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Jeep Grand Cherokee Performance Upgrades

Jeep 4 0l Engine With Airaid Intake
Christian Lee | Writer
Posted September 13, 2013

Hopping Up a High-Mileage Jeep 4.0L

When is stock not enough? For our ’99 Jeep Grand Cherokee, it came at 137,000-plus miles when power began waning, even while using synthetic oil for regular oil changes and an open-element air filter in place of the factory air filter. We could have survived as is, but we needed some seat-of-the-pants, if not documented, performance improvements to keep our WJ maintaining at our desired level.

With a venerable engine like the 4.0L I-6, it doesn’t take much to eke out extra power. With that in mind, we decided to install a free-flowing exhaust system, a throttle-body spacer, fresh plugs and wires, and a cold-air intake system to really awaken it to its true potential. With all in place, we felt the improvements immediately. It isn’t an engine-swap-to-V-8 improvement but improvement nonetheless. Throttle response is much better and the throaty-without-drone exhaust note certainly makes the rig feel quicker, especially when laying down the skinny pedal to pass another vehicle on the highway.

We leisurely completed the installation of all of these engine performance parts in half a day. None of the new parts presented any difficulties going in and those with decent mechanical skills could carry out the installation of parts without issue. Check out the photos and captions to see how it’s done.

Step By Step

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  • Our ’99 4.0L remained stock since it left the factory. With more than 137,000 on the odometer it was time for some power improvements.

  • Airaid’s QuickFit Intake System with SynthaMax filter creates a huge air inlet that provides a steady, free-flowing supply of cold air. Intakes are made in the United States and include all the necessary installation hardware as well as easy-to-understand instructions written with the do-it-yourselfer in mind.

  • The Airaid PowerAid throttle body spacer was developed to enhance off idle to midrange performance and efficiency. Torque gains are most noticeable between 800 to 2,500 rpm. PowerAid Throttle Body Spacers are manufactured from aircraft-grade billet 6061-T6 aluminum and include the necessary gaskets, hardware, and precision brackets.

  • The Rugged Ridge exhaust system was designed in conjunction with Cherry Bomb Exhaust and features a Vortex muffler. The cat-back system features the Vortex muffler, stainless steel hardware, and a mandrel-bent aluminized pipe.

  • We selected Champion 7034 Double Platinum spark plugs for use in our 4.0L engine. The plugs feature a copper core for increased conductivity, improved heat control, and improved resistance to fouling. The platinum-tipped center electrode and platinum pad on the ground electrode is said to provide long life and minimal wear.

  • To begin the installation, we removed the stock air filter box lid, air filter, and air duct. The stock air duct was retained and will be used with the Airaid intake.

  • The old spark plugs were removed. We weren’t certain about the last time they were replaced but they looked to be cold detonating for some time and the electrodes were well worn. The proper spark gap for our 4.0L application is 0.036 but four of the old plugs were gapped at 0.050 and two were at 0.054. When removing the old plugs it’s a good idea to number them so you know which cylinder each was firing. In our case we didn’t have a felt pen handy so we laid them out on the ground in sequence as they were removed. Unfortunately, the line of old plugs was too close to the work area and were accidently kicked out of place so we weren’t able to determine where the two most worn plugs came from. Live and learn!

  • The Airaid PowerAid throttle body spacer easily bolts in place between the throttle body and intake manifold.

  • Its unique helix-bore design helps to provide a superior atomized mixture of fuel and air all the way into the combustion chamber.

  • The throttle body includes two new gaskets, longer mounting bolts, and riser spacers and bolts for the throttle linkage.

  • The Airaid QuickFit Intake System with SynthaMax filter was assembled for installation. From this point it basically snaps into place atop the factory airbox and is connected to the factory intake using the factory duct.

  • The Rugged Ridge Vortex muffler is constructed from heavy-duty, 14-gauge aluminized steel and is seam welded and offers reinforced inlets and outlets. The muffler offers a deep, throaty sound, without drone, that could easily be mistaken for a V-8. We had planned for a bolt-on install but decided to have the muffler and pipe welded in when it became obvious that a new catalytic converter was needed. The heavy-duty muffler should also withstand trail abuse much better than the factory setup.

  • The new Rugged Ridge mandrel-bent aluminized pipe was welded to the muffler and supported from the factory hanger. Long Beach Advance Tire & Muffler in Long Beach, California, completed the installation.

  • The installation of a new catalytic converter helped get the most out of the Rugged Ridge exhaust.

  • We woke up the aged 4.0L engine in our ’99 Jeep Grand Cherokee with a variety of performance parts, including an Airaid intake kit and filter, a PowerAid throttle body spacer, new Champion spark plugs, and a Rugged Ridge exhaust system. The seat-of-the-pants improvement was very noticeable.


Suwanee, GA 30024
Airaid Filter Company
Champion Spark Plugs
Sun City, CA 92586
Long Beach Advance Tire & Muffler