Why Upgrade the Intercooler on Your Mitsubishi Eclipse?

The turbocharged Mitsubishi Eclipse—especially the 1G (1990-1994) and 2G (1995-1999) models equipped with the 4G63T or the later 420A turbo setups—relies on efficient charge air cooling to produce safe, repeatable power. The factory intercooler was designed for stock boost levels and quickly becomes a bottleneck when you increase boost pressure, upgrade the turbo, or push the engine harder. Hot intake air reduces oxygen density, raises the risk of detonation, and forces the ECU to pull timing. Installing an aftermarket intercooler drops intake air temperatures (IATs) by 30-50°F or more under boost, allowing the engine to run more aggressive timing and produce more horsepower without knocking.

This step-by-step guide walks you through the entire process of replacing the factory intercooler with an aftermarket unit on a Mitsubishi Eclipse. While the instructions focus on the 2G Eclipse GSX or GST (4G63 platform), the general steps apply to other generations and turbocharged models. Always reference your specific kit’s instructions and your vehicle’s service manual for torque specs and exact fastener locations.

Tools and Materials Needed

Gather these items before you begin. Some specialty tools will save hours of frustration.

  • Aftermarket intercooler kit – Choose a kit designed for the Eclipse (e.g., ExtremePSI or MAPerformance front-mount intercooler kit). Ensure it includes core, piping, silicone couplers, T-bolt clamps, and mounting hardware.
  • Socket and wrench set – Metric sizes from 8mm to 17mm, including extensions and a universal joint for hard-to-reach bolts.
  • Screwdrivers – Flathead and Phillips for clips and trim.
  • Trim panel removal tool – Helps pop plastic clips without breaking them.
  • Hose clamps and silicone couplers – Spare 2.5″ and 2.75″ sizes if your kit doesn’t include extras. T-bolt clamps are preferred over worm-gear clamps for high boost.
  • Drill and step drill bit – For enlarging mounting holes or removing rivets.
  • Dremel or cutoff wheel – For trimming the bumper support or crash bar (common on front-mount installs).
  • Measuring tape or ruler – For centering the core behind the bumper opening.
  • Jack and jack stands – To raise the front and gain access underneath.
  • Boost leak tester – Essential after installation to verify every connection is sealed.
  • Safety glasses and gloves – Always protect yourself when cutting or working under the car.
  • Coolant catch pan – If your intercooler piping interferes with the radiator or coolant lines, you may need to drain some coolant.

Preparation Steps

Work in a well-ventilated area. Allow the engine to cool completely before beginning. Disconnect the battery negative terminal first to prevent any accidental short circuits while unplugging sensors or wiring near the front bumper.

1. Raise the Front of the Car

Position the Eclipse on level ground. Use a hydraulic jack to lift the front crossmember and secure it on jack stands. Remove the front wheels if you need easier access to the inner fender liners and bumper bolts. Many factory intercooler pipes and mounting points are behind the wheel well liners.

2. Remove the Front Bumper and Support

The front bumper on a 2G Eclipse is held by several 10mm bolts and push clips. Start by removing the lower splash shield (if equipped) using a flathead screwdriver or trim tool. Then remove the screws along the top edge under the hood, the two bolts on each side inside the wheel well, and the bolts underneath the bumper. Have a helper support the bumper as you pull it forward to disconnect any fog light connectors. Set the bumper aside in a safe place.

Next, remove the foam absorber or the metal crash bar if your kit requires it. On many front-mount intercooler (FMIC) kits, the crash bar needs to be trimmed or replaced with a tubular bar that clears the new core. Use a Dremel or reciprocating saw to cut the steel bar, leaving enough material to support the bumper cover. Apply rust-preventive paint to any bare metal cuts.

3. Drain Coolant (If Necessary)

Some intercooler pipe routing passes near the radiator or lower coolant hose. To avoid a mess, place a drain pan under the radiator petcock and drain about a gallon of coolant. This isn’t always required—check the kit’s piping path first.

4. Disconnect Factory Intercooler Piping

Locate the stock intercooler mounted in front of the radiator (factory sidemount on 1G and 2G models). Loosen the clamps on the rubber couplers connecting the piping from the turbocharger compressor outlet, through the intercooler, and to the throttle body. Remove the piping completely and set it aside. Keep the factory rubber grommets if they are still good; they may be reused on the new piping.

Removing the Stock Intercooler

The stock intercooler is typically secured by two bolts at the top and two at the bottom, plus a couple of push clips. On a 2G, you may need to unbolt the A/C condenser support bracket to slide the intercooler out. Carefully wiggle the intercooler free; if it’s stuck, check for hidden screws. Do not force it or you may damage the radiator fins. Once removed, inspect the stock piping for any cracks or damaged O-rings—this is often a source of boost leaks in older cars.

Installing the Aftermarket Intercooler Core

5. Position and Test Fit the Core

Slide the new intercooler core into place behind the bumper opening. For a front-mount setup, the core should sit directly in front of the radiator, with the inlet and outlet facing toward the passenger side (on most 2G kits). Center it by eye or measure from the frame rails. Make sure the core is level and not touching the A/C condenser or radiator—leave at least ½ inch of clearance for airflow and thermal expansion.

Many aftermarket kits include custom brackets that mount to the core’s side tanks and bolt to the frame or the radiator support crossmember. Attach these brackets loosely and verify alignment. If no brackets are included, use 2x2″ aluminum L-brackets and stainless steel hardware to create a sturdy mount. Do not let the core hang only from the piping—mechanical strain can cause cracking.

6. Trim the Bumper Reinforcement (If Needed)

With the core positioned, reinstall the bumper cover loosely to check clearance. If the intercooler hits the cover or pushes it outward, you need to trim the inside of the bumper skin or cut more of the crash bar. Use a marker to outline the interference area, remove the bumper, and trim with a Dremel, jigsaw, or utility knife. Clean up the edges so they don’t rub the intercooler fins.

7. Secure the Intercooler Permanently

Once the test fit is perfect, tighten the mounting brackets fully. Use Loctite (blue) on bolts that vibrate. Apply a thin strip of weatherstripping or rubber trim along the contact points between the core and the brackets to prevent metal-on-metal vibration.

Installing the Intercooler Piping

8. Route the Cold-Side and Hot-Side Pipes

Most FMIC kits come with pre-bent aluminum piping painted black or polished. Begin with the “hot side” (pipe from turbo compressor outlet to intercooler inlet). On a 2G Eclipse, this pipe typically goes straight down from the turbo, across the lower part of the engine bay, and up into the passenger-side intercooler tank. Use the provided silicone couplers and T-bolt clamps. Slip the couplers onto the pipe ends and tighten lightly—you will tighten everything after all pipes are attached.

Route the pipe so it does not contact the lower radiator hose, battery tray, or engine mounts. Use the factory plastic grommets or add a split rubber hose around the pipe at any potential rub point.

9. Connect the Cold-Side Pipe (Out to Throttle Body)

The “cold side” runs from the driver-side intercooler outlet up to the throttle body inlet. On 2G models, this pipe usually goes along the top of the radiator near the driver side, then crosses over to the throttle body. Be careful not to block the radiator cap or interfere with the hood latch. Some kits include a “short route” pipe that stays low in the engine bay; follow the kit’s routing instructions. Secure the pipe with the included bracket or use a zip tie to hold it away from hot surfaces.

10. Install the Blow-Off Valve (If Applicable)

If your new piping includes a flange for a blow-off valve (BOV), install the BOV before final tightening. Use the supplied gasket or a thin layer of silicone sealant. Position the BOV so the vacuum line can reach the intake manifold nipple without kinking. Many Eclipses benefit from a recirculating BOV if the car retains the stock mass airflow sensor (MAF). If you have a standalone ECU, venting to atmosphere is acceptable.

11. Tighten All Clamps and Couplers

Go back over every connection. Ensure each silicone coupler is pushed fully over the bead or step on the pipe—at least 1 inch of overlap. Tighten T-bolt clamps with a 1/4″ or 5/16″ socket. The clamp should be snug, but not so tight that you deform the pipe. Double-check that no coupler is twisted or pinched.

Reassembly and Final Checks

12. Reattach the Front Bumper

With the intercooler core and piping fully installed, reinstall the front bumper cover. Align it carefully to avoid scratches. If you trimmed the crash bar, you may need to add new bracket holes or use self-tapping screws to re-secure the bumper. Tighten all bolts and reattach fog light connectors.

13. Reconnect the Battery and Refill Coolant

Reconnect the battery negative terminal. If you drained coolant, refill with a 50/50 mix of ethylene glycol and distilled water. Bleed the cooling system by running the engine with the radiator cap off until the thermostat opens and you see steady flow.

14. Test for Boost Leaks

Build a simple boost leak tester from a PVC cap, a Schrader valve, and a coupler to attach to the turbo inlet. Pressurize the system to 20 psi (or your intended maximum boost) and listen for hissing. Use soapy water in a spray bottle to find leaks at every coupler, BOV flange, and throttle body shaft seal. Tighten any leaking clamps or replace damaged O-rings. This step is critical—a boost leak after an intercooler install will cost horsepower and may cause the car to run rich or lean.

15. Test Drive and Verify IATs

Take the Eclipse on a calm test drive. Monitor intake air temperature on a scan tool or aftermarket sensor if possible. You should see a noticeable drop in IATs compared to the stock setup. Listen for any unusual noises—if you hear a high-pitched whistle, check for a boost leak or a misaligned pipe. After a few miles, turn off the car and feel the intercooler core. It should be cool or warm, but not hot. If the core is hot to the touch, the engine is likely still running rich or the intercooler is heat-soaked from poor airflow—check the bumper for blockages.

Tuning Considerations After Intercooler Upgrade

An aftermarket intercooler alone does not require a tune, but it changes the air density at the MAF sensor (if using a mass-air system) and can lean out the mixture slightly. For best results, consider getting a custom ECU tune or a reflash (via ECMLink for 2G Eclipses). You can safely increase boost pressure by 3-5 psi with a larger intercooler without raising exhaust gas temperatures. Retard timing slightly if running pump gas with high boost.

For more information on tuning the Mitsubishi Eclipse 4G63 engine with a front-mount intercooler, consult the DSMtuners community or a professional tuner. External resources like MAPerformance offer plug-and-play intercooler kits and supporting upgrades.

Benefits of Improved Charge Cooling

  • Increased horsepower and torque – Cold air is denser, allowing more oxygen into the cylinders. Gains of 15-30 whp are common on a turbo Eclipse with a properly sized FMIC, even without other mods.
  • Reduced risk of knock – Lower IATs mean less thermal stress on the engine. You can run higher boost on pump gas without detonation.
  • Consistent power in summer heat – An aftermarket core sheds heat faster than the factory unit, so performance doesn’t degrade as much on hot days.
  • Better spool characteristics – Some aftermarket piping has smoother bends and larger diameter, which reduces back pressure on the turbo and helps the turbo spool slightly quicker.

Potential Installation Pitfalls to Avoid

  • Insufficient clearance to the hood – Cold-side pipes that run over the radiator may hit the hood latch. Check clearance before closing the hood; you may need to dent the hood bracing slightly.
  • Leaking BOV – If the BOV flange is welded unevenly, you’ll have a persistent boost leak. Use a quality gasket and tighten the bolts evenly.
  • Over-tightening clamp on aluminum pipe – T-bolt clamps can crush thin-wall aluminum. Tighten just enough to prevent air escape, do not “gorilla” the ratchet.
  • Not centering the intercooler – A misaligned core can block airflow to the radiator, causing overheating in traffic. Always center behind the bumper opening.
  • Ignoring the need for a tune – While the car may run okay, a tune will maximize gains and prevent lean conditions.

Conclusion

Installing an aftermarket intercooler on your Mitsubishi Eclipse is one of the most rewarding modifications for a turbocharged engine. The process is straightforward for a DIY mechanic armed with the right tools, patience, and attention to detail. By following this step-by-step guide, you’ll enjoy lower intake temperatures, more horsepower, and greater reliability—especially when combined with other bolt-on upgrades. Whether you’re building a daily driver or a weekend track car, the improved charge cooling from a quality intercooler kit will keep your 4G63 or 420A running strong mile after mile.

Always verify fitment with your specific year and model. For more detailed wiring, tuning, and troubleshooting tips, visit DSMtuners and ExtremePSI.