MiataRoadster

Changing the Fuel Filter

by Jim Creer

Intro:

I've done enough fuel filter changes to decide there is an easier way to do it than following manual instructions. I can change one in about 10 minutes, without the use of golf tees or line clamps. When finished, the dirty fuel is still trapped inside the filter for easy disposal, and the fuel spilled will be a few drops. We will then test the new filter connections for leaks with at least twice the pressure found during any actual road use. The manual also calls for things like disconnecting the battery, using line clamps, and wearing protective goggles and gloves. If you feel more comfortable doing it that way... please do so! I do also fully agree to work in a ventilated area.

Tools Needed:

Hunk of 5/16" fuel line in the event that you may need to replace a section, rag, socket for the filter bracket, assorted pliers and 4 in 1 screwdriver, since we're unsure of what clamps will be on the hoses.  The knife can be used to cut hoses as needed and also serves as a lever to pry up any damaged plastic screws securing the cover. You may not need all of the stuff, but plan to make a minimum amount of trips underneath.

The hardest part of the job is getting these old bones up and down from the ground.  The taller ramps provide enough space to work easily by crawling under the passenger rear bumper.  Not shown, but the front wheels are chocked in both directions.  Big difference in being lazy versus being stupid! Have all tools handy for easy reach.

Procedure:

The replacement filter came with one cap missing. Not an issue, unless you're wishing to follow remaining instructions. I made a cap with a short hunk of line and fat screw. During the install, we will be switching one line at a time and replacing each cap to the old filter. The is no need to drain the lines, contaminated fuel is trapped inside the old filter, and with a small amount of dexterity, we will only lose a few drops of fuel. But, this will not work unless the fuel system is properly depressurized.

Removing the fuel cap does not depressurize fuel to the filter or the fuel rail. It merely relieves pressure in the fuel tank and return fuel line from the stock FPR. This step is rather minor as we are working on the other side of the FPR. Remove it anyway, but remember we will still have nearly 30 PSI remaining at the fuel filter after engine is turned off. With an electronic fuel pressure gauge installed, I've found the pressure will slowly dissipate over time but can still show some pressure after 8 hours.

With engine at idle, disconnect the clunky yellow plug under the dash near the steering column. This is the Circuit Opening Relay (1.6), or the Fuel Pump Relay (1.8). Slightly different function, but the end result is the same. It shuts off power to the fuel pump, so in a few moments the engine will stall. When that happens, replace the plug and switch ignition key to OFF. Fuel to the filter is now nearly depressurized. Lines are still filled with fuel and can dribble out by gravity. Still not 100 % depressurized, since will still retain a tiny amount of pressure. If you wrap a rag around the filter line when twisting the first fuel line loose, the tiny spurt of fuel will be absorbed by the rag.

Crawling under the passenger side rear bumper, locate the fuel filter cover. The cover is held in place by 4 or 5 plastic type screw clamps. Frequently they become stripped, so use the knife edge to pry them out slightly, and use cross tip screwdriver to remove. If you find a few broken ones, you should be able to go to your local auto parts store for suitable replacements. Once cover is removed, your lines will probably look a bit different from mine, since I routed my fuel lines to other side of PPF for a Pierberg secondary fuel pump.

Here is what we see with the cover off. Loosen both hose clamps and slide up the lines a few inches to get them out of the way.The fuel lines will probably be stuck on there fairly tight. Use the rag to pad the pliers jaws and break them loose. This is where the last tiny bit of pressure is released. If you have not depressurized the fuel system as indicated above, you will receive a liberal dousing of pressurized fuel. Do not completely remove the lines, just loosen them up enough so they can easily be removed by hand. Does not matter if the metal filter lines are smashed a little, as we will be throwing the old filter away. Just don’t damage the hoses.

Here the hose clamps are slid up the hoses, the fuel lines are broken loose but not removed, and old filter is removed from bracket and dangling loose on the fuel lines. Next step is to put new filter inside bracket, but leave it loose so you can slide it around a bit as needed.

Here the new filter is loosely held in place. I have already swapped out one of the lines. Dexterity is needed here to cap the filter and the fuel line with your fingers until you get one line on the new filter, and put the cap on the old filter. If you don't trust your dexterity, you can use some line clamps or golf tees, as desired. There is no pressure on the lines and you're only clamping off flow from gravity.

Here, both lines are swapped over and everything is secured in place. Dirty fuel is trapped in the old filter for no spills and simplifies proper disposal. Prior to replacing cover, I suggest testing it for leaks.

I'm unaware of anyone else suggesting doing this, but it makes sense to me! Providing an optional additional test for leaks: Strap the F/P and GND in the diagnostic test box under hood. This allows the fuel pump to run when ignition switch is in the ON position when engine is not running.

A very minor benefit here is priming the fuel lines to remove any air to save a bit of wear and tear on the starter.  Minor yes, but still a freebie!

With the fuel pump running, put a padded clamp on the fuel return line. Pinching the line shut will drive the fuel pressure to the filter to around 78 - 85 PSI. This is about double what it would see in any actual driving test. If you try this with the engine running, it will probably stall the engine.

It should not take more than 10 minutes to do this job... Well, the second time you do it!


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16 January, 2007



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