We all have read time and again that oil is the lifeblood of an engine, but without fuel, the engine doesn’t run, and if it doesn’t run, well, the oil just lies dormant in the sump. In its own way, fuel is the lifeblood of an engine.
Most modern cars have two-stage fuel filtration systems, one in the tank and one on the underside of the car for easy replacement. In the Mazda Miata, that first filtration element also is critical to keeping the fuel pump pickup surrounded with enough fuel to feed the engine.
When that fuel sock is working properly, you can run the car really low on fuel — a half gallon or less — before it begins to starve in hard cornering. In stock form, it’s remarkably efficient, but if that sock develops a pinhole, or or a seam separates, or deteriorates due to age, it doesn’t work nearly as well, and you need to keep it above a half a tank to keep the pickup fed.
In a car with as little horsepower as a Miata, those extra six pounds per gallon can be the difference between first and second, so it’s important to be able to run the car as low on fuel as possible, which requires a good fuel pump sock.
It’s critical to use only the factory sock. The filtration material is some serious black magic — really. It allows fuel in more quickly than it lets fuel out. For example, if you pull the fuel pump assembly out of the tank, and the photos in this feature will show this, the fuel stays in the sock for several minutes before it drains out. That’s what keeps the pickup fed even when there’s hardly any fuel in the car. Aftermarket socks don’t do that, and you can get starvation even with a new aftermarket part.
With a hardtop and a rollcage, the job is cramped and more difficult than it should be, so it’s easier with the top off. All the hardware used on the fuel pump assembly is tiny, so take your time, and have your magnetic pickup tool handy.
Power tools are great for their torque and speed, but once you get the cover over the fuel tank off, this job is best done with hand tools. A “manual” Phillips screwdriver, quarter-inch ratchet with a 7 mm socket, a pick and a fuel-line removal tool are all you’ll need. Here’s how the job goes.
Disconnect the battery first, then pinch the tab and pull the fuel pump electrical connector to disconnect it. I like to dab some dielectric grease in there before reconnecting it because it makes it easier to remove next time.
The fuel lines use those quick-connector clips that require a special removal tool. The line toward the rear of the car leaves enough room to get the whole tool onto it. Just slip the tool between the clip and the fuel nipple and slide it off.
The forward fuel nipple on the fuel pump assembly is a bit of a bugger. The bend in the fuel line prevents you from getting the clip removal tool around the nipple on both sides. I could buy another tool and grind it down to fit, but until that happens, I use half the clip-removal tool on one side and a pick tool on the other side. It works, but be careful not to poke holes in yourself or the plastic fuel lines.
There is one 7 mm nut that holds on a strap on the left side of the top plate.
The rest of the top plate is attached to the top of the fuel tank with small Phillips head screws. Take your time, skip the power tools and keep your magnet handy.
Here’s where you can see the black magic of the factory fuel sock material. It lets fuel in faster than it lets it out, keeping fuel concentrated around the pickup.
This is the part number and fuel sock for the BP4W Mazda engine. If you want reliability and the ability to carry as little fuel as possible, this is the only fuel sock you should use in a Miata.
The fuel pump is held in place with a “shoe” at the bottom, which is attached with a Phillips head screw. A manual, properly sized Phillps screwdriver works best to keep from rounding off the screw head.
The shoe has two oblong holes for the rubber insulator at the bottom of the fuel pump assembly.
The fuel pump sock slips over the fuel pump pickup and a locating pin, which uses a circlip retainer to hold the sock in place. You can use a pick tool to remove the circlip. Keep a magnetic pickup tool on it to keep the circlip from flying across the garage and getting lost.
Once the circlip comes off, you can remove the fuel sock.
Slide the new sock over the fuel pump inlet and snap the circlip on to hold it in place.
Put the rubber insulator over the bottom of the fuel pump and replace the shoe. Tighten the screw as much as possible without rounding the head.
You have to do a twist and turn to get it back in place. Then, replace all the retainer screws, the 7 mm nut and reconnect the fuel lines and the electrical connector last.
Brett, is this something you change proactively? Or do you just wait until it feels like you are starting to run out of gas, when you know there is enough gas in the tank?
Hi Louis,
Whenever I build a new car, I replace the fuel pump assembly with a new one from Mazda. The new assembly comes with a sock already installed, and it usually prevents trouble before it starts.
If you are getting starvation at low fuel levels in left-hand turns, the first thing I’d change is the sock, and I’d have a close look at the one that comes out. They do get brittle and deteriorate with age, which is what allows the fuel to flow away from the pickup on the pump itself.