S85 Fuel Filter Upgrade DIY

Why the S85 Needs a Post-Pump Inline Filter

I’ve owned this car for over a decade and have witnessed many fellow owners experience injector failures, often resulting in catastrophic engine damage. This issue has always been a serious concern for me. While I acknowledge the presence of a factory filter in the tank, it is clearly not sufficient—otherwise, injector failures wouldn’t be as common, even among those who routinely replace their filters.

To better understand the issue, I consulted with multiple fuel system engineers and fuel injection specialists in Michigan. Their consensus was clear: an additional layer of filtration is necessary after the fuel pump. They specifically recommended a 10-micron filter as the correct rating to provide adequate protection. At the end of the day, this is a practical and cost-effective safeguard. My motivation isn’t profit—it’s my passion for preserving these vehicles and preventing avoidable failures. I didn’t invent this solution—it’s been proven in the domestic car industry, notably in the 2009-2013 Corvette ZR1, which shares a similar fuel system relying on a single filter. The fix? A post-pump inline fuel filter, like the Dedicated Motorsports Auxiliary Fuel Filter Kit for the ZR1.

After testing various off-the-shelf options, I struggled to find one that fit securely and installed easily with OEM fuel lines. As a DIYer, I wanted a hassle-free solution. I’ve installed this on my own car, alongside a new pre-pump filter, and I consider it a must-have preventative mod.We designed and manufactured this system in the USA, using an easily accessible 10-micron Earls filter element. By bulk-ordering machined parts, we keep costs low while ensuring top quality

Below, you’ll find a detailed DIY guide on how to install a kit designed specifically for the S85 to follow along with our video.

S85 Fuel Filter Kit DIY Installation Guide

Time Required: ~20 minutes
Skill Level: Moderately easy (even for below-average DIYers!)

Tools Needed:

1. Socket Set (metric)
2. Wrenches (including an AN wrench if available)
3. Needle-Nose Pliers or Small Pick (for C-clip removal/install)
4. Rags/Shop Towels (fuel drips are likely)
5. Gloves & Eye Protection (recommended)

1. Remove the Two Fuel Pump Fuses

1. Open the glove box fuse panel and remove Fuse #30 (20A).
2. Open the trunk fuse panel and remove Fuse #72 (20A).
   • The E60 M5 has two fuel pumps, so there are two separate fuses.
3. Depressurize the fuel system by starting the engine and letting it run until it stalls.

Why This Step Matters
Removing the fuel pump fuses and depressurizing the system prevents excessive fuel spray when you disconnect the lines.

2. Remove/Move the Air Box & Coolant Reservoir

1. Air Box
   • Unbolt/unclip and carefully lift it. Disconnect MAF sensors or any other connectors that may be in the way.
2. Coolant Reservoir
   • Unbolt or unclip the reservoir from its bracket and move it aside.
   • Do not remove the coolant lines; you only need enough space to comfortably access the fuel line near the firewall.

Tip
Keep everything organized. Bag and label bolts, nuts, or clips you remove so you can quickly reassemble later.

3. Disconnect the OEM Fuel Line

1. Locate the Hard Line on the driver’s side (for LHD cars) near the firewall. You’ll see a hard line feeding into a flexible line.
2. Remove the Factory C-clip by using needle-nose pliers or a small pick to pop out the OEM clip.
3. Separate the Fuel Line by pulling the flexible line off the hard line. Expect some residual fuel to drip.

Safety Note
Have a rag or shop towel handy to catch any spills and wear safety glasses whenever you’re working with fuel lines.

4. Install the Kit’s New C-clip & AN Fitting

1. Place the Kit’s C-clip around the hard line where the OEM clip was.
2. Slide the AN Fitting onto the exposed hard line. Make sure it locks in place with the new C-clip.
3. Hand-Tighten first to ensure proper alignment, then use a wrench (preferably an AN wrench) to snug it down.

5. Attach the 180° Fitting & Mock Up the Filter

1. Thread the 180° Fitting onto the newly installed AN fitting. Keep it just hand-tight so you can rotate it.
2. Mock the Filter Placement to see how it will sit. In many installs, the filter tucks up top with a loop that mates well to the flexible line.
3. Tighten the 180° Fitting once you’re satisfied with the clearance and angle.

6. Mount & Tighten the Fuel Filter

1. Thread the Filter onto the 180° fitting.
2. Secure & Tighten the filter with a wrench to ensure a leak-free seal.
3. Double-Check Orientation so the flexible line can reattach without any kinks.

7. Reconnect the Flexible Fuel Line

1. Slide the Flexible Line onto the filter outlet (or second AN fitting if included).
2. Reuse/Install C-clip to lock the line in place. Confirm everything is firmly secured.
3. If your kit includes a fire sleeve, wrap it around the OEM rubber line for extra heat or abrasion protection.

8. Reassemble & Prime

1. Return the Coolant Reservoir to its original position and secure it.
2. Reinstall the Air Box, ensuring all bolts, clamps, and sensors/MAFs are reconnected.
3. Reinsert Both Fuel Pump Fuses (#30 in the glove box, #72 in the trunk).
4. Prime the Fuel Pumps by turning the key to the “ON” position a few times (without starting). This helps pressurize the lines.

9. Final Check & Start Up

1. Start the Engine and let it idle.
2. Inspect around the new filter, fittings, and lines for leaks.
3. Celebrate if everything is dry and secure—you’ve successfully upgraded your S85 with proper post-pump filtration!

Upgrading to a post-pump inline fuel filter on your S85 is one of the best “preventive maintenance” mods you can do. Not only does it address a known weak point in the fuel system, but it also helps extend the life of your injectors and engine internals. Plus, the installation is straightforward enough for most DIY enthusiasts.

By following the steps above, you’ll protect your S85 from debris-related injector failures and enjoy the peace of mind that comes with delivering cleaner fuel to each cylinder. Good Luck!

- Abdul