The Milwaukee-Eight (M8) engine, introduced by Harley-Davidson in 2017, promised enhanced power, performance, and smoother riding experience. Despite these improvements, many owners have reported recurring issues that have raised concerns about its reliability. This article outlines the most common problems encountered with the Milwaukee-Eight engine, based on real-world experiences from numerous riders.
1. Sumping Issues
One of the most widespread and concerning problems with the M8 engine is sumping. This occurs when oil accumulates in the crankcase rather than circulating properly through the engine. Riders experiencing sumping often notice:
- Loss of power, especially at higher speeds.
- Excessive engine heat, making the ride uncomfortable.
- A sluggish, lethargic feel when accelerating.
This issue tends to surface as early as 5,000 miles for some riders, while others report problems around 12,000 miles. The root cause is poor oil scavenging, where the oil pump fails to return oil quickly enough from the crankcase. As oil builds up, the engine overheats and performance declines.
Potential Fixes:
Many riders have had success upgrading to newer oil pump designs, especially those released after 2020. While this has mitigated the issue for some, there are still reports of sumping even after installing aftermarket pumps.
2. Transmission Clunking
Harley-Davidson transmissions have always had a reputation for being noisy, but transmission clunking on the M8 has raised concerns among riders. The problem is particularly noticeable when shifting into first gear or during hard shifts. Symptoms include:
- A pronounced “clunk” sound when shifting into first gear.
- Stiffer gear changes, especially at lower speeds.
While some riders have adjusted clutch tension or upgraded their transmission fluid to alleviate the noise, it appears to be more of a characteristic of the design rather than a sign of mechanical failure.
Solutions:
- Adjusting clutch tension.
- Using higher-quality transmission fluid to smooth gear transitions.
3. Oil Leaks
Another common issue reported by M8 owners is oil leaks, particularly around the rocker boxes and cam chest. These leaks typically appear within the first few thousand miles, causing concerns about the durability of engine seals. Common areas where leaks occur include:
- Rocker box gaskets.
- Cam chest seals.
While many of these leaks have been addressed under warranty, the recurrence of the issue has frustrated some riders. Regular oil changes and inspections are essential for catching leaks early before they lead to more severe problems.
Potential Fixes:
- Replace the faulty gaskets or seals.
- Regularly inspect the engine for early signs of oil leaks.
4. Excessive Heat
Harley-Davidson’s air-cooled engines are known to run hotter than liquid-cooled engines, but the M8 has been criticized for producing excessive heat, especially in stop-and-go traffic or warmer climates. Many riders report unbearable heat from the rear cylinder. Solutions that riders have found useful include:
- Installing aftermarket exhaust systems to improve airflow.
- Adding heat shields to direct heat away from the rider’s legs.
- Using synthetic oils designed to handle higher temperatures.
While these modifications can help, the heat issue remains one of the more persistent complaints about the Milwaukee-Eight engine.
5. Valve Train Noise
Valve train noise is another frequently reported issue with the M8 engine. Although Harley engines are traditionally loud, many riders believe the M8’s valve train is noisier than previous models, particularly at idle or during acceleration. Causes of the noise could include:
- Hydraulic lifters not maintaining proper pressure.
- Inconsistent oil delivery to the top end.
While some owners have replaced their lifters with aftermarket options, others suggest that the noise is simply part of the engine’s design and may not indicate a significant problem.
Potential Fixes:
- Replacing hydraulic lifters.
- Ensuring proper oil delivery with regular maintenance.
6. Compensator Failures
The compensator is a critical component responsible for dampening the shock from the engine to the transmission, ensuring smooth power delivery. However, some M8 owners have reported premature compensator wear, leading to clunking noises and rougher shifts. Symptoms include:
- A loud clunk or knock when starting the bike.
- Increased vibration during operation.
Replacing the stock compensator with a heavy-duty aftermarket version has been a popular solution, though it can be an expensive fix.
Solutions:
Regular inspection of the compensator to catch wear early. expensive fix.
Install a heavy-duty aftermarket compensator to handle the engine’s power better.
7. Cold Start Issues
Some owners have reported rough idling or misfiring during cold starts, especially in colder climates. This could be due to insufficient fuel mapping in the ECM (Engine Control Module) or issues with the fuel injectors.
Potential Fixes:
- Tuning the ECM for colder climates.
- Installing upgraded spark plugs or fuel injectors.
8. Starter Motor Problems
A small but noticeable number of M8 owners have experienced issues with the starter motor, particularly with it struggling to turn the engine over, especially after the bike has been running for a while and is heat-soaked. This could be related to the engine’s high compression.
Common Symptoms:
- Slow cranking or clicking sounds when attempting to start.
- Starter fails to engage after long rides.
Potential Solutions:
- Upgrading to a high-torque starter.
- Installing a compression release system.
9. Fuel Pump Failures
Some riders have encountered fuel pump issues, particularly after several thousand miles. This can lead to inconsistent fuel delivery, which can cause stalling or erratic throttle response.
Symptoms:
- Sudden loss of power.
- Difficulty starting the bike.
Possible Fix:
- Replacing the fuel pump with a more robust aftermarket option.
10. Throttle-by-Wire Sensitivity
The Milwaukee-Eight engine employs a throttle-by-wire system, which has led some riders to complain about either too much sensitivity or lag in throttle response. This could be due to calibration issues or the electronic throttle control’s nature.
Signs of Throttle Sensitivity Issues:
- Jerky or lagging acceleration.
- Delayed throttle response, particularly at low speeds.
Solutions:
- Aftermarket throttle tuners or remapping the system for a smoother response.
11. Cam Chain Tensioner Wear
In the long run, the M8’s cam chain tensioners may wear out prematurely, causing the chain to become loose and leading to possible engine damage. Previous generations of Harley engines, like the Twin Cam, were notorious for this issue, and while it has improved with the M8, it can still occur.
Indicators of Cam Chain Tensioner Issues:
- Rattling noise from the cam chest.
- Reduced performance or irregular engine running.
Fixes:
- Inspecting the tensioner regularly and replacing it as needed.
- Upgrading to an aftermarket cam chain tensioner system.
12. Exhaust Valve Problems
Over time, some owners have noticed that their exhaust valves can become damaged or burnt, leading to a loss of compression and reduced performance. This could be caused by excessive heat in the exhaust system or improper fueling.
Symptoms of Exhaust Valve Issues:
- Loss of power.
- Excessive backfiring or popping in the exhaust.
Potential Fix:
- Replacing damaged valves and improving heat management with upgraded exhaust systems or ceramic coatings.
13. ECM/Software Updates
Some riders may not be aware that Harley-Davidson regularly issues ECM (Engine Control Module) software updates to address performance and reliability issues. Not keeping the ECM up to date can lead to unresolved issues that could have been fixed through a simple reflash.
Importance of Regular ECM Updates:
- Enhanced throttle response.
- Better fuel efficiency and smoother engine operation.
How to Check for Updates:
- Regularly visit a Harley-Davidson dealership for diagnostics or use third-party tuners to check and install updates.
14. Suspension Compatibility with M8’s Power Output
With the increased torque and horsepower of the Milwaukee-Eight engine, some riders have reported that the stock suspension components, especially on touring models, struggle to handle the power, resulting in a less stable ride or bottoming out.
Suspension Symptoms:
- Excessive bouncing or bottoming out, especially under acceleration.
- Unstable handling in corners.
Potential Solutions:
- Upgrading to high-performance shocks or adjustable suspension systems.
15. Air Intake Restrictions
While the M8 engine is more powerful, some owners have pointed out that the stock air intake system can be restrictive, preventing the engine from breathing freely at higher RPMs. This can lead to reduced performance potential.
Symptoms:
- Lag in acceleration, particularly at high speeds.
- Lower-than-expected top-end power.
Potential Solutions:
- Installing a high-flow aftermarket air intake system.
Read more: Best Air Cleaner for Milwaukee 8
16. Clutch Wear
With more torque being generated by the M8 engine, the clutch can wear out faster, especially if the bike is ridden aggressively. Riders have reported clutch slippage, particularly during hard accelerations or towing.
Symptoms:
- Noticeable slippage during acceleration.
- Difficulty shifting smoothly, especially in higher gears.
Fix:
- Replacing the stock clutch with a performance-oriented aftermarket option.
Conclusion
While the Milwaukee-Eight engine offers significant improvements in power and performance, it’s not without its challenges. Sumping, transmission clunking, oil leaks, excessive heat, valve train noise, and compensator failures are among the most commonly reported issues. Understanding these problems and addressing them with timely upgrades or modifications can help M8 owners maintain their bikes and enjoy a smoother, more reliable ride.
For those experiencing any of these issues, regular maintenance, software updates, and aftermarket upgrades are often effective solutions. If the problems persist, it’s always a good idea to consult with a Harley-Davidson specialist for more personalized advice.