The 5.7 HEMI is a staple of the MOPAR lineup and has been since it was introduced in 2003. It has provided more than enough power to put rubber to the road in vehicles like the Charger and Challenger R/T, Chrysler 300C, and a bunch of RAM trucks. Despite being such a fan-favorite engine, it does have its fair share of issues, including a particularly destructive one.
Early 5.7 HEMI engines are known to drop valve seats, especially if the engine is overheated, doing catastrophic damage to the cylinder heads and even the long block in some instances. That can lead to some pretty hefty repair bills. In this guide, we’ll cover the infamous 5.7 HEMI dropped valve seats issue, discussing what causes the problem and what you can do to prevent it from happening.
If you are interested in learning more about common 345 HEMI issues, take a look at our 5.7 HEMI Common Problems Guide as well.
What Causes The 5.7 HEMI To Drop Valve Seats?
The main factor that causes the 5.7 to drop valve seats is heat. Ultimately, the issue is pretty straightforward and has a lot to do with the materials that Chrysler used to construct the engine. In order to save weight, the third-generation HEMI uses aluminum cylinder heads. While that isn’t a problem on its own, as most modern engines use aluminum heads, aluminum has a lower expansion threshold compared to other materials like steel or iron. That means that as aluminum gets hot, it expands much easier than harder metals.
In contrast, Chrysler uses steel to construct the HEMI’s valve seats. Valve seats are a crucial component in the cylinder heads, as they are where the intake and exhaust valves rest when they are in their closed position. That keeps the combustion chamber airtight and doesn’t allow any valve leakage when the valves are closed. Since the valve seats are made from steel, they are much harder than the aluminum heads that they are pressed into, meaning that they don’t expand as much under high heat as the heads themselves.
If engine temperatures get too high, the 0.005” of interference between the heads and valve seats isn’t enough to hold the valve seats into the expanding cylinder heads, allowing them to fall out and into the combustion chamber. Not only does the valve no longer have anything to seal against in the head, but the dropped valve seat can do serious damage to the pistons and block as a whole. While that rarely occurs at regular engine operating temperatures, it is very common for a 5.7 HEMI to drop a valve seat if the engine overheats.
MDS Might Play A Role Too
There is actually a lot of speculation surrounding the exact cause of valve seat drop in the 5.7 HEMI. While overheating is the most common explanation, some say that the MDS system, or multi-displacement system, has something to do with it too. Chrysler developed the HEMI’s MDS system to improve fuel economy when the engine isn’t under load. The system shuts down half of the engine’s cylinders when the engine is under light load, like during idle or at a steady cruising speed.
While the system operates as intended, one side effect of the MDS might lead to an increase in dropped valve seats. Since some of the cylinders are operational and some aren’t, there is a big temperature difference between the active and deactivated cylinders. It has been commonly reported that the dropped valve seats, in many instances, occur in cylinders that are deactivated by the MDS. That could be caused by the temperature change once the cylinders are deactivated.
Although there isn’t solid evidence or proof blaming the MDS system, it is commonly cited as another potential contributor to the issue.
Which Engines Are Affected By Dropped Valve Seats?
While it isn’t ideal that any of the 5.7 variants were affected by dropped valve seats, Chrysler eventually solved the issue on later 345 engines. The 5.7 Eagle HEMI was released in 2009 and featured a new variable valve timing system.
With the new system came entirely redesigned cylinder heads with increased flow and a new valve seat design. The revised heads pretty much eliminated the risk of the engine dropping valve seats. The 5.7 HEMI is still in production today with those revised heads and none of the engines produced after 2009 suffer the same issues as the pre-VCT engines. The following vehicles were affected by the issues:
- 2003–2009 Ram Pickup
- 2004–2009 Dodge Durango
- 2005–2008 Dodge Magnum R/T
- 2005–2009 Chrysler 300C,
- 2005–2009 Jeep Grand Cherokee
- 2006–2009 Dodge Charger R/T
- 2006–2009 Jeep Commander
- 2007–2009 Chrysler Aspen
- Typically happens suddenly and without warning
- Tends to happen when engine is hot, gets turned off, then is started again
- Tends to affect vehicles with 60,000 miles or more on the odometer
- Significantly higher chance if the cooling system isn’t functioning correctly
One of the most troubling aspects of valve seat drop is the fact that there are rarely any signs or symptoms leading up to it actually happening. Since the problem itself is just a small metal piece falling into the combustion chamber, there aren’t any sensors to alert you that there is anything wrong until the valve seat drops. Once it does, you’ll likely hear a terrible pinging/clinking noise if you try to start the car before the engine stops and cannot be restarted.
It is important to mention that in most cases, the valve seat actually drops when the engine isn’t running following a period of hard driving. Turning the engine off after it is hot can cause heat spots to develop, allowing the heads to expand to where the valve seat drops.
As we covered earlier, overheating is the main cause of dropped valve seats. For that reason, it is important to make sure that your 5.7 HEMI’s cooling system is functioning properly and that there aren’t any coolant leaks, failed components (like a thermostat or water pump), or clogs in the system. Keeping your cooling system in good shape is the best way to prevent dropped valve seats on the 5.7L HEMI.
Finally, mileage also seems to play a role in dropped valve seats, although not as much as the other factors we’ve discussed. In most cases, dropped valve seats tend to occur past the 60,000-mile mark, although that isn’t a failproof metric.
How Do You Prevent 5.7L HEMI Valve Seat Drop?
- Replace the factory valve seats with the revised valve seat design from later model engines
- Disable MDS with a handheld tuning device
- Keep up with cooling system maintenance
Since the HEMI’s valve seat drop issue is caused by the design of the engine’s cylinder heads themselves, there isn’t much that you can do in terms of easy solutions. This isn’t a problem that is cheap or simple to remedy, unfortunately. As a matter of fact, the only true way to solve the issue for sure is to remove the affected cylinder heads and either replace them with heads from a 2009+ 5.7 HEMI, or have the stock heads revised by a reputable shop.
While either of those solutions is cheaper than a new engine, it still isn’t a very cost-effective fix. Completely new heads run in the ballpark of $1,300 a piece or $2,600 for both while rebuilt heads cost around $900 including new-style valve seats. Despite being relatively expensive, you’ll be more than thankful that you did the preventative maintenance.
Since the MDS system is another commonly pointed area for concern, disabling the system with a handheld tuning device. Most aftermarket tuning devices for the 5.7 HEMI come preinstalled with software to disable MDS. While overheating is the main cause of dropped valve seats, disabling MDS is another worthwhile safety measure.
Last but not least is maintaining your cooling system. Like I said earlier, keeping up with cooling system maintenance is the second most fool-proof way of preventing dropped valve seats outside of replacing the heads. By ensuring your car doesn’t overheat, you are significantly reducing your risk while also preventing other cooling-related problems from occurring.
Dropped Valve Seats Are A Ticking Time Bomb On Early 5.7 HEMIs
While the 5.7L HEMI has very few serious issues, dropped valve seats are unquestionably the one that keeps most early-model HEMI owners awake at night. Dropped valve seats have to do with the material that both the heads and seats are made from, as the pressed seats can fall out of the aluminum cylinder heads as they heat up and expand. While dropped valve seats occur most often because of overheating, it has also been known to happen when the engine has been run hard, stopped, and then turned on again.
One of the worst aspects of the issue is the fact that there is no warning before the 5.7 HEMI drops a valve seat. The lack of symptoms makes it a really unpredictable issue that can’t really be anticipated. As a result, some HEMI owners argue that it is a better idea to do preemptive maintenance and repairs before the damage is done. While rebuilding your cylinder heads, or replacing them entirely, is no cheap endeavor, it is significantly cheaper than if you were to drop a valve seat and destroy the bottom end.
It is important to mention that dropped valve seats are not a guarantee on early model 5.7L HEMIs. There are plenty of them out there with hundreds of thousands of miles on the odometer that haven’t encountered the issue. The problem is that you can’t know if yours is one of those lucky engines.