Jeep 4.0 Straight-Six Engine

The Jeep 4.0 isn’t only a legend in the Jeep world, but also the automotive industry as a whole. AMC developed this engine prior to Chrysler buying them out in 1987. It’s a strong and reliable engine, however, the 4.0L straight-6 is also known for a lack of power. Regardless, the 4.0 is an excellent all around engine that many appreciate to this day. Good enough that Chrysler and Jeep kept it around for nearly 20 years. In this article, we discuss Jeep 4.0 engine problems, reliability, specs, and more.

*Some also refer to the 4.0 inline-6 engine as the PowerTech or 4.0 PowerTech. We may refer to the Jeep 4.0L engine by these names in the article. We might also call it an AMC, Jeep, or Chrysler engine.

What Cars Use the Jeep 4.0?

AMC/Chrysler/Jeep 4.0L straight-6 engines are in the following years and models:

• 1987-2001 Jeep Cherokee
• 1993-2004 Jeep Grand Cherokee
• 1987-1990 Jeep Wagoneer
• 1987-1992 Jeep Comanche
• 1991-2006 Jeep Wrangler

4.0 Straight-Six Engine Specs

Initial introduction of the 4.0L inline-6 engine was in 1986 – just one year before Chrysler bought out AMC. Despite this fact, Chrysler opted to keep the AMC designed 4.0 around for the long haul. Though, Chrysler did make a number of updates to the straight-6 over the years. We’ll circle back to this point and cover some in-depth specs in a moment. The below chart shows a few of the base specs for the Jeep 4.0 engine:

Engine	AMC/Jeep/Chrysler 4.0
Configuration	Straight-6 (Inline-6)
Displacement	3,965 cc (4.0L)
Aspiration	Natural
Block Material	Cast iron
Head Material	Cast iron
Valvetrain	OHV
Bore x Stroke	98.4mm x 86.7mm
Compression	8.8 : 1 to 9.2 : 1
Power	173-190 HP
Torque	220-235 lb/ft

As the engine name suggests it’s a 4.0L inline-6 engine. Actual displacement is around 3,965cc which is rounded up to 4.0L. Cast iron heads and blocks were normal back in the 1980’s and provide plenty of strength. They’re often not found in modern times due to weight. Jeep 4.0 engines are overhead valves unlike the standard overhead or double overhead cams of the modern days.

Power began at 173hp in the original 1987 models. Output was quickly increased to 177hp in 1988 and then 190hp in 1991. Torque also improved over the years. Of course, during its nearly 20 year production the 4.0 straight-6 saw a number of updates. Some helped increase power and torque while others were intended to improve efficiency, strength, and reliability. We’ll cover a few of these updates quickly in the next section.

Jeep 4.0L Inline-6 Updates

It’s important to note – this is not an exhaustive list of all updates. 20 years is a long time for an engine to be around, and many tweaks occur during that kind of timeframe. Anyways, a few notable updates for the Jeep 4.0 straight-6 engine include:

• 1988 – Higher flowing fuel injectors
• 1991 – Multi-port fuel injection replaces RENIX, intake ports raise 3.2mm, larger throttle body, intake & exhaust manifold updates, higher flowing injectors, cam timing updates
• 1995 – Changes to cylinder head
• 1996 – Stronger engine block
• 2000 – Cylinder head design changes for greater efficiency & distributor-less coil on plug ignition system

Again, there are other minor tweaks and changes over the years. One year after introduction the Jeep 4.0 engine receives higher flowing injectors which help add a few horsepower and torque. 1991 saw a handful of updates that allowed the engine to make 190hp. A few small updates to the head and block occurred in the following years. Eventually, in 2000, Jeep 4.0L engines saw a few design changes to help support lower emissions and greater overall efficiency.

Jeep 4.0 Straight-6 Engine Problems

Alright now we’re moving onto a few of our larger focal points for this article. As legendary as the 4.0 inline-6 may be it’s still prone to running into engine problems. This is true for any automotive engine, and especially so when talking about a 15-30+ year old engine. A few of the most common engine problems on the Jeep 4.0 include (in no specific order):

1. Exhaust manifold cracks
2. Oil leaks
3. Water pump & thermostat

We’ll break the above issues down in greater depth throughout this article. At the end we’ll finish up with thoughts on overall reliability. That will also lead us into our final discussion about rebuild kits. Many owners still adore the Jeep 4.0 straight-6 but age is a big factor here. There are likely tons of Jeeps out there with the 4.0L straight-6 that are running out of time due to old age and high mileage. As such, Jeep 4.0 rebuilds and rebuild kits are a common theme.

1) Jeep 4.0L 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 4.0L straight-6 engines 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 inline-6 exhaust manifold heats up and expands. This expansion can help close the crack, so symptoms may go away once the engine warms up.

AMC 4.0 Exhaust Manifold Crack Symptoms

A few symptoms of a cracked exhaust manifold on the Jeep 4.0L PowerTech engine include:

• 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.

4.0L Inline-6 Exhaust Manifold Repair

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 Jeep 4.0L straight-6 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) Jeep 4.0 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 the AMC 4.0 straight-6 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.

Chrysler 4.0L Engine Oil Leak Symptoms

Symptoms of oil leaks may be obvious to some. Nonetheless, look for the following symptoms that may point to an oil leak on the Jeep 4.0 engine:

• 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.

Jeep 4.0 Oil Leak Repairs

The DIY crowd will appreciate the fact that these Jeep 4.0L 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) 4.0 Straight-6 Water Pump Failures

Overheating is another common issue on the Jeep 4.0 straight-6 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.

Water pump issues are a pretty urgent repair on any engine including the 4.0L Chrysler PowerTech engine. The engine is known to develop head gasket problems on occasion, too. Those failures become a lot more common when the 4.0 liter engine is subject 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.

Jeep 4.0 Water Pump Failure Symptoms

Symptoms of water pump problems on the Chrysler/Jeep 4.0 engine include:

• 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 Cherokee and 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 the 4.0L engine. As such, AMC opted to use some older but proven and simple engineering for the straight-6 engine. 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 AMC/Chrysler/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 Jeep 4.0 engine problems.

Then there’s age which is a big factor when it comes to the 4.0L inline-6. 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.

Jeep 4.0L Inline-6 Rebuild Kits

Given the allure of the Jeep 4.0 engine opting for a rebuild kit is an increasingly common choice. Most parts for the straight-6 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

This Jeep 4.0 rebuild kit is a good example of what they often look like. 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.

4.0 Straight-6 Engine Summary

It’s no secret the 4.0L straight-6 build by AMC is somewhat a legend in the Jeep world. The 4.0 liter engine certainly doesn’t put out much power by modern standards, but it offered solid performance for its era. Jeep 4.0 engines also use 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 to the AMC inline-6 engine 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.

What’s your experience with the Jeep 4.0L straight-6? Are you considering one?

Leave a comment and let us know!