The 5.9 Magnum is a naturally aspirated 5.9L V8 engine that was produced by Chrysler from 1992 up until 2003. With 360 cubic inches of displacement, the engine is also commonly referred to as the 360 Magnum. The 5.9 has a little brother – the 5.2 Magnum – that was used as the base engine option for Dodge’s and Jeep Grand Cherokees during the same time frame.
Magnum engines aren’t the most powerful or efficient for their time, but do offer a good balance of reliability. However, the 5.9 Magnum is still prone to a number of problems including the plenum gasket, cylinder heads, timing chain, and transmission.
Dodge 5.9 Magnum Common Problems
- Plenum Gasket Leak
- Cracked Cylinder Heads
- Timing Chain Failure
- Weak Front End
The two biggest issues with the 5.9 Magnum are plenum gasket leaks and weak cylinder heads. Outside of these two issues, the 5.9 is generally a stout and reliable engine. Often, owners either have no issues at all or they have endless problems. However, the people who have no issues at all outweigh the people with a lot of issues.
1. Plenum Gasket Leak
The intake manifold, also known as an intake plenum, is made of two separate pieces on the 5.9 Magnum. The top section of the plenum is a cast aluminum manifold and the bottom section is a stamped steel plate. A plenum gasket is used to seal the top and bottom sections together.
Gaskets are notorious on any engine for deteriorating and wearing out over time. On the Dodge 5.9 Magnum, the plenum gasket is prone to failing frequently. When the gasket fails, an air leak will develop and the engine will lose vacuum pressure leading to various performance issues.
While there are a handful of techniques for identifying a bad plenum gasket, most don’t realize the gasket is bad until the catalytic converter gets clogged up. When the cat gets clogged, you will notice a massive loss of power. With clogged cats, engine pressure increases as air cannot efficiently leave the engine which leads to a very high likelihood of the cylinder heads cracking as they are already weak.
Plenum Gasket Leak Symptoms
- Engine pinging noise while accelerating
- Excess oil consumption
- Fouled spark plugs
- Rough idling
- Cylinder misfires (commonly cyl #8)
- Poor overall performance & lack of power
- Bad gas mileage
- Fouled O2 sensors
Diagnosing a Leaking Plenum Gasket
Unfortunately, there isn’t a foolproof way of telling whether your plenum gasket is bad. Experiencing some of the symptoms above is a good indicator. There are a few diagnostic techniques that can give you a pretty good idea of whether the gasket is bad or not, but none are guaranteed.
Option 1: Open the throttle body and look into the manifold with a flashlight. The manifold should be relatively clean inside without any standing oil or sludge built up inside of it. If there is some oil or other buildup, then the gasket is likely bad.
Option 2: Remove the PCV valve from the valve cover and check for air pressure. There should be air coming out of the valve cover; you will know that the gasket is leaking if there is a vacuum coming from the cover.
2. Cracked Cylinder Heads
The biggest downside of the 5.9 Magnum is the weakness of the cylinder heads. While the heads received a new design over the LA 360 engine, including increased air flow, larger valve sizes, and an improved combustion chamber configuration, they are known to be very weak.
The most common cause of cracked cylinder heads is excess heat. Coolant loss, bad water pumps, serpentine belt failures, etc. can all lead to increased engine temperatures. When engine temps increase, the cast iron heads expand and crack. The cracks most commonly occur between the valve seats since the seats are induction hardened into the heads instead of pressed in.
While some shops estimate about 50% of 5.9 heads are cracked, the good news is the majority of the cracks tend to be very small. While we don’t recommend this, we’ve seen people driving around on cracked heads for years without any major issues.
Cracked Cylinder Head Symptoms
- Engine overheating
- Water in the oil (thick white stuff)
- Coolant or oil leaking from cylinder head
- Spark plugs are coated in gas
- Engine misfires
Performing a cylinder leak-down or compression test is the best way to identify a cracked head.
Cylinder Head Replacement Options
While you can continue to drive on small cylinder head cracks, it is a dangerous game to play. Cracked heads can lead to serious internal engine damage and ultimately require a brand-new engine. Because of this, we recommend upgrading cracked heads with stronger, aftermarket replacement heads.
Due to the frequency of this problem, there are dozens of affordable aftermarket heads available for the Dodge 5.9 Magnum. Aftermarket heads have thicker metal in the combustion chambers and decks which eliminates the cracking issue.
3. Timing Chain Failure
Timing chains connect the camshaft and the crankshaft, which are responsible for opening and closing the valves. The intake valves open and let air and gas into the engine while the exhaust valve remains closed. When the intake valve closes, the exhaust valve opens and pushed exhaust gases out. The timing chain is responsible for controlling this process for each of the cylinders and valves.
On the 5.9 Magnum, as with most timing chain engines, the chain stretches over time. While a lack of frequent oil changes can accelerate this problem, the timing chain on the 5.9 Magnum usually starts to stretch around the 100,000-mile mark. While it doesn’t need to be replaced at this interval exactly, I would consider replacing the chain at the 150,000-mile mark if it hasn’t shown any signs of wear or stretching prior to that.
When the chain becomes too stretched it can jump a few teeth and cause significant performance issues. When the timing chain stretches, the timing of the opening and closing of each of the valves gets out of whack and the engine begins to run very poorly. If you have serious stretch or the chain stretches to the point it falls off its guides, it can cause the engine internals to bang against each other, leading to the need for a full engine rebuild.
Symptoms of Timing Chain Failure
- Frequent cylinder misfires (most common)
- Lack of power
- Rough idling/rattling while idling
- Overall poor performance
- Engine no start
If you have a stretched timing chain the only real fix is replacing the chain. When you replace the chain we recommend replacing it with a double roller chain. The double roller adds extra strength and reliability and is only a couple bucks more than a stock/OEM timing chain.
4. 5.9 Magnum Weak Front End
The Magnum 360 is a pretty large engine and therefore has some additional weight. Unfortunately, the front ends, specifically on the Dodge Ram’s, are a weak point of the truck.
While these problems aren’t necessarily engine related, I thought it was worth mentioning that there might be some additional maintenance required with respect to these trucks. Obviously, as is with anything old, parts need to be replaced.
With that being said, the front-end components on the Dodge Ram are known for being weak and needing replacement once the truck reaches higher mileage. These repairs aren’t anything out of the ordinary for high-mileage trucks, just be aware that issues with these parts may begin to develop slightly earlier than you would expect.
- Axle seals and bearings
- Wheel bearings and joints
- Tie rods
- Wheel hubs
5. Transmission Problems
Although not an engine issue again, a common problem that needs mentioning is transmission reliability. Overall, the 46RH and 46RE transmissions used with the 5.9 are very shotty at best.
Outside of poor gearing ratios, the torque converters and transmission cooling lines are known to go bad. On 4×4 transmissions, the reverse and overdrive assembly commonly fails as well. People who do a lot of towing, especially with overdrive, will experience issues a lot sooner than those who do not tow and tend to drive conservatively.
When these engines are properly rebuilt with upgraded internal components they are rock solid. You have to keep in mind that these are now 20+ year-old trucks and the small internal items like seals have degraded and worn down significantly since they were new. Overall, the transmission without a rebuild is likely going to be an unreliable one, especially if you are pushing past the 150,000-mile mark.
One common trick used by some Dodge 5.9 Magnum owners is to shift into neutral before a cold start to allow transmission fluid to circulate through the system from the get-go. In park, the pump doesn’t circulate fluid, so putting the transmission in neutral provides a bit of extra lubrication before setting off. This can help with internal wear and potentially prolong any issues.
Dodge 5.9 Magnum Reliability
Overall the 5.9 Magnum can be a reliable engine with proper maintenance and care. Reliability reviews tend to be mixed, with a lot of people reporting no issues at all. However, the ones that do report issues tend to report a lot of issues. One of the biggest factors of this is likely the fact that these trucks are 20+ years old now.
While the plenum gasket is virtually a guaranteed problem, cracked cylinder heads are less common, albeit still a concern and the cracks tend to be less severe. Outside of these two issues, there aren’t any cruxes of the 5.9 Magnum. Fortunately, the plenum gasket can be fixed with an aftermarket kit for less than $200, and cylinder head replacements are relatively cheap compared to other engines. Front-end components will definitely need replacing at some point and the transmission can be another question mark depending on how the truck has been driven over its lifetime.
With proper maintenance, these engines can last up to 350,000 miles. However, it is worth noting that there are plenty of typical maintenance items that will need to be repaired for the 5.9 to make it this long. As is normal with old cars, water pumps, oil pumps, gaskets, seals, belts and chains, etc. will probably go out once or twice along the journey to that kind of mileage.