A leaking intake manifold gasket on the 4.0L shows up as a rough idle, a lean code, and an engine that won't settle down — this guide covers replacing the intake/exhaust combination gasket on 1987–2001 XJs.
If your XJ idles rough, surges, throws a lean code (P0171 on OBD-II trucks), or makes a hissing sound that changes with throttle, a failed intake manifold gasket is a strong suspect. On the 4.0L the intake and exhaust manifolds share the same studs on the side of the head and seal against one combination gasket, so the two jobs are linked — which is why most people do this gasket at the same time they chase a cracked exhaust manifold (the XJ's other famous leak in this area). A leaking intake gasket pulls unmetered air into the engine, leans out the mixture, and makes the truck run poorly cold and hot. Ignore it long enough and you'll be cleaning up burned valves and a confused fuel trim.
Before you tear into it, confirm the leak. Spray a little brake cleaner or carb cleaner along the intake-to-head seam with the engine idling — if the idle changes, you've found unmetered air getting in there. That five-minute test saves you from pulling a manifold that wasn't the problem.
This is an intermediate job, not because any one step is hard, but because there's a lot crossing the manifolds and the fasteners have been heat-cycled for decades. Budget time for stuck studs, plan to replace the exhaust manifold if it's cracked while you're in there, and respect the torque spec — the front and rear studs snap if you lean on them.
Steps
Disconnect the negative battery terminal and let the engine sit cold. Working on hot exhaust studs is how they break.
Relieve the fuel system pressure (pull the fuel pump fuse and run the engine until it stalls on 1991+ HO trucks), then remove the air intake tube and air box bracket to clear access.
Disconnect everything crossing the intake: throttle and cruise cables and their bracket, the TPS and IAC connectors, the MAP sensor, the fuel injector harness, the fuel rail feed line, vacuum lines, and the CCV/PCV hoses. Label them. A phone photo before you start beats guessing later.
Unbolt the fuel rail (with injectors) and set it aside on a clean rag — you don't have to pull the injectors out of it. Cap the lines.
Soak every manifold stud and nut with penetrating oil and give it 15 minutes. The 4.0L manifolds bolt to the head with a row of bolts and studs/nuts; many are dual-purpose fasteners that clamp the intake and exhaust together with a special OEM washer or clamp. Note where each washer/clamp lives — you cannot substitute hardware-store washers, they won't hold the seal.
Remove the fasteners from the outside in, working toward the center, to relieve the clamping load evenly. If a stud starts to turn instead of the nut, stop and add heat/penetrant — backing a seized nut off a stud snaps the stud in the head, which turns a four-hour job into a drill-and-extract afternoon.
Separate the intake manifold from the exhaust manifold (they're bolted together with a few bolts between them) and lift the intake off. If you're replacing or resurfacing the exhaust manifold, pull it now.
Scrape the old gasket material off the head's machined face and off both manifold flanges. Get them clean, flat, and dry. Check the exhaust manifold for cracks — held up to light, hairline cracks near the runners are common on these. If it's cracked, replace it; a new gasket won't fix a leaking manifold.
Lay a straightedge across the intake manifold flange. A warped intake won't seal. Mild warp can be resurfaced; severe warp means a replacement manifold.
Set the new combination gasket onto the studs dry — no RTV on these gaskets. Reinstall the exhaust manifold and intake manifold together with the correct dual-purpose washers/clamps in their original spots, and start every fastener by hand.
Torque in the factory sequence, working from the center outward. Intake manifold bolts go to 23 ft-lb. The exhaust manifold's center nut goes to 30 ft-lb, and the two outer exhaust nuts to 23 ft-lb. Run the fasteners up in two or three passes rather than one — even clamping is what makes the seal. Do not overtighten; the end studs are the ones that break.
Reinstall the fuel rail, reconnect the fuel line, every electrical connector, every vacuum and CCV/PCV hose, the throttle bracket and cables, and the air intake. Work backward through your photos.
Reconnect the battery and reinstall the fuel pump fuse. Cycle the key a few times to prime the fuel system before cranking. Start it, let it idle, and recheck for leaks with the brake-cleaner test once warm. Clear any stored codes and confirm the lean code stays gone after a drive.
Why it works
Fixes the root cause of a rough idle, surging, and lean codes instead of masking it.
Done alongside an exhaust manifold replacement, you handle both of the XJ's classic side-of-head leaks in one teardown.
Gasket cost is modest ($40–120 with hardware); the value is in the labor you keep.
Trade-offs
Seized and broken exhaust studs are the real risk — a snapped stud in the head can add hours and an extraction.
A lot of components cross the manifolds, so reassembly demands care to avoid pinched harnesses or missed vacuum lines.
The dual-purpose OEM washers/clamps are quick to lose track of and can't be substituted — keep them organized or the manifolds won't seal.
Tools required
Metric socket set with extensions and a swivel/U-joint
Torque wrench (ft-lb range)
11mm and 13mm sockets (manifold fasteners)
Gasket scraper or plastic razor
Penetrating oil
Brake cleaner and shop rags
Flare-nut wrenches (fuel line, if disconnecting the rail)
Parts
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Written and maintained by an AZ wheeler and driveway wrencher. Always cross-reference your factory service manual — modifications affect vehicle safety and warranty. Work at your own risk.