The Jeep 4.0-liter Straight-Six Engine. The Jeep 4.0 is a legend not only in the Jeep world, but also in the automobile business. AMC created this engine before Chrysler bought them out in 1987. The 4.0L straight-6 is a powerful and durable engine, but it is also noted for its lack of power. Regardless, the 4.0 is a terrific all-around engine that many people still love today. It was good enough that Chrysler and Jeep kept it for nearly two decades. This article discusses Jeep 4.0 engine difficulties, dependability, specifications, and more.
Jeep 4.0 Engine Reliability, Issues, and Specifications
*The 4.0 inline-6 engine is also known as the PowerTech or 4.0 PowerTech by some. In the article, we may refer to the Jeep 4.0L engine by several designations. It could alternatively be referred to as an AMC, Jeep, or Chrysler engine.
What Vehicles Make Use of the Jeep 4.0?
AMC/Chrysler/Jeep 4.0L straight-6 engines are available in the years and models listed below:
- 1987-2001 Jeep Cherokee
- Jeep Grand Cherokee 1993-2004
- 1987-1990 Jeep Wagoneer
- 1987-1992 Jeep Comanche
- 1991-2006 Jeep Wrangler
4.0 Straight-Six Engine Specifications
The 4.0L inline-6 engine was first introduced in 1986, just one year before Chrysler purchased AMC. Despite this, Chrysler chose to keep the AMC-designed 4.0 on the market for the long term. Chrysler did, however, make a number of changes to the straight-6 over the years. In a bit, we’ll return to this subject and go over some specifics. The table below displays some of the basic specifications for the Jeep 4.0 engine:
It’s a 4.0L inline-6 engine, as the name says. The actual displacement is around 3,965cc, which has been rounded up to 4.0L. Cast iron heads and blocks were common in the 1980s and offer a lot of strength. Due of their weight, they are rarely seen in modern times. Jeep 4.0 engines are overhead valves, as opposed to modern overhead or double overhead cams.
The original 1987 versions had 173hp of power. In 1988, output was swiftly boosted to 177hp, and subsequently to 190hp in 1991. Torque has also improved over time. Of course, the 4.0 straight-6 received a number of upgrades throughout its nearly 20-year lifespan. Some contributed to increased power and torque, while others aimed to improve efficiency, strength, and reliability. In the following part, we’ll go over a few of these modifications fast.
Updates to the Jeep 4.0L Inline-6
It should be noted that this is not a full list of all updates. 20 years is a long time for an engine to exist, and numerous changes occur over that time. Anyway, here are a few important upgrades for the Jeep 4.0 straight-6 engine:
- 1988 – More powerful fuel injectors
- 1991 – Multi-port fuel injection replaces RENIX, intake ports are raised 3.2mm, the throttle body is enlarged, the intake and exhaust manifolds are updated, higher flowing injectors are installed, and cam timing is updated.
- 1995 – Cylinder head modifications
- 1996 – More powerful engine block
- 2000 – Improved cylinder head efficiency and a distributor-less coil-on-plug ignition system are introduced.
Other modest modifications and changes have occurred over the years. One year after its debut, the Jeep 4.0 engine obtains greater flow injectors, which contribute to an increase in horsepower and torque. A few upgrades in 1991 allowed the engine to produce 190hp. In the years that followed, the head and block received a few minor changes. Finally, in 2000, Jeep 4.0L engines received a few design improvements to assist lower emissions and improved economy.
Problems with the Jeep 4.0 Straight-6 Engine
Now we’ll get into some of the most important elements of this article. Even though the 4.0 inline-6 is famous, it is prone to engine troubles. This is true for any car engine, but it is especially true for engines that are 15-30 years old or older. The following are a some of the most prevalent engine problems on the Jeep 4.0 (in no particular order):
- Cracks in the exhaust manifold
- Oil spills
- Thermostat and water pump
Throughout this essay, we’ll go over the issues raised above in further detail. We’ll wrap up with some remarks on overall dependability. This will also lead us to the conclusion of our examination of rebuild kits. Many owners still enjoy the Jeep 4.0 straight-6, although age plays a significant role in this. There are probably a lot more Jeeps out there with the 4.0L straight-6 that are nearing the end of their lives owing to age and mileage. As a result, Jeep 4.0 rebuilds and rebuild kits are a popular choice.
1) Cracks in the Jeep 4.0L Exhaust Manifold
One of the most common difficulties with the Jeep 4.0 engine is exhaust manifold cracking. This is especially true given the age and mileage of today’s 4.0L straight-6 engines. Fortunately, it’s a small concern in the larger scheme of things. Exhaust manifolds are often called headers or exhaust headers.
Sometimes the problem is only the manifold gasket or bolts. However, it is not uncommon for the manifold to develop cracks. The symptoms are particularly obvious when the engine is cold. As the exhaust manifold on the Jeep 4.0 inline-6 heats up and expands. Because the expansion can help close the crack, symptoms may disappear as the engine warms up.
Symptoms of an AMC 4.0 Exhaust Manifold Crack
A fractured exhaust manifold on the Jeep 4.0L PowerTech engine might cause the following symptoms:
- Ticking noises
- The odor of raw exhaust
- There is visible cracking.
Exhaust manifold cracks are commonly responsible for unusual engine sounds, most notably a ticking noise. Again, this may be apparent just on a colder engine as the cracks shrink with heat. You may also note that the Jeep 4.0 smells extremely pungent, similar to raw exhaust. With minor cracks, this can be difficult to notice. Otherwise, a visual inspection may reveal whether and where fissures exist.
Repair of the 4.0L Inline-6 Exhaust Manifold
If you have a fractured exhaust manifold on your Jeep 4.0 engine, you have various options. You can go with an OEM option, but new parts can be difficult to come by. Used Jeep exhaust manifolds may experience the same problems, so proceed with caution.
Aftermarket Jeep 4.0L straight-6 exhaust headers are an excellent option. However, altering the exhaust may be considered illegal and may result in emissions difficulties. Nonetheless, it’s a viable approach for avoiding more numerous leaks in the functionality.
Finally, your cracked Jeep 4.0 manifold may be repairable. This option has worked for some people, but it is only a short-term answer for others. This could be a nice, low-cost solution for AMC straight-6 owners who don’t mind a do-it-yourself job.
2) Jeep 4.0 Oil Leak Issues
As huge BMW fans, this one hits close to home. Many of the same jokes regarding oil leaks circulate in the Jeep world as well. Oil leaks affect numerous engines, therefore the Jeep 4.0 engine is not alone in this regard. Some Jeep oil leaks are merely the result of age and mileage, while others have been known to be defective since the beginning.
Some of the most typical oil leak issues are the Jeep 4.0 valve cover, valve cover gasket, and rear main seal. These seals and gaskets deteriorate and fracture over time. Oil leaks typically start tiny and worsen over time as cracks spread and new ones form.
Given the age of the AMC 4.0 straight-6, it’s possible that many engines are leaking or have previously repaired oil leaks. whether you’re looking for a 4.0L engine, search for leaks and determine whether any of these repairs have been performed in the past.
Symptoms of a Chrysler 4.0L Engine Oil Leak
Some people may notice oil leak symptoms right away. Nonetheless, keep an eye out for the following signs, which may indicate an oil leak in the Jeep 4.0 engine:
- Visible dripping
- The odor of burning oil
- Light smoke is coming from the engine bay.
Visible drops of oil on the ground indicate that oil is leaking someplace. When the 4.0L inline-6’s rear main seal leaks, it frequently pours onto the ground. However, because valve cover gaskets are located at the top of the engine, minor leaks may not reach the ground.
Check for probable burning oil smells or light smoke from the engine bay when replacing Jeep valve cover gaskets. You may also notice some oil accumulating near the valve cover gasket.
Repairs for a Jeep 4.0 Oil Leak
The DIY audience will appreciate how inexpensive these Jeep 4.0L oil leak remedies are. Gaskets and seals are frequently less than $20, so a few hours in the garage can save you a lot of money. Those that visit repair shops will discover their bills to be somewhat expensive due to labor charges.
A repair business can charge $300-500+ to replace the rear main seal. It’s a physically demanding job. The simple engine configuration of an inline-6 allows for faster valve cover gasket replacement.
3) Failures of 4.0 Straight-6 Water Pumps
Another typical concern with the Jeep 4.0 straight-6 engine is overheating. Not usually are the water pump and thermostat to blame. They are, however, common causes of overheating and are often appropriate beginning places. When these engines were newer, this wasn’t a big deal. Water pumps, on the other hand, endure a lot of abuse over time and are regular repairs on many aging engines.
Water pump failure is a critical repair on any engine, including the 4.0L Chrysler PowerTech engine. On occasion, the engine has been known to have head gasket problems. When the 4.0 liter engine overheats, these breakdowns become much more common.
Aside from actual water pump failure, another concern is pump flow. Some OEM pumps may not flow well at low RPMs during idle, which can lead to overheating. Fortunately, there are various aftermarket options that can assist in resolving this issue with the Jeep straight-6 engine.
Symptoms of Jeep 4.0 Water Pump Failure
The following are symptoms of water pump failure on the Chrysler/Jeep 4.0 engine:
- Leaks in the cooling system
- The engine produces steam.
If you observe a coolant leak, the water pump is likely to be to blame. In some circumstances, coolant loss can be extremely rapid, and you may even detect steam coming from the engine as it leaks out and burns off. On the 4.0 Grand Cherokee and Wrangler, this can cause rapid overheating.
It’s critical to avoid overheating the engine, therefore if you notice this symptom, turn it off as soon as possible. Running the engine for extended periods of time raises the chance of additional difficulties.
Reliability of the Jeep 4.0 Engine
Is the 4.0 straight-6 engine in the Jeep dependable? Yes, we believe this engine gets high points for dependability. When the 4.0L engine was created, AMC was in financial trouble. As a result, AMC chose to use some older but tried and true tech for the straight-6 engine. The 4.0 is widely regarded as one of the best Jeep engines due to its simplicity, dependability, and 4×4 performance.
However, no engine is flawless, and the iconic AMC/Chrysler/Jeep 4.0L engine is no exception. Exhaust manifold cracks, oil leaks, and water pump failures have all been reported in these engines. This is not an exhaustive list, either. Other Jeep 4.0 engine problems include leaking fuel injectors, a leaking head gasket, and a leaking piston skirt.
Then there’s age, which is a major deal with the 4.0L inline-6. The engine frequently travels well over 200,000 miles, and in some cases even 300-400k miles. That’s an impressive amount of lifespan and dependability for the Jeep 4.0 engine. Still, given their age, these engines are likely to require some extra TLC and even a rebuild.
Rebuild Kits for Jeep 4.0L Inline-6
Given the appeal of the Jeep 4.0 engine, a rebuild kit is an increasingly popular option. Most straight-6 parts are reasonably priced, so it’s not a bad deal for people who can do it themselves. Of course, rebuilds are time-consuming and necessitate extensive engine knowledge. In general, rebuild kits include the following components:
- Gaskets and seals
- Rings and pistons
- Bearings for rods
- The main bearings
- Lifters, camshafts, and cam bearings
- The oil pump
- Timing equipment
This Jeep 4.0 repair kit is a nice example of how they often appear. During a rebuild, you have the option of doing less or going even further. There are numerous possibilities for internal upgrades, stroker kits, forced induction, and so on. Again, if you are unable to complete the work on your own, this might get costly. At that point, you might be better off finding an already rebuilt engine and swapping it out.
Summary of the 4.0 Straight-6 Engine
It’s no secret that AMC’s 4.0L straight-6 engine is something of a Jeep legend. The 4.0 liter engine didn’t produce much power by current standards, but it provided adequate performance for the time. Jeep 4.0 engines have a straightforward design that ensures long-term durability and dependability. There’s a reason Chrysler and Jeep maintained an AMC engine in their lineup for 20 years.
However, no engine is flawless, and the AMC inline-6 engine is no exception. The exhaust manifold, oil leaks, and water pump are all common problems with the Jeep 4.0. Other common issues include head gaskets, piston skirts, and crank position sensors. A lot of reliability is simply down to maintenance, so make sure you look after the 4.0L engine.
The Jeep 4.0 engine frequently lasts well over 200k miles, and some even make it to 300-400k with few issues. Not terrible in terms of durability and dependability. However, as these engines age, it is becoming more typical for them to require rebuilds. It might be an expensive venture, but some believe it is worthwhile to acquire one of the greats.