Jeep 4.0 cracked exhaust manifold symptoms

Welcome back to our series on properly diagnosing mechanical problems rather than blindly chucking money at replacement parts hoping to stumble on a fix. Granted, a cracked exhaust manifold doesn’t rank very high on the tech-o-meter, but it’s directly related to last month’s topic of diagnosing a bad O2 sensor. And you’d be surprised at just how many fuel-injected 4.0L, 2.5L, and 3.8L-powered Jeeps are running around with a cracked exhaust manifold.

Cracked Manifold Causes and Effects
Jeeps are big, unaerodynamic boxes. To eke out as much EPA-regulated fuel economy as possible and meet emissions regulations, the ECU is programmed to run the engine on the lean side of the air/fuel curve. Running lean improves mileage, reduces emissions, and increases horsepower slightly, but it also contributes to higher exhaust gas temperatures. Adding things like a cold-air intake and after-cat exhaust only serve to further lean things out and increase temps. When you factor in aftermarket items such as bigger tires and heavy armor, the increased load put on the engine when climbing grades, towing, or accelerating can bring the exhaust manifolds close to cherry red. This constant cycle of superheating and cooling eventually fatigues the metal and cracks develop in the exhaust manifold.

Diagnostics
A cracked exhaust manifold won’t always be audible. However, if you find your mileage plummeting, your catalytic converter is running unusually hot (function of more unburned fuel and hydrocarbons in the exhaust stream), and/or your exhaust is overly rich chances are good air is entering the exhaust upstream of the O2 sensor and tricking the ECU into thinking the vehicle is running leaner than it actually is. To compensate, the ECU dumps fuel in an attempt to bring the air/fuel ratio into spec killing fuel mileage and exacerbating the problems listed above.

There are many old-school and Internet diagnostics that range from probing with an unlit propane torch to shoving rags into the tailpipe to force the exhaust out the cracks. In reality, the best diagnostic is visual. For engines with tubular manifolds, like most 4.0L engines, the cracks are usually visible with a flashlight. Check the areas where tubes come together. Normally they’ll crack right in the valleys on the intersections at or right next to the welds. For cast iron manifolds, such as on earlier 2.5L and later 3.8L engines, it’s a bit more complicated thanks to the integrated heat shrouds.

The leaks will be most evident at first startup with the engine cold. That’s because as the manifolds heat up the metal expands and can diminish or even completely seal a leak that’s evident with the engine cold. If you have an electric fan, complete this test before the engine comes up to temp. On a mechanical fan, either unbolt the fan or remove the fan belt so the pulley no longer spins. If you pull the belt, don’t run the engine for more than a minute or so or you can roast it. So, with no fan blowing air across the bay, fire the engine and listen and feel for a blast of exhaust gas coming from between the manifold and shroud. Let your ears guide you to the right spot.

After chasing an audible exhaust leak on our ’89 YJ and trying to seal the manifold-to-exhaust downtube several times, we eventually unbolted the fan and performed this test only to find a huge whack of exhaust coming from between the heat shield and manifold that wasn’t noticeable when the cooling fan was spinning. Removing the manifold and snipping off the heat shield revealed major fissures throughout the cast iron.

The Fix
If you have a tubular exhaust manifold you can remove the manifold and weld up the cracks. This fix will hold for a while, but it’s been our experience that the cracks will eventually return upstream or downstream of the welded fixes. The only real solution we’ve found is to replace the factory manifold with a quality aftermarket header.

On the cast iron manifolds welding is an option with a high-nickel rod and an arc welder and some have even had good luck slathering the cracks with JB Weld. Neither gives us a warm and fuzzy feeling. Some applications have aftermarket tubular headers available, but if not, Quadratec carries quality OE-spec replacements, so that’s what we went with.