The Jeep 4.0 isn’t only a legend in the Jeep world, but also the automotive industry as a whole. While it’s not very powerful it is absolutely strong and reliable. These engines are known for their longevity, being able to make it 300,000+ miles with minimal problems along the way.
Not to burst it’s bubble, but the Jeep 4.0 engine is still prone to a few common problems. These include exhaust manifold cracks, oil leaks, and water pump and thermostat failure. Fortunately, these problems are not very severe at all. We will cover each problem in depth below as well as provide some more context on this engines brutal reliability.
Jeep 4.0 Engine Problems
- Exhaust manifold cracks
- Oil leaks
- Water pump & thermostat
1) Exhaust Manifold Cracks
Exhaust manifold cracking is without question one of the most common problems on the Jeep 4.0 engine. This is especially true with the age and mileage of most around today. Fortunately, it’s a pretty minor issue in the grand scheme. Exhaust manifolds may also be known as headers or exhaust headers.
Sometimes it may just be the manifold gasket or bolts causing the issues. However, it’s also common for the manifold to actually develop cracks. Symptoms are most noticeable on a cold engine. As the Jeep 4.0 exhaust manifold heats up and expands. This expansion can help close the crack, so symptoms may go away once the engine warms up.
Symptoms
- Ticking sounds
- Raw exhaust smell
- Visible cracking
Exhaust manifold cracks typically cause odd engine sounds – notably a ticking noise. Again, this may only be present on a colder engine as the cracks become smaller with heat. You might also notice the Jeep 4.0 smells unusually stronger like raw exhaust. Though, this can be hard to notice with small cracks. Otherwise, a visual inspection might confirm if and where cracks exist.
Repair Options
There are several viable options if you run into a cracked exhaust manifold on the Jeep 4.0 engine. You can opt for an OEM solution, but sometimes new parts can be tough to track down. Used Jeep exhaust manifolds may quickly run into the same issues so be cautious.
Aftermarket exhaust headers are a good choice. However, modding the exhaust may be seen as illegal and can cause emissions concerns. Nonetheless, it’s a good option to hopefully avoid running into more manifold leaks in the feature.
Finally, you may be able to repair your cracked Jeep 4.0 manifold. Some have success with this option, but others find it to be a short-term solution. This can be a good, cheap option for AMC straight-6 owners who don’t mind a DIY project.
2) Oil Leak Problems
Well, this one hits close to home as big BMW enthusiasts. Many of the same jokes about oil leaks float about in the Jeep community, too. In reality, oil leaks affect many engines so the Jeep 4.0 engine is not alone here. Some Jeep oil leaks are simply due to age and mileage while others were known to be faulty from the early days.
The Jeep 4.0 valve cover, valve cover gasket, and rear main seal are a few of the most common oil leak problems. Over time, these seals and gaskets wear down and begin cracking. Oil leaks generally start as a small leak and gradually become worse as cracks expand and new ones develop.
With the age of these engines it’s likely many engines are leaking or had previous oil leaks that were repaired. If you’re in the market for the 4.0L engine definitely look for any leaks and see if any of these repairs were done in the past.
Oil Leak Symptoms
- Visible leak
- Burning oil smells
- Light smoke from engine bay
Visible drops of oil on the ground is a dead give-away oil is leaking from somewhere. When the 4.0L inline-6 is leaking from the rear main seal it often drips onto the ground. However, valve cover gaskets lie at the top of the engine so small leaks may not make it to the ground.
With Jeep valve cover gaskets check for potential burning oil smells or light smoke from the engine bay. You might also notice some oil pooling around the valve cover gasket area.
Oil Leak Repairs
The DIY crowd will appreciate the fact that these oil leaks are cheap repairs. Gaskets and seals often come in for less than $20, so a couple hours in the garage can save a lot of money. Those going to repair shops will find bills to be a bit pricey due to labor costs.
Rear main seal replacement can run $300-500+ at a repair shop. It’s a fairly labor intensive job. Valve cover gaskets are quicker thanks to the simple engine design of an inline-6.
3) Water Pump Failures
Overheating is another common issue on the Jeep 4.0 engine. The water pump and thermostat aren’t always to blame. However, they’re typically good starting points and a common cause of overheating. This wasn’t a major issue when these engines were newer. However, water pumps take a lot of abuse over the years and are common repairs on many aging engines.
The engine is known to develop head gasket problems on occasion, too. Those failures become a lot more common when the engine is subjected to overheating.
Another issue outside of actual water pump failure is the pump flow. Some OEM pumps don’t flow very well at lower RPM’s and idle, which can also cause overheating. Fortunately, there are some aftermarket solutions that help solve this problem on the Jeep straight-6 engine.
Water Pump Failure Symptoms
- Coolant leaks
- Steam from engine
- Overheating
If you notice a coolant leak then the water pump may be to blame. In some cases the coolant loss can be incredibly rapid and you’ll even notice steam from the engine as it leaks out and burns off. This can lead to rapid overheating on the 4.0 Grand Jeep Cherokee and Jeep Wrangler.
It’s important to ensure the engine does not overheat, so if you notice this symptom shut off the engine as soon as possible. Running the engine too hot for long periods increases the risk of additional problems.
Jeep 4.0 Engine Reliability
Is the Jeep 4.0 straight-6 engine reliable? Yes, we believe this engine earns good remarks for reliability. AMC was struggling financially when they built this engine. As such, AMC opted to use some older but proven and simple engineering. Many consider the 4.0 among the best Jeep engines for its simplicity, reliability, and 4×4 capability.
However, no engine is perfect and that applies to the legendary Jeep 4.0L engine. These engines are known to run into problems such as exhaust manifold cracks, oil leaks, and water pump failures. This also isn’t an exhaustive list. It’s not totally uncommon to see head gasket, piston skirt, and leaking fuel injectors among some other engine problems.
Then there’s age which is a big factor. The engine often doesn’t have much trouble making it beyond 200,000 miles and even 300-400k+ miles in some cases. That’s pretty good longevity and reliability for the Jeep 4.0 engine. Still, with the current age of these engines it’s likely they’ll need some extra TLC and potentially a rebuild.
Rebuild Kits
Given the allure of the Jeep 4.0 engine opting for a rebuild kit is an increasingly common choice. Most parts are pretty inexpensive, so it’s not a bad deal for those who can DIY. Of course, rebuilds are very labor intensive and require a good bit of knowledge around an engine. Generally, rebuild kits include something along the lines of the following parts:
- Seals & gaskets
- Pistons & rings
- Rod bearings
- Main bearings
- Camshaft, lifters, & cam bearings
- Oil pump
- Timing kit
During a rebuild you can choose to do a bit less or take things even further. Plenty of options exist for internal upgrades, stroker kits, forced induction, etc. Again, this stuff can add up to be expensive if you’re not capable of doing the work on your own. At that point, you may be better off sourcing an already rebuilt engine and simply swapping.
Summary
It’s no secret the 4.0L straight-6 build by AMC is somewhat a legend in the Jeep world. They certainly don’t put out much power by modern standards, but they offered solid performance for its era. It used a simple design that provides lasting longevity and reliability. There’s a reason Chrysler and Jeep kept an AMC built engine around for 20 years.
However, no engine is perfect and that applies here too. A few common issues with the Jeep 4.0 include the exhaust manifold, oil leaks, and water pump. Other problems like head gaskets, pistons skirts, and crank position sensors also aren’t uncommon. A lot of reliability simply comes down to maintenance, so ensure you take good care of the 4.0L engine.
The Jeep 4.0 engine often holds up easily to 200k miles and some even make it to 300-400k with few problems along the way. Not bad for longevity and reliability. However, with the age of these engines it’s becoming more common that they require rebuilds. It can be a costly endeavor, but some believe its worth the cost to own one of the greats.
