Typically when you overhear somebody talking about a super reliable inline-6 engine, they’ll refer to the 2JZ, RB26, or Ford Barra. While those engines are awesome and reliable as we’ve discussed in other articles, there is another ultra-reliable inline-6 engine out there, which is the AMC but there is another awesome inline-6 which is often overlooked, which the Jeep 4.0L.
While this engine might not have the performance specs or capabilities of the other inline-6 engines we’ve covered on the channel, it’s arguably the most reliable. Fair warning, this is one of my favorite engines because of my extended hands-on time with these engines in the past, but I’ll try to keep the article as unbiased as possible.
Where it Came From
What’s particularly interesting Jeep 4.0L is that it can trace its roots back to 1964, with the AMC Rambler American, which was a small compact car that was available with a few different engines, including a 232 cubic inch Typhoon Six. While the modern-day Jeep doesn’t share a massive amount with those older AMC engines, they laid the groundwork that the 4.0L was built on.
Throughout the years, AMC updated their inline-six engines with various displacements and design features until 1986 when they launched the modern-day 242 ci / 4.0l engine. During the development period of the 4.0L, AMC wasn’t exactly thriving incredibly well, so take save development costs the 4.0L used many of the same parts and dimensions as the other inline-sixes in the 232/258/4.0 family.
This evident by the fact that the heads can be bolted to the earlier 232 and 258 blocks with some minor modifications. What’s even more interesting, is that according to AMC historian Frank Swygert, it’s totally possible and relatively common to put a 258ci crank and rods in a 4.0L to make a 280ci engine. This can be done with stock 258ci crank and rods and 4.0L block and pistons.
Before we move too much further, I would to briefly cover some basic info and specs on this engine for those of you out there who are completely new to this engine. As the name suggests, the engine is a 4.0L inline-6. It features a heavy cast-iron block, cast iron cylinder head, 2 valves per cylinder, it lacks any kind of variable valve timing or variable valve lift.
Bore measures at 98.4mm and stroke at 86.7mm, which equates to 3962cc of displacement which is rounded up to 4 liters. Depending on the year of the engine, compression from 9.2:1 to 8.8:1.
What’s interesting about this engine that’s completely different than every other inline-six engine we’ve featured on the channel is that this engine does not use overhead cams. Instead, it features a single camshaft inside the block which uses hydraulic lifters, pushrods, and rocker arms to operate the valves.
Something else which is way different than anything you’ll typically see on modern engines is the fact that the intake and exhaust ports of the cylinder head are on the same side. With most engines, air enters on one side of the head, and exhaust exits on the opposite side. On the 4.0L, air comes in and exhaust goes out on the driver’s side of the cylinder head.
As you can probably imagine, putting the exhaust ports and intake ports directly next to each other isn’t exactly the most thermally efficient design, which is probably why you no longer see this design with modern engines.
Again, this is way different than most modern inline-six engines which almost always use a single or dual overhead cam design with the intake and exhaust ports on opposite sides of the heads, which shows how old the internal designs of this engine are.
Something you might have noticed by the specs we covered a moment ago is that this engine has a surprising bore to stroke ratio. With a lot of engines used in SUVs and trucks, you’ll find a larger stroke than the bore to help increase low-end torque, however, the Jeep 4.0L is opposite with a much larger bore than stroke.
When the 4.0L appeared on the scene in 1987, it was surprisingly powerful for an engine of that size used in SUVs or trucks. Upon initial release, it output 173hp and 215lb-ft, which was quite a bit more than the six-cylinder engines found in Ford, GM, or Nissan trucks and SUVs.
Throughout the years, Jeep increased power output until the final High Output version which outputs a whopping 190hp and 235lb-ft of torque. Ironically, the 4.0L was pretty impressive upon its first release in 1987 but the power wasn’t increased all the much in the 20 years that Jeep used it.
Renix vs H.O.
Something which a lot of truck engines didn’t have at the time was an advanced fuel injection system. At the time, the large majority of truck engines were carbureted, however, AMC sent the 4.0L to Renault-Bendix the fuel injection system and electronics.
It’s important to note that the carbureted 258ci engine was produced alongside the 4.0L, with the carbureted engine being used in the YJ Wrangler from 1987 to 1990. The Renault-Bendix engine management, also known as RENIX, was advanced at the time that it was introduced, but it’s been known as an overly complicated system which is super difficult to diagnose issues with because there aren’t many tools which can connect to the RENIX system and the RENIX system didn’t store codes.
Luckily, Chrysler figured out that the RENIX system was kind of trash and eventually replaced it with a new engine management system in 1991. Aside from just changing the engine management in 1991, Chrysler also changed the location of the intake ports, added a larger throttle body, improved the intake and exhaust manifolds, and changed the camshaft.
The result of all the changes in 1991 was a power increased to 190hp and 225lb-ft. These engines are now known as the High Output engines, with many of the Jeeps using the 1991 and later engines having a high output badge on the side.
From 1991 to 1995 there were pretty much no changes to the 4.0, but in 1995 the cylinder got a few small updates and then in 1996 the engine block was upgraded with more webbing and a stud girdle for added rigidity. In 2000, there were some changes in the cylinder head which had a bad design and lead to cracking in the head.
Not all the heads had this issue, just the heads with the 0331 casting number. The head will typically cack between cylinder #3 and #4. Then midway through 2001, the bad casting was fixed but still retained the new cylinder head design.
Throughout the years, Jeep made a lot of other small changes to the 4.0L, but there weren’t any major changes to the engine design. Something really interesting is how closely related the 4.0 is to the 2.5 liter 4-cylinder. The 4.0 and 2.5 have the same architecture and design, but the 2.5 simply has the two center cylinders removed.
Looking at some of the common problems and issues of the 4.0, there aren’t that many major issues. As mentioned earlier, the RENIX 4.0 motors are very difficult to diagnose, the later engines supposedly have fuel injector issues, the rear main seals can be leaky, the 0331 cylinder heads can crack, and the oil pumps are known for occasionally wearing out prematurely.
That’s it for the common issues, which is the main reason people love these engines so much, there is very little on them that breaks and they can easily survive conditions which would destroy most engines. Because of the simplicity and lack of any advanced features, there isn’t much that can go wrong with a Jeep 4 liter.
Just in my personal experience, I’ve had 4.0 liters run 5 cylinders for extended periods of time, run way below operating temperature from lack of thermostat, run on basically no oil, etc. I’ve even put a hole and dented an oil pan on a 4.0 liter bad enough that the crank started hitting the pan, and the motor still ran just fine after welded the hole shut. The points I just made are anecdotal, but I thought they were worth mentioning.
As far as performance goes, the 4.0 was never designed to be a performance engine and while you can get more power out of them, they’ll never be a good performance engine, they simply weren’t designed with high-performance in mind. Some of the popular builds you’ll see are 4.7-liter stroker motor which will make around 210hp and upwards of 300lb-ft of torque.
There are quite a few people who have turbocharged these engines which is interesting because of how wild the 4.0 sounds when you add some boost to it. There are also some supercharger options, but most of them won’t push the 4.0 past 250hp or so.
So, that’s pretty much everything you need to know about the Jeep 4.0. I tried to keep this unbiased, but I won’t lie, I love these engines and I’ve had quite a bit of experience with them. The reason I love these motors is the same reason most people like them: they simply don’t break.