Clean the mounting surface. Wipe the metal flange on the intake pipe with brake cleaner and a lint-free cloth. Remove any old gasket material or debris. A clean surface ensures a leak-free seal with the new gasket.

Installing the HKS Super Sequential BOV

Now comes the exciting part. The HKS kit comes with a flange adapter that matches the stock Celica mounting pattern. Most kits include a 3-bolt flange—if yours does not match, you may need a separate adapter plate (available from HKS or aftermarket suppliers).

  1. Attach the flange adapter. Place the new gasket on the clean intake pipe flange. Then position the HKS adapter flange over the gasket. Insert the supplied bolts and tighten them to 10 Nm using a torque wrench. Apply a drop of blue Loctite to each bolt to prevent loosening from vibration.
  2. Mount the HKS BOV onto the adapter. The Super Sequential BOV has a large threaded collar that screws onto the adapter. Hand-tighten it firmly until snug—do not overtighten. If your kit uses a separate locking nut, tighten that as well.
  3. Connect the vacuum source. The HKS BOV requires a manifold vacuum reference. Locate a suitable vacuum port on the intake manifold (the stock BOV vacuum line is perfect). If the factory line is too short, extend it with the supplied silicone hose. Some installers prefer to tee into the brake booster line or use a dedicated port on the intake plenum. Ensure the hose is routed away from hot surfaces (exhaust manifold, turbo). The HKS valve has two small vacuum nipples: one for the primary diaphragm and one for the secondary. Connect the primary (usually the one closer to the valve body) to your vacuum source. The secondary (if present) can be left open to atmosphere or connected to a boost reference—consult your kit instructions. For most Celica installations, only the single vacuum line is needed; the secondary nipple is for additional boost response tuning.
  4. Secure all hose connections. Use the provided spring clamps or small zip ties to ensure the vacuum line does not pop off under boost. If using silicone couplers for the BOV outlet (if your kit is weld-on type), tighten T-bolt clamps to 3–4 Nm.

Note: If your HKS kit came with an optional dump tube (for recirculating), you can install it to keep the sound quieter and maintain a closed system, but most GT-Four owners run it fully atmospheric for the sound.

Reconnecting the Battery and Initial Checks

With the HKS BOV physically installed, double-check all connections:

  • Ensure the vacuum hose is securely attached and not kinked.
  • Verify that all bolts are torqued to spec.
  • Look for any loose clamps or hoses near the intake.

Reconnect the negative battery terminal. Start the engine and let it idle for 2 minutes. Listen for any hissing sounds that indicate a vacuum leak. If the idle is rough, there may be a leak at the flange or vacuum line. Use a can of brake cleaner or a smoke machine to pinpoint leaks. Rev the engine gently to 2000–3000 RPM and listen for the BOV opening—you should hear a distinct pshhh when you snap the throttle shut. If you hear flutter (repetitive stuttering sound), the BOV spring may be too stiff or the vacuum line is restricted. HKS offers different spring inserts for high-boost applications; the factory spring works well up to 1.5 bar.

Post-Installation Tuning and Test Drive

After confirming no leaks, take the Celica for a careful test drive. Start with light throttle and low boost to allow the BOV to seat properly. Gradually increase boost to full throttle in third gear. Observe the following:

  • Idle stability: The car should idle as smoothly as before. The HKS Super Sequential is designed to minimize idle disruption, but if you experience stalling or surging, check for vacuum leaks or try repositioning the vacuum source.
  • Shift response: Between gear changes, the BOV should vent with a crisp sound, and boost should build quickly in the next gear. If boost takes longer to build, the BOV may be leaking under boost. Perform a boost leak test using a homemade tester (PVC cap with a Schrader valve) or a smoke machine to verify.
  • Fuel trims: Since the GT-Four uses a MAP sensor, running VTA does not affect fueling. However, if you have upgraded your ECU or are running a piggyback, monitor air-fuel ratios via a wideband gauge. The BOV should not change AFRs unless there is a massive leak.

If everything checks out, congratulations—you have successfully installed an HKS Super Sequential BOV. Enjoy the improved throttle response, reduced turbo lag, and aggressive sound that turns heads at the track and on the street.

Troubleshooting Common Issues

SymptomLikely CauseSolution
BOV flutters or chattersSpring too stiff for boost level; vacuum line too long or restrictedInstall a softer spring (available from HKS); shorten vacuum line
BOV leaks under boost (no sound or power loss)Flange gasket not sealing; o-ring damaged; bolts looseRetorque bolts; replace gasket; inspect o-ring
Idle drops or stalls after decelerationVacuum leak at BOV flange; vacuum line collapsedRe-seal flange; replace vacuum hose with reinforced silicone
BOV opens at idle (constant hiss)Spring too soft; vacuum line incorrectly connected to boost source instead of manifold vacuumCheck vacuum source; install stiffer spring

Maintenance and Long-Term Care

The HKS Super Sequential BOV is a precision component that requires minimal maintenance, but a few checks will keep it operating perfectly for years:

  • Annual inspection: Remove the BOV and clean the piston and housing with brake cleaner. Inspect the O-ring for cracks or flattening. Lubricate the piston lightly with silicone grease—do not use petroleum-based lubricants.
  • Vacuum hose: Replace the rubber vacuum hose every 2–3 years as it hardens with heat and can crack, causing boost leaks.
  • Flange bolts: Check torque after the first 100 miles and again after a few track days. Heat cycles can loosen fasteners.

Conclusion

Installing an HKS Super Sequential Blow Off Valve on your Celica GT-Four is a straightforward upgrade that delivers noticeable performance and acoustic benefits. By following this step-by-step guide—preparing properly, removing the stock unit carefully, and tuning the vacuum system—you will enjoy a reliable, powerful, and responsive turbo setup. For further reading, check out the official HKS Super Sequential product page for specifications and spring options, and visit the GT-Four Forum for community tips and build threads. If you encounter persistent issues, a visual installation video can help clarify tricky steps. Now get out there and let that turbo sing.