Fuel Pressure Regulator Replacement

Replacing the Fuel Pressure Regulator

Fuel injected bus... This is done on the Guac, a 78 with stock fuel injection.

Time: Approx 20 minutes

Tools: 17mm open end wrench, 2 line clamps, screwdrivers for worm clamps, dykes for crimp clamps if necessary.

My fuel pressure regulator (FPR) had developed a vacuum leak. So a couple months ago I pulled the vacuum line and plugged it. Bus ran fine so I forgot about it. Until I was discussing on the Itinerant Air Cooled Forum my fuel filter issues. It was suggested that maybe a bad pressure regulator could be the culprit. By releasing pressure too early, it could be causing a vapor lock condition, allowing the fuel to boil in the lines. Remembering that my FPR was leaking vacuum, I thought maybe it was affecting other things too. I decided to replace it. Just in case...

How a Fuel Pressure Regulator Works

In the Bosch L-Jetronic systems, the fuel pressure regulator is a mechanical device that uses a spring to release overpressure in the fuel system.

With everything in place, the regulator holds fuel pressure as follows:

  • Engine under load: 2.5 bar

  • Engine at idle: 2 bar

  • Engine under decel: 1.5 bar

What`s happening here? Just like a power brake booster, vacuum pulls against the diaphragm in the regulator, aiding the fuel in pushing the spring away from the valve and allowing the fuel to return to the tank. At full load, wide open throttle conditions there is little vacuum available to assist the fuel. So it takes more pressure to open the valve. Under decelleration there is HUGE vacuum available, so it takes much less pressure to open the valve.

Why do we want to vary the pressure needed to open the return valve?

When idling or under decel, we use very little fuel. There`s no need for lots of pressure at the injectors. So by reducing the pressure at the injector, we put less fuel into the engine when we don`t need it anyway. So it helps with both emissions and fuel economy.

Now let`s swap regulators:

  1. Crawl under bus with 17mm open end wrench and a line clamp. I prefer to go under the bus forward of the rear wheel on the passenger side, instead of going under the engine. Too much grease on the engine that gets all over me.

  2. Loosen the nut so it can be turned with your fingers. Don`t remove it yet.

    Fuel Pressure Regulator from under the bus
  3. Clamp the fuel line. There`s more room to clamp between the FPR and the steel line, though you can clamp the hose from the fuel tank as well if you prefer.

  4. Loosen the worm clamp at the FPR and pull the fuel line loose.

  5. Remove the 17mm nut with your fingers. Use your thumb and index finger to hold the fuel line nipple while using your middle or ring finger to spin the nut the rest of the way off.

  6. Crawl out from under the bus and open the engine hatch.

  7. Inside the engine compartment, attach the line clamp at the high pressure hose from the fuel injection rail to the side of the regulator.
    Fuel Pressure Regulator from inside the engine compartment

  8. Loosen the worm clamp (if used) or cut the pinch clamp. Pull the fuel line off the regulator. Pull the vacuum line off as well.
  9. The regulator should just fall out of the firewall tin.

  10. Installation is the reverse of removal. (Don`t you hate when manuals say that?)


This is a GREAT time to inspect your fuel lines. The high pressure line to the fuel rail looked pristine from the outside. But here`s what the inside looked like:

Cracked fuel line

Obviously not reusable.

The reason you loosen the nut before removing any hoses is the @#$%^ regulator can sometimes turn with the nut. The lines hold it in place.

The crimp clamps are a pain to remove. There are two ways to do it. If you have a pair of cutters ("Dykes") you can cut the part that is crimped. If you don`t the other end of the clamp is just two overlapping ends held with a hook. Pry with a thin screwdriver to get them separated. Cutting is easier.

The FPR isn`t mounted square. In the photo above you see the cable grommet is sticking out at a different angle than the regulator nipple. The tin it`s mounted to has a bend in it setting the regulator at that angle. It`s SUPPOSED to be like that, but can be disconcerting when putting the nut on by touch. Yes, it`s dark under there. Yes, it would have been nice to have a flashlight. But my flashlight batteries were dead. Sigh.

The vacuum lines in the above pic are shot. I need to order some new ones...

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