Jeep 4.0 Straight-Six Engine: Everything You Need to Know

AMC/Chrysler/Jeep 4.0 Engine

The Jeep 4.0 is a legend not only in the Jeep community but throughout the automobile industry. This engine was created by AMC before Chrysler purchased them in 1987. The 4.0L straight-6 is a strong and dependable engine, but it’s also criticized for its lack of power.

Whatever the case may be, the 4.0 is still a well-liked all-arounder. To the point where it lasted nearly two decades after its introduction, Chrysler and Jeep decided to keep it. Jeep 4.0 engine issues, reliability, specifications, and more are covered in this article.

The 4.0-liter inline-6 engine is sometimes referred to as the PowerTech or simply the 4.0 PowerTech. In the article, the Jeep 4.0L engine may be referred to by any of these names. There are several other names for this engine.

What Cars Use the Jeep 4.0?

The following years and Jeep models have 4.0L straight-6 engines:

  • 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

4.0 Straight-Six Engine Specs

The 4.0L inline-6 engine was first introduced in 1986, a year before Chrysler acquired AMC. But Chrysler decided to keep the AMC-designed 4.0 in production for the foreseeable future. However, over time, Chrysler made a number of improvements to the straight-6 engine.

We’ll come back to this later and go over some specifics in more detail. The following table lists some of the Jeep 4.0’s most important specifications:

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

The engine is a 4.0L inline-6, as the name implies. The actual displacement is approximately 3,965cc, but the 4.0L measurement is rounded to the nearest hundredth of a liter. Back in the 1980s, cast iron blocks and heads were commonplace, and they offer plenty of strength.

Due to their heaviness, they are rarely seen today. Overhead valves, as opposed to modern standard overhead or double overhead cams, are found in Jeep’s 4.0-liter V8 engine.

The initial 1987 models had a maximum power output of 173 horsepower. A quick boost in horsepower brought the engine’s output to 177 horsepower in 1988 and then 190 horsepower two years later in 1991. The torque has also gotten better over time. A number of improvements were made to the 4.0 straight-6 during its nearly two-decade production run.

Several of these contributed to increased power and torque, while others aimed to enhance efficiency, toughness, and dependability instead. In the next section, we’ll take a quick look at a few of these changes.

Jeep 4.0L Inline-6 Updates

It should be noted that this is not a complete list of all changes. For an engine, 20 years is a long time, and a lot can change in that time. Anyway, the Jeep 4.0 straight-6 engine has received a number of upgrades, including the following:

  • 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

Other tinkering and tweaking has taken place over the years, as well. Higher flowing injectors are added to the Jeep 4.0 engine a year after its debut, helping to boost output by a modest amount.

In 1991, a few improvements allowed the engine to reach 190 horsepower for the first time. The following years saw a few minor revisions to the head and block. The design of Jeep 4.0L engines was modified a bit in 2000 to help with reduced emissions and improved overall efficiency.

Jeep 4.0 Straight-6 Engine Problems

Jeep 4.0 Straight-6 Engine Problems

After this, we’ll move on to a few of the article’s more important themes. Despite its legendary status, the 4.0 inline-6 is still prone to malfunctions. If you’re talking about a 15-30 year old engine, this is especially true. The following are a few of the most frequent Jeep 4.0 engine problems:

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

We’ll go into more detail about each of these concerns later on in the article. We’ll wrap things up with some thoughts on overall reliability. Finally, we’ll talk about rebuild kits when we get to that point.

The Jeep 4.0 straight-6 engine continues to be beloved by many owners, but it’s beginning to show its age. Due to age and high mileage, there are probably a large number of Jeeps still on the road with the 4.0L straight-6 engine. In this way, Jeep 4.0 rebuilds and kits are a frequent theme in the Jeep aftermarket.

1) Jeep 4.0L Exhaust Manifold Cracks

One of the most common issues with the Jeep 4.0 engine is exhaust manifold cracking. This is particularly valid given the age and mileage of the majority of today’s 4.0L straight-6 engines. However, this is a relatively insignificant problem in the grand scheme of things. Headers or exhaust headers are other terms for exhaust manifolds.

The manifold gasket or bolts may be to blame in some cases. Although the manifold can develop cracks, it does so frequently. When the engine is cold, the symptoms are more obvious. Exhaust gases expand and heat up in the manifold of the Jeep 4.0 inline-6 engine. Once the engine has warmed up, the crack may close and the symptoms will go away.

AMC 4.0 Exhaust Manifold Crack Symptoms

Cracks in the Jeep 4.0L PowerTech engine’s exhaust manifold can cause a variety of symptoms.

  • Ticking sounds
  • Raw exhaust smell
  • Visible cracking

Cracks in the exhaust manifold often lead to strange engine noises, such as a ticking. The cracks become smaller as the engine heats up, so this may only be visible on a colder engine. The Jeep 4.0 also has an unusually strong raw exhaust smell. However, small cracks can make this difficult to detect. A visual inspection, on the other hand, may reveal whether or not cracks exist.

4.0L Inline-6 Exhaust Manifold Repair

If the exhaust manifold on your Jeep 4.0 engine cracks, you have a few options. While OEM solutions are available, new parts can be hard to come by. Be cautious when purchasing used Jeep exhaust manifolds because they may have the same problems.

Aftermarket Exhaust headers for a Jeep 4.0L straight-6 engine are a good option. Modifying the exhaust, on the other hand, may be considered illegal and raise questions about emissions. In spite of this, it’s a viable option for avoiding further feature leaks.

Jeep 4.0 manifolds are notoriously difficult to repair. Some people find success with this approach, while others see it as a stopgap measure. AMC straight-6 owners who don’t mind taking on a do-it-yourself project may find this to be a good, affordable option.

2) Jeep 4.0 Oil Leak Problems

We’re big BMW fans, so this one’s especially poignant for us. In the Jeep community, many of the same stories about oil leaks are told. The Jeep 4.0 engine is not the only one susceptible to oil leaks. Some Jeep oil leaks can be attributed to the vehicle’s age and mileage, while others have been a problem since the beginning.

A common oil leak point is the Jeep 4.0 valve cover, which includes the valve cover gasket as well as the rear main seal. Seals and gaskets degrade and crack over time. When an oil leak begins, it’s usually a small one that gets worse over time as cracks widen and new ones form.

Because the AMC 4.0 straight-6 is so old, it’s likely that many of them leak oil or have had oil leaks in the past that were fixed. It’s important to look for previous repairs and leaks in a 4.0L engine before making a purchase.

Chrysler 4.0L Engine Oil Leak Symptoms

Some people will be able to detect oil leak symptoms right away. Nevertheless, keep an eye out for the following signs of an oil leak in your Jeep 4.0 engine:

  • Visible leak
  • Burning oil smells
  • Light smoke from engine bay

It’s an obvious sign of oil leakage if there are visible drops of oil on the surface of the ground. Leaks from the rear main seal of the 4.0L inline-6 often dribble to the ground when it is malfunctioning. Small leaks, on the other hand, may not reach the ground because valve cover gaskets are located at the very top of the engine.

Check the engine compartment for any burning oil odors or light smoke when using Jeep valve cover gaskets. It’s also possible that you’ll see some oil accumulating near the valve cover’s gasket.

Jeep 4.0 Oil Leak Repairs

Jeep 4.0L oil leaks are simple and inexpensive to fix, which will appeal to the do-it-yourself crowd. When you consider that most gaskets and seals cost less than $20, you can save a significant amount of money by doing it yourself. Those who take their vehicles to repair shops will find that labor costs make their bills quite expensive.

The cost of replacing the rear main seal at a repair shop can range from $300 to $500 or more. It takes a lot of time and effort to do this job well. Because of the inline-6’s simple design, valve cover gaskets can be replaced more quickly.

3) 4.0 Straight-6 Water Pump Failures

The Jeep 4.0 straight-6’s tendency to overheat is another problem. Not all problems can be traced back to the water pump or thermostat. Overheating is common when using them, but they’re good starting points. When these engines were newer, this wasn’t a big deal. In spite of this, water pumps take a beating over time and are a common source of engine repair work.

Any engine’s water pump needs immediate attention, but the 4.0L Chrysler PowerTech’s are no exception. The engine’s head gasket has been known to fail from time to time. When the 4.0-liter engine overheats, these failures become far more frequent.

Pump flow is an issue that is separate from actual water pump failure. In addition to causing overheating, some original equipment manufacturer (OEM) pumps run poorly at low RPMs and idle. Fortunately, there are some aftermarket options for the Jeep straight-6 engine that can help solve this issue.

Jeep 4.0 Water Pump Failure Symptoms

Problems with the Chrysler/Jeep 4.0 engine’s water pump can manifest as the following symptoms:

  • Coolant leaks
  • Steam from engine
  • Overheating

A faulty water pump may be the cause of a coolant leak. When the coolant leaks and burns off, you may notice steam coming from the engine as a result of the rapid coolant loss. This can cause the 4.0 Grand Cherokee and Wrangler’s engine to overheat very quickly.

Make sure the engine doesn’t overheat by turning it off as soon as you notice this symptom. Leaving the engine running at a high temperature for an extended period increases the risk of developing further problems with it.

Jeep 4.0 Engine Reliability

Jeep 4.0 Engine Reliability

Is the 4.0-liter, straight-six engine in the Jeep dependable? Yes, we believe the reliability of this engine is excellent. Back when the 4.0L engine was being developed, AMC was in a tight financial spot.. As a result, the straight-6 engine was designed by AMC using some proven and simple older technology. The 4.0 is widely regarded as one of the best Jeep engines due to its ease of use, dependability, and four-wheel drive capabilities.

No engine, even the legendary AMC/Chrysler/Jeep 4.0L engine is perfect. The exhaust manifolds, oil leaks, and water pumps on these engines have been known to fail. Also, this is not a comprehensive list. Other Jeep 4.0 engine problems include leaking fuel injectors, a blown head gasket, or a piston skirt that is cracked.

There’s also mileage, which has a significant impact on the 4.0L inline-6. It’s not uncommon for the engine to go well over 200,000 miles before needing to be replaced. The Jeep 4.0 engine has an impressive lifespan and reliability record. In spite of this, these engines are likely to require some additional TLC and/or a rebuild due to their age.

Jeep 4.0L Inline-6 Rebuild Kits

Jeep 4.0 engine rebuild kits are becoming increasingly popular due to their allure. Most of the straight-6’s components are reasonably priced, making it a good deal for those with some mechanical know-how. Rebuilds, on the other hand, are time consuming and necessitate extensive engine knowledge. For the most part, rebuild kits include the following components:

    • Seals & gaskets
  • Pistons & rings
  • Rod bearings
  • Main bearings
  • Camshaft, lifters, & cam bearings
  • Oil pump
  • Timing kit

This Jeep 4.0 rebuild kit serves as an excellent illustration of what these kits typically appear to be. While rebuilding, you have the option of doing less or more than before. Internal upgrades, stroker kits, forced induction, and other such options abound.

Again, if you can’t do the work on your own, it will cost you a lot of money. If that’s the case, you might be better off finding a pre-rebuilt engine and just swapping that in instead.

4.0 Straight-6 Engine Summary

In the Jeep community, the 4.0L straight-6 engine built by AMC has become something of a cult classic. Compared to modern engines, the 4.0 liter engine was underpowered, but for the time it was installed, it performed admirably. The simple design of Jeep 4.0 engines ensures their long life and dependability. That Chrysler and Jeep used an AMC-built engine for 20 years is because they trusted the company’s quality.

There are flaws in every engine, however, and the AMC inline-6 engine is no exception. Oil leaks, water pump failure, and exhaust manifold corrosion are all common problems with this model of Jeep SUV. Crank position sensors, piston skirts, and head gaskets are all common failures. Maintaining the 4.0L engine properly is critical to its long-term dependability.

In many cases, the Jeep 4.0 engine can go 200k miles or even 300-400k miles without experiencing any issues. Longevity and dependability go hand in hand nicely. However, as these engines get older, rebuilding them becomes more common. Owning a great work of art can be expensive, but some people believe it’s well worth the investment to do so.

What are your thoughts on the Jeep 4.0L straight-six engine? Is one something you’re thinking about doing?

Let us know what you think by leaving a comment below!

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