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Jeep Grand Cherokee Performance Upgrades

Hopping Up a High-Mileage Jeep 4.0L

Christian LeePhotographer, Writer

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.